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Niacin is the most effective lipid-modifying agent for raising high-density lipoprotein cholesterol levels, but it also causes cutaneous vasodilation with flushing. To determine the frequency of flushing in clinical trials, as well as to delineate counseling and treatment approaches to prevent or manage flushing, a MEDLINE search was conducted of English-language literature from January 1, 1985, through April 7, 2009. This search used the title keywords niacin or nicotinic acid crossed with the Medical Subject Headings adverse effects and human. Niacin flushing is a receptor-mediated, mainly prostaglandin D2–driven phenomenon, the frequency, onset, and duration of which are largely determined by the distinct pharmacological and metabolic profiles of different niacin formulations. Subjective assessments include ratings of redness, warmth, itching, and tingling. In clinical trials, most (>60%) niacin users experienced mild or moderate flushing, which tended to decrease in frequency and severity with continued niacin treatment, even with advancing doses. Approximately 5% to 20% of patients discontinued treatment because of flushing. Flushing may be minimized by taking niacin with meals (or at bedtime with a low-fat snack), avoiding exacerbating factors (alcohol or hot beverages), and taking 325 mg of aspirin 30 minutes before niacin dosing. The current review advocates an initially slow niacin dose escalation from 0.5 to 1.0 g/d during 8 weeks and then from 1.0 to 2.0 g in a single titration step (if tolerated). Through effective counseling, treatment prophylaxis with aspirin, and careful dose escalation, adherence to niacin treatment can be improved significantly. Wider implementation of these measures should enable higher proportions of patients to reach sufficient niacin doses over time to prevent cardiovascular events.
GPR = Gi protein–coupled receptor; HDL-C = high-density lipoprotein cholesterol; NSAID = nonsteroidal anti-inflammatory drug; PG = prostaglandin; RCT = randomized controlled trial
Reducing low-density lipoprotein cholesterol levels is the primary treatment for preventing cardiovascular disease, largely because of robust evidence from randomized controlled trials (RCTs) involving statins.1,2 However, many patients optimally treated with statins have residual cardiovascular risk3 associated with low levels of high-density lipoprotein cholesterol (HDL-C), elevated levels of triglycerides, and a preponderance of small, dense (atherogenic) low-density lipoprotein particles.
Niacin (3-pyridine-carboxylic acid; C6H5N02) is the most effective medication to raise HDL-C levels. A water-soluble B-complex (B3) vitamin used to treat pellagra, niacin was discovered to lower cholesterol levels at higher (gram) doses in 1955.4 It was also the first lipid-lowering therapy proven in RCTs to significantly lower (by 27%) the risk of myocardial infarction and the risk of all-cause mortality in the long term (by 11%),5,6 as well as confer significant angiographic benefits (Table 1).7,10-15
Niacin improves the entire lipid panel in patients with dyslipidemia, lowering apolipoprotein B–containing lipoproteins and raising apolipoprotein A–containing lipoproteins (eg, high-density lipoproteins).16 Putative mechanisms for these multidimensional lipid benefits involve interactions of niacin with its Gi protein–coupled receptor (GPR109A) in adipose tissue, reducing free fatty acid release.17,18
One potential obstacle to effective use of lipid therapies is suboptimal treatment adherence and long-term persistence.19-27 In recent studies, approximately 53% of niacin users did not reach recommended daily maintenance doses of 1.0 g or higher, 92% did not reach doses of 2 g, and flushing severity significantly predicted niacin treatment discontinuation.28,29 This systematic review (1) explores the pathophysiology of niacin flushing, (2) characterizes its typical clinical presentation, (3) assesses the approximate frequency of niacin flushing and of discontinuations ascribed to flushing in clinical trials, (4) explores potential strategies to prevent or minimize flushing, and (5) surveys ongoing controversies.
An English-language MEDLINE search dating from January 1, 1985 (the year of a key active-comparator trial of immediate- and sustained-release niacin30), to April 7, 2009 (date of searches), was performed. This search used the title keyword niacin or nicotinic acid crossed (using the Boolean operator "AND") with the Medical Subject Heading adverse effects and the Medical Subject Heading term human, also joined by the Boolean operator "AND." These sets were crossed with the publication types of correspondence, case reports and series, and comments/editorials using the operator "NOT," thereby excluding these types of publications. Also excluded using "NOT" were articles related to smoking (NOT: title keyword nicotin*, smok*, cigar*) or schizo* (the normal flushing response is blunted in many patients with schizophrenia). This search returned 205 citations, of which 56 are included in this review. Many of the remaining articles, including reviews and meta-analyses, were subjected to "saturation searches," which produced additional articles of interest. Other articles were known to the author or identified by using title keywords, including flush*, vaso*, cutan*, prostagl*, Niasp*, and larop*. Other sources of data included full US-prescribing information for niacin-containing treatments and online reports from regulatory agencies.
The earliest form of niacin was immediate-release niacin, which is also termed plain or crystalline niacin. Some formulations are available as dietary supplements, whereas others have Food and Drug Administration–approved labeling as lipid-modifying therapies (Niacor; Upsher-Smith, Minneapolis, MN; Nicolar, sanofi-aventis, Bridgewater, NJ).11 Early attempts to reformulate niacin largely to limit flushing resulted in sustained-release niacin, which releases niacin more slowly via combination with an inert (resinous) base.11 Examples include Slo-Niacin (Upsher-Smith) and Nicobid (sanofi-aventis). Endurance Products Company (Tigard, OR) manufactures niacin formulated in a wax matrix (ENDUR-ACIN). Finally, inositol hexani-cotinate (Niacinol; Integrative Therapeutics, Green Bay, WI) is a synthetic prodrug that consists of 6 molecules of niacin esterified to 1 molecule of inositol. Often advertised as "flush-free" niacin, it also appears to be "effect-free" because the esterified niacin is not bioavailable.31
Food and Drug Administration–approved, prescription niacin-containing treatments include extended-release niacin (Niaspan; Abbott Pharmaceuticals, Abbott Park, IL), extended-release niacin and lovastatin (Advicor; Abbott Pharmaceuticals), and extended-release niacin and simvaastatin (Simcor; Abbott Pharmaceuticals).32-34 Although the terms extended-release, sustained-release, slow-release, controlled-release, long-acting, and timed-release niacin all denote different forms of extended-release niacin products, for the purposes of this review, the term extended-release niacin is used only for references specifying any of these terms (or Niaspan, Advicor, or Simcor).
Interactions between niacin and GPR109A receptors on epidermal Langerhans cells result in increased cytosolic calcium; this increase triggers increases in phospholipase A2 activity and expression of prostaglandin (PG) D2 synthase, with elevated mobilization of arachidonic acid into vasodilator eicosanoids and prostanoids.35-37 Lipid-lowering doses of niacin are associated with 430- to 800-fold increases in circulating levels of a stable PGD2 metabolite (9-α-11-β-PGF2).38 PGD2 binds to DP1 receptors on local capillary smooth muscle cells,37,39 causing capillary vasodilation in the papillae of the upper dermis.40 Acetylsalicylic acid (aspirin) and nonsteroidal anti-inflammatory drugs (NSAIDs) can prevent or blunt flushing by blocking PGD2 synthesis via cyclooxygenase (chiefly, cyclooxygenase 1). However, other PGs (eg, PGE1), other mediators (eg, serotonin), and other signaling pathways may also be involved. (An elaborate consideration of mechanism exceeds the scope of this review, which is clinically focused and hence includes only human studies.)
The pharmacodynamic effects of niacin are linked to its hepatic metabolism via 1 of 2 saturable pathways41,42 (Figure 1): a low-affinity, high-capacity pathway, in which niacin is conjugated with glycine to form nicotinuric acid, and a high-affinity, low-capacity amidative pathway, in which niacin forms nicotinamide. It was once thought that the conjugative pathway, and particularly formation of nicotinuric acid, was associated with flushing and that the amidation pathway, and particularly formation of nicotinamide, was associated with hepatotoxicity; the metabolic fates of niacin formulations via either of these pathways are in turn influenced by these formulations' dissolution and pharmacokinetic profiles. However, a more recent consideration of the evidence by Stern43 emphasized that the parent compound nicotinic acid, through its interaction with the GPR109A receptor, is the chief mediator of flushing and not any particular metabolite, such as nicotinuric acid, formed via the conjugative pathway. This review also concluded that "unlike flushing, the mechanism of nicotinic acid hepatotoxicity has not been determined. Specifically, there is no evidence that a metabolite is responsible … ."43
Because immediate-release niacin is completely absorbed from the gastrointestinal tract in 1 to 2 hours, immediate-release niacin rapidly saturates the low-capacity amidation pathway, such that most of the drug is metabolized via the conjugative pathway.11 The dissolution and gastrointestinal absorption of sustained-release niacin are not complete until after more than 12 hours. Sustained-release niacin is chiefly metabolized by the amidation pathway and has been associated with hepatic dysfunction (and potentially related gastrointestinal effects) in up to 52% of patients.44,45 With an intermediate dissolution and absorption profile (complete in 8-12 hours), extended-release niacin has a better balanced hepatic metabolism across the conjugative and amidative pathways.19,41
Although flushing typically occurs as levels of circulating niacin increase, long-term tolerance to flushing results not from decreased niacin levels per se but rather from reduced output of PGD2 over time with continued dosing.46
Virtually all niacin users initially experience flushing,19 typically commencing 15 to 30 minutes after ingesting immediate-release niacin, 30 to 120 minutes after ingesting extended-release niacin, or at more variable times after sustained-release niacin ingestion. Niacin users often experience "prickly heat" or a sense of warmth in the face, neck, ears, trunk, and, less frequently, the upper or lower extremities.47 Other common features include erythema, itching, and tingling. Symptoms typically last for less than 1 hour to 2.5 hours.48,49 A meta-analysis demonstrated that the overall incidence of flushing with extended-release niacin was similar in women (79%) and men (82%), but women were significantly less likely to complete extended-release niacin studies (68% vs 79%; P=.014) and spent significantly less time using these medications (mean, 15.5 vs 17.3 weeks; P<.001).50
A self-administered questionnaire with up to 22 items, the Flushing Assessment Tool evaluates the severity and troublesomeness of cutaneous warmth, redness, itching, and/or tingling. Individual and overall symptoms are rated on a scale of 1 to 10, in which 1 through 3 signify mild flushing and 10 indicates very severe flushing.51 An 8-week validation trial was conducted in 276 adults with dyslipidemia.51 The overall troublesomeness of flushing was significantly correlated with treatment dissatisfaction. However, changes in most Flushing Assessment Tool measures from treatment week 1 to 6 suggested minimal effects of flushing on quality of life.51
The Flushing Symptom Questionnaire comprises 11 items, each of which is scored as 0 (no symptom) to 10 (extreme symptoms). Item 3 concerning the severity of overall flushing symptoms is also termed the Global Flushing Severity Score. A similar 8-week RCT involving 175 candidates for niacin therapy was conducted to determine the validity of the Flushing Symptom Questionnaire.52,53 During the first niacin treatment week, 47% to 58% of patients experienced daily flushing episodes, most of which were mild and minimally bothersome, and 16% had sleep disturbances. In all, 43% to 49% of those taking 1.0 g of niacin had moderate or greater mean flushing (severity score of ≥4) compared with less than 15% of placebo recipients.52 The mean proportion of days with moderate or greater flushing decreased from 34% in treatment week 1 to 18% at week 2 or afterward.53 Fourteen extended-release niacin users (8%) discontinued treatment because of flushing, including 11 (79%) in the first treatment week.
Most of these studies evaluated the effects of immediate-release niacin, which in some cases was combined with bile acid resins and/or diet, on clinical, angiographic, and/or imaging end points (Table 1).5-10,12,15 In these studies, flushing was reported by up to 92% of patients receiving niacin-containing regimens but caused treatment discontinuation in smaller proportions (2.5%-10%).
Studies summarized in Table 254-64 evaluated the effects of a combination regimen of extended-release niacin and a statin as well as of other combination regimens on surrogate (lipid or lipoprotein) outcome measures. These studies demonstrated that many patients (29%-74%) in these trials experienced at least 1 flushing episode, 75% to 92% of episodes were rated as mild or moderate, flushing frequencies declined over time, and approximately 4% to 12% of patients discontinued treatment because of flushing.54-60 The long-term OCEANS (Open-Label Evaluation of the Safety and Efficacy of a Combination of Niacin Extended-Release and Simvastatin in Patients with Dyslipidemia) study showed that less than 40% of patients who experienced flushing in the first 12 weeks of treatment, when extended-release niacin/simvastatin doses were being escalated from 0.5 g/40 mg to 2.0 g/40 mg, did so during the following 12 weeks.57
In the SEACOAST (Safety and Efficacy of a Combination of Niacin ER and Simvastatin in Patients With Dyslipidemia) trial, 27% of patients who experienced flushing while taking 1.0 g/20 mg of extended-release niacin/simvastatin (and 30% of patients taking 2.0 g/20 mg of extended-release niacin/simvastatin) during the first 12 weeks but who completed the study did not flush during the second 12 weeks.56 In the recent SUPREME (Study to Compare the Lipid Effects of Niacin Extended-Release and Simvastatin to Atorvastatin in Subjects With Hyperlipidemia or Mixed Dyslipidemia) trial, extended-release niacin was up-titrated more aggressively, from 1.0 g for treatment weeks 1 to 4 to 2.0 g for weeks 5 to 1254; despite this, approximately 47% of patients experienced no flushing during weeks 1 to 4, 55% experienced no flushing during weeks 5 to 8, and 70% experienced no flushing during weeks 9 to 12 (Figure 2).54 The overall incidence of flushing was approximately 39% during treatment week 1 and 26% at week 5 (when the dose was advanced).
Studies summarized in eTable 165-73 (available online, linked to this article) evaluated the effects of niacin monotherapy on surrogate (lipid or lipoprotein) end points and patient-related flushing per se (ie, flush provocation studies). In 1 long-term study, more than 40% of patients using extended-release niacin experienced flushing at treatment week 4 compared with less than 20% at week 96.65 The mean per-patient incidence of flushing per month decreased approximately 10-fold: from 1.9 episodes to 0.19 episode.
In a meta-analysis74 of HDL-C–raising therapies involving more than 4000 patients using immediate-release niacin (n=3062),75-83 sustained-release niacin (n=536),66,67,77,84-86 or extended-release niacin (n=648),68-71,87,88 some of the results of which are summarized in eTable 1, most patients (70%) receiving niacin experienced flushing (vs 4% with placebo; P<.001). This proportion included 85% of patients receiving immediate-release niacin, 66% receiving extended-release niacin, and 26% receiving sustained-release niacin.74 Approximately 13% of niacin users discontinued treatment (vs 6% of those receiving a placebo).
Extended- or Prolonged-Release Niacin. Eleven additional published reports provided data on flushing in more than 7000 patients.89-99 A particularly low frequency of flushing was observed in the IMPACT (Impact of Medical Specialty on Patient Compliance to Treatment) study, a 12-week, open-label trial involving 4499 patients with dyslipidemia receiving combination therapy with extended-release niacin and lovastatin (0.5 g/20 mg to 1.0 g/40 mg) once nightly.90 Before beginning therapy, patients received a recorded message about controlling cholesterol, dietary and physical activity counseling, and extended-release niacin's safety profile, with a focus on flushing. Patients also received written reminders to take their medications. At treatment week 8, patients were reevaluated, with laboratory monitoring, queries about adverse events, and adherence reminders, including dosing at bedtime with a low-fat snack and taking aspirin or an NSAID 30 minutes before niacin dosing. Only 18% of extended-release niacin users reported daily flushing, and 6% of patients discontinued niacin because of flushing.
Sustained-Release Niacin. Three prospective trials44,100,101 and 2 retrospective studies102,103 of patients receiving sustained-release niacin were identified. In a study by McKenney et al,44 higher proportions of patients receiving immediate-release niacin (22%-53%) than sustained-release niacin (13%-22%) at daily doses of 0.25 to 3.0 g experienced vasodilator symptoms. In the other prospective studies, 0% to approximately 7% of patients discontinued use of sustained-release niacin because of flushing.100,101 Other studies did not report flushing data specifically by niacin formulation and were excluded.104-107
Immediate-Release Niacin. Nine smaller studies assessed the effects of immediate-release niacin, alone or in combination, on more than 600 patients receiving immediate-release niacin-containing regimens in prospective108-115 and retrospective116 studies. Many involved high starting doses of immediate-release niacin or rapid titration schedules. The percentage of patients discontinuing use of immediate-release niacin because of flushing ranged from 0% to 21%. Other studies without specific flushing data were excluded.104,117-119
According to the Health Belief Model,120-123 patients are more likely to adhere to their treatments if they understand that they are susceptible to a disease, that treatment will reduce this susceptibility (without causing offsetting untoward effects), and if they are personally able to adopt the treatment. Given that dyslipidemia is a largely asymptomatic condition, it can be difficult to persuade a patient with no observable symptoms to take a medication that initially causes discomfort. Accordingly, it is important to explain the potential benefits of niacin therapy; the typically transitory, reversible, and harmless nature of flushing; and, in contrast, the stable and lasting coronary preventive benefits of continued niacin administration (Table 3). Patients need to know that niacin for dyslipidemia is essentially a "lifetime drug."
Engaging the patient in this sort of dialogue can help to cement a stronger physician-patient bond. Patients are advised to take niacin with a low-fat snack (eg, applesauce) at bedtime or with a meal, which can improve both vasocutaneous and gastrointestinal tolerability. Taking niacin with food may also increase niacin's bioavailability,124 whereas taking it at bedtime allows many patients to sleep through any flushing and may blunt nocturnal release of free fatty acids.
Counseling approaches recommended by the National Lipid Association to prevent or minimize flushing are presented in Table 4.125 As suggested by the IMPACT study,90 ongoing patient education and reminders may assist in promoting niacin adherence. Patients are asked to describe the symptoms of flushing by formulation; for instance, flushing with immediate-release niacin is probably the shortest lived and may be a more consistent experience compared with other formulations.126 Immediate-release niacin should be administered in divided doses, typically in the middle of breakfast and dinner, and after taking 325 mg of aspirin on awakening (at least during the early weeks of treatment).126 One approach is to initiate immediate-release niacin treatment at 0.05 to 0.10 g (50-100 mg) twice daily for the first week, then double the daily dose each week until the daily dosage is raised to 1.0 to 2.0 g.127
Patients should be advised that flushing is a nonallergic, typically time-limited phenomenon; although uncomfortable, it is not usually harmful. Flushing will not occur with each dose and should subside during a period of weeks if not days with continued dosing. Failure of flushing to abate with time may reflect inconsistent dosing. If the patient has not taken niacin in 3 or more days, it may be necessary to restart treatment at a low dose and reescalate. To improve patient expectations of flushing, it may be useful to administer the starting niacin dose in the office. In a number of clinical trials, patients took either 325 mg of a non–enteric-coated aspirin or an NSAID (eg, 200 mg of ibuprofen) 30 to 60 minutes before (but not 15 or 120 minutes before) niacin use, and this approach has been endorsed by the National Lipid Association.125 Some authorities recommend taking 325 mg of aspirin the day before the first niacin dose and continuing its use for the first month of treatment.126 Given the perception of heat associated with flushing, patients may wish to take their niacin with a cold (non-alcoholic) beverage. They should avoid alcohol, spicy foods, hot beverages, and hot showers or baths around the time of dosing. Patients should also be advised to avoid missing niacin doses and to report any such lapses to their clinicians.
In contrast to certain RCTs that did not allow back-titration (down-titration), the physician and patient have wide latitude to individualize the schedule of niacin dose escalation. Some sensitive patients will need to "start lower and go slower" or even "start low, go slow, and go back." To assist in this process, it is important to schedule a follow-up visit with laboratory monitoring approximately 4 to 8 weeks after initiating niacin treatment and to identify other patient concerns about flushing. Doing so enables the physician to assess patient safety and toleration of niacin, make any needed medication adjustments, and review key counseling messages. The follow-up visit also offers an occasion to revisit the lipid and coronary preventive benefits of niacin, focus patients on attainable intermediate objectives (eg, achievement of cholesterol level goals), and hence promote adherence.128
Optimized Niacin Formulation. Extended-release niacin was reformulated with a different coating to optimize its dissolution, absorption, and metabolic profiles. In a flush provocation study, 53% of patients receiving 650 mg of aspirin before treatment and 61% receiving 650 mg of concomitant aspirin experienced flushing with 2.0 g of reformulated extended-release niacin.129 Aspirin before treatment reduced the median duration of the first flush by approximately 43% (37 vs 65 minutes for extended-release niacin alone; P=.008), the mean intensity by 34% (P<.001), and the proportion of patients with moderate or greater flushing by 62% (14% vs 37%; P<.001).
Aspirin and NSAIDs. Findings from studies specifically evaluating adjunctive or prophylactic aspirin or NSAIDs to attenuate niacin flushing are summarized in eTable 2 (available online, linked to this article).129-134 Of note, 325 mg of aspirin significantly (P<.001 vs placebo) attenuated flushing symptoms when administered 30 to 60 but not 15 or 120 minutes before immediate-release niacin, and an aspirin dose of 650 mg was not superior to 325 mg.132 In another study, pretreatment with 325 mg of aspirin 30 minutes before niacin was significantly superior to 200 mg of ibuprofen in blunting flushing and itching in patients experiencing these manifestations.133 In a previous study, 325 mg of aspirin was superior to 80 mg of aspirin in mitigating flushing with 0.5 g of immediate-release niacin.134
The flushing-specific discontinuation rate in a recent 4-week study was 1.8% among patients randomized to receive 325 mg of aspirin before extended-release niacin treatment compared with 9.4% among those receiving extended-release niacin without aspirin (P=.007).130 In this study, 15% of patients receiving aspirin prophylaxis with a 0.5- or 1.0-g starting dose of extended-release niacin experienced moderate or greater flushing during the first treatment week (vs 29% not receiving aspirin; P=.01). Pretreatment with aspirin significantly reduced mean maximum flushing severity, number of moderate or greater flushing episodes, and the incidence of flushing by 18% (P=.014; Figure 3).130 The incidence of flushing was also significantly lower among patients receiving an extended-release niacin starting dose of 0.5 g (32%) compared with 1.0 g (53%; P=.05).
A literature review135 demonstrated that the mean rate of treatment discontinuation because of niacin flushing was approximately 6.5% in studies in which 325 mg of aspirin was recommended as needed or mandatory to prevent or minimize flushing: 8% in studies using extended-release niacin vs 18% in those using immediate-release niacin. The mean incidence of niacin flushing across the studies was 29% (range, 18%-100%). In a recent study, 23% of patients receiving extended-release niacin and aspirin experienced flushing during the first treatment month compared with 32% of those without prior aspirin therapy (P=.02).89
Laropiprant. A selective PGD2 receptor (DP1) antagonist approved as a treatment adjunct to mitigate niacin flushing in certain non-US markets, laropiprant has been combined with extended-release niacin (modified- or prolonged-release niacin; Tredaptive; Merck, Whitehouse Station, NJ). The proprietary extended-release niacin formulation in Tredaptive is not identical to the proprietary formulation in Niaspan, Advicor, and Simcor.
In healthy volunteers, laropiprant is rapidly absorbed (median time to maximum drug concentration, 0.8-2.0 hours), with no effects of food on absorption, and the apparent terminal elimination half-life is 10 to 18 hours.136
In a single-dose study, adjunctive (concomitant) laropiprant significantly reduced flushing associated with 1.5 g of extended-release niacin: by 47% for a 30-mg dose of laropiprant, by 67% for a 100-mg dose, and by 74% for a 300-mg dose.137 Coadministration of extended-release niacin with either 30 mg or 100 mg of laropiprant decreased peak malar (cheek) skin blood flow (objective measure of vasodilation) by 50% and 70%, respectively. Reductions in patient-rated flushing correlated significantly with decreases in malar blood flow (r=0.64-0.68; P<.001).137 In multiple-dose studies, 200 mg of adjunctive laropiprant reduced time-weighted average flushing scores by 65% (vs 1.5 g of extended-release niacin alone) on day 1 and by 52% on day 3.137 In a phase 1 study (study 044), coadministration of 325 mg of aspirin did not confer significant incremental benefits in reducing the incidence and severity of residual flushing associated with 40 mg of laropiprant and 2.0 g of extended-release niacin.138
In a phase 2, dose-ranging study, adjunctive laropiprant (18.75 to 150 mg [pooled]) resulted in lower proportions of patients reporting moderate or greater flushing during the first week of treatment with 1.0 g of extended-release niacin (vs 1.0 g of extended-release niacin alone).139 In a pivotal phase 3 study (study 020), approximately 31% of patients who were using adjunctive laropiprant and extended-release niacin (1.0 g) experienced moderate or greater flushing during treatment week 1 compared with 56% receiving monotherapy with 1.0 g of extended-release niacin (P<.001) and 6% taking placebo.140 Corresponding proportions of patients with severe or extreme flushing were 14% in the laropiprant and extended-release niacin group and 33% in the extended-release niacin group (P<.001).140
During treatment weeks 2 to 24 in study 020, 14% of patients taking laropiprant and extended-release niacin experienced 1 day per week or more with moderate or greater flushing compared with 29% of those taking extended-release niacin monotherapy (P<.001) and approximately 2% of those taking placebo.140 During the maintenance phase, patients in the laropiprant and extended-release niacin group experienced approximately 1 day of moderate or greater flushing per patient-month compared with 1 day per patient-week in the extended-release niacin group. Commenting on these data, the authors contended that "tolerance to this level of flushing is incomplete during at least a 6-month period."141 Ten percent of the extended-release niacin and laropiprant group discontinued treatment because of flushing compared with 22% of the extended-release niacin group (P<.001) and less than 1.0% of the placebo group. The combination of laropiprant and extended-release niacin is being evaluated in the ongoing phase 3 HPS2-THRIVE (Heart Protection Study 2–Treatment of HDL to Reduce the Incidence of Vascular Events).
One ongoing controversy concerns schedules for extended-release niacin dose escalation. For many years, the adage about niacin dosing has been to "start low and go slow[ly]." More recently, however, studies have suggested that a shorter titration interval, on the order of 8 or 12 weeks (vs 16 weeks), to increase the niacin daily dose from 0.5 to 2.0 g may be associated with comparable or even superior flushing data. In the OCEANS study, 68% of patients using an 8-week extended-release niacin and simvastatin titration schedule experienced flushing compared with approximately 74% of those using a 12-week titration.57 Regimens of patients receiving the PGD2 antagonist laropiprant along with extended-release niacin have consisted of a 2-step schedule: initiating niacin treatment at the relatively high daily dose of 1.0 g, which was maintained for 4 weeks, and then escalating to 2 g at treatment week 4 and maintaining this dose for 12 weeks.142 In my practice, we use a combination of these 2 approaches, starting low (0.5 g/d) and going slowly (for a total of 8 weeks) until a stable dose of 1.0 g has been reached. At this juncture, the daily niacin dose is increased directly from 1.0 g to the maintenance daily dose of 2.0 g (if tolerated).
Although neither aspirin nor laropiprant influences the lipid-modifying effects of niacin, neither agent completely abrogates the flushing response. Aspirin can also saturate the glycine elimination (conjugative) pathway shared with niacin and, hence, in theory, actually increase plasma niacin levels. This could, in turn, contribute to increased GPR109A receptor–mediated, PGD2-driven flushing.134,143 However, this probably has never been observed because the net effect of 325 mg of aspirin is to reduce PGD2 levels and hence contribute to the development of tachyphylaxis.134 Furthermore, aspirin can increase the risk of gastrointestinal events and/or bleeding. Niacin is often associated with gastrointestinal effects, and both active peptic ulcer disease and arterial bleeding contraindicate treatment with niacin-containing regimens.33-35
In a clinical pharmacology study, laropiprant at supratherapeutic doses had no significant effect on bleeding time, whereas 325 mg of aspirin prolonged bleeding time by 44% (P=.004 vs placebo) 24 hours after administration.136 However, laropiprant also causes adverse gastrointestinal effects. In the pivotal laropiprant phase 3 study (study 020), 2.9% of the laropiprant and extended-release niacin group experienced nausea (vs 2.0% with extended-release niacin alone) and 2.6% experienced diarrhea (vs 1.8%).140 Across laropiprant studies 020, 022, and 054, 16.5% of the laropiprant and extended-release niacin groups experienced gastrointestinal adverse events (vs 11.7% of the extended-release niacin group).144
Despite a lower rate of discontinuation because of flushing in the laropiprant and extended-release niacin groups (approximately 4% vs 9% with extended-release niacin), a higher proportion of the laropiprant and extended-release niacin group discontinued treatment because of gastrointestinal (2.5% vs 1.5%) or laboratory adverse events (1.3% vs 0.7%), particularly abnormal liver function test results. In its Committee for Medicinal Products for Human Use assessment, the European Medicines Agency concluded that "the lower discontinuation rate due to flushing [with laropiprant and extended-release niacin vs extended-release niacin alone] is partly offset by other side effects, in particular the occurrence of gastrointestinal symptoms."144 In a recent study (reported after the literature search had been conducted), frequencies of gastrointestinal adverse events were comparable in patients randomized to laropiprant and extended-release niacin (1.0 g for 4 weeks followed by 2.0 g for 12 weeks) or extended-release niacin monotherapy (0.5 g titrated to 2.0 g at 0.5-g monthly increments) when data were normalized by time of exposure to extended-release niacin at 2.0 g (1.7% vs 1.8%).142 In this study, approximately 7% of patients in the laropiprant plus extended-release niacin group discontinued treatment because of flushing, compared with 12% in the extended-release niacin group (P=.002).142
In contrast, patients receiving laropiprant and extended-release niacin may have increased frequencies of niacin-related, nonflushing adverse events (vs extended-release niacin monotherapy) because of dose inequities; recipients of laropiprant and extended-release niacin might experience a somewhat higher frequency of nonvasodilatory untoward effects because they are able to achieve higher niacin doses without flushing. Treatment adjuncts such as laropiprant and aspirin typically need to be used for a relatively short period to tide patients over the early niacin dosing period, hence limiting overall exposure to these adjuncts. Given a low percentage of patients discontinuing laropiprant and extended-release niacin therapy after treatment week 16 and the dubious effects of long-term withdrawal of laropiprant on extended-release niacin flushing, the European Medicines Agency also concluded that the efficacy of laropiprant had not been demonstrated for more than 24 weeks.144
Perhaps the most appropriate use of aspirin and laropiprant, along with counseling and other measures, should be as temporary "bridges" that are targeted to patients during the first several weeks of treatment, when niacin doses are being advanced, as well as when restarting niacin treatment after a lapse in dosing. Studies are needed to predict which patients are most likely to experience poor vasocutaneous toleration of niacin and target the most intensive counseling and pharmacological support to these individuals. However, no study to my knowledge has yet demonstrated an "at-risk genotype" for flushing. Studies are also needed to compare the effects of aspirin prophylaxis and adjunctive laropiprant on flushing, as well as to establish an optimal dosing paradigm (eg, via studies comparing extended-release niacin dosing with dinner or at bedtime).
Other potential treatment adjuncts to mitigate niacin flushing include agents targeting the serotonergic component of the vasocutaneous reaction (eg, cyproheptadine [off-label indication]). Other candidates include flavonoids (eg, quercetin [Algonot Plus; Algonot LLC, Sarasota, FL] and luteolin [Luti-Max; SYNORx Inc, San Clemente, CA]), which also inhibit mast cell PGD2 secretion.145,146 Another possible approach is to develop new niacin receptor (GPR109A) agonists with a superior therapeutic index for inhibiting lipolysis (and hence improving lipid levels) over vasodilation, even though both of these processes are receptor-mediated.147
Limitations of this review include potential publication bias; the decision not to include case reports and case series; a focus on data from RCTs, in which patients are often highly selected for superior treatment adherence (potential selection bias); and inclusion of healthy volunteers in some of the studies. Many studies reviewed reported frequencies of flushing separately from those of the individual symptoms (eg, pruritus [itching] or paresthesia [tingling]). These factors may have caused an underestimation of the true incidence and severity of flushing associated with niacin compared with more naturalistic clinical settings.
Many RCTs in this review did not determine whether decreases in mean incidences of flushing over time with niacin truly resulted from tachyphylaxis in most patients or from a survivor effect in which patients with poor toleration were more likely to discontinue treatment in the early weeks, leaving a more flushing-resistant phenotype in the later weeks. In future trials, conducting an analysis of "time to first flush" or "time [or cumulative niacin dose] to flushing-related discontinuation" might help to adjust for this potential statistical artifact.142,148,149
Data on available niacin receptor agonists (Acipimox [Pfizer]) or from proprietary databases (eg, EMBASE) were not reviewed. Many reports providing data on flushing did not explicitly state whether the aspirin administered was enteric-coated. Combined with cellulose and/or other inert ingredients that resist gastric disintegration, coated aspirin is absorbed more slowly than other forms, with a time to peak concentration of approximately 3 to 4 hours compared with 0.25 hour to 2 hours for uncoated aspirin, which is closer to the time at onset of flushing with many niacin formulations. Hence, coated aspirin may need to be administered more than 30 minutes before niacin, although no clinical study has evaluated the comparative antiflushing efficacy of coated (vs uncoated) aspirin.150
Niacin is the most effective agent for raising HDL-C levels, but its use has been constrained by potentially dose-limiting, receptor-mediated, mainly PG-driven cutaneous vasodilation with flushing. Although typically mild or moderate and reversible with continued dosing, niacin flushing warrants effective counseling, treatment adjuncts or prophylaxis, careful dose escalation with follow-up monitoring, input from the patient, and regimen adjustments as needed. Wider implementation of these measures should enable higher proportions of patients to reach and persist with sufficient niacin doses over time to prevent cardiovascular events.
Assistance in research and editorial support were provided by Stephen W. Gutkin, BA, Rete Biomedical Communications Corp, Wyckoff, NJ.
PubMed Central Canada is a service of the Canadian Institutes of Health Research (CIHR) working in partnership with the National Research Council's national science library in cooperation with the National Center for Biotechnology Information at the U.S. National Library of Medicine(NCBI/NLM). It includes content provided to the PubMed Central International archive by participating publishers.


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University of Toronto, Division of Biophysics and Bioimaging, Ontario Cancer Institute/University Health Network and Department of Medical Biophysics, 610 University Avenue, Toronto, Ontario M5G 2M9 Canada
University of Toronto, Division of Urology, Developmental & Stem Cell Biology, Sick Kids Hospital, 555 University Avenue, Toronto, Ontario M5G 1X8 Canada
University of Toronto, Department of Radiation Oncology, 610 University Avenue, Toronto, Ontario M5G 2M9 Canada
This article was corrected | View correction
Microstructural remodelling in epithelial layers of various hollow organs, including changes in tissue anisotropy, are known to occur under mechanical distension and during disease processes. In this paper, we analyze how bladder distension alters wall anisotropy using polarized light imaging (followed by Mueller matrix decomposition). Optical retardance values of different regions of normal rat bladders under different distension pressures are derived. Then optical coherence tomography is used to measure local bladder wall thicknesses, enabling the calculation of the tissue birefringence maps as a measure of the tissue anisotropy. Selected two-photon microscopy is also performed to better understand the compositional origins of the obtained anisotropy results. The dome region of the bladder shows maximum birefringence when the bladder is distended to high pressures, whereas the ventral remains roughly isotropic during distension. In addition, the average anisotropy direction is longitudinal, along the urethra to dome. The derived wall anisotropy trends are based on birefringence as an intrinsic property of the tissue organization independent of its thickness, to aid in understanding the structure-functions relation in healthy bladders. These new insights into the wall microstructure of ex vivo distending bladders may help improve the functionality of the artificially engineered bladder tissues.
In several diseases such as bladder obstruction, spinal cord injuries and diabetes, normal bladder function changes as a result of bladder wall morphology alterations.1–6 Although these are known to be the main reason for bladder dysfunction, their exact relation to altered bladder functionality is still unclear.4 Bladder wall is composed of extracellular matrix (including collagen and elastin) and smooth muscle.7 During outlet obstructive diseases, the bladder wall experiences smooth muscle overgrowth (hypertrophy and hyperplasia) or changes in smooth muscle cell orientation, an increase in total wall thickness, and excessive extracellular matrix deposition and remodelling.1,3 In many cases, over the long-term these alterations become irreversible, decrease patient quality of life, and increase risk of kidney failure and urinary tract infections. One promising solution is replacing the bladder with engineered tissues.8,9 A major outstanding issue in this approach is ensuring comparable functionality through comparable engineered architecture, required for a healthy normal bladder. Thus, currently ambiguous relationships between distension-induced changes in the organization of smooth muscle and extracellular matrix tissues that comprise the bladder wall, and the resulting alterations in bladder function, warrant further investigation.
Several groups have taken a biomechanical characterization approach in quantifying the bladder wall displacement (strain) and correlating it to the liquid pressure inside the bladder. Korossis et al.,10 mechanically stretched trabeculae from different regions of porcine bladder, and based on the resultant thickness changes in different directions, identified regions with distinct anisotropy and elasticity. To relate the anisotropy to the pressure inside the bladder, approximations such as a spherical bladder shape have been assumed, and Laplace's law has also been invoked to describe pressure-shape relationships.10 In a recent study, displacement has been measured from the surface of the bladder wall while bladders were distended ex vivo under controlled pressure and volume of liquid,11 from which regional anisotropy and strain have been calculated using Laplace's law. Although such displacement measurements coupled with bio-mechanical models are useful to detect elasticity and anisotropy, they do not provide any information on the underlying tissue organization. Therefore, this missing organizational information about bladder wall morphology and microstructure is acquired from thin samples using optical microscopy techniques.10,12
Polarized light imaging in bulk intact tissues is beginning to be used for quantitative assessment of morphology in biological tissues.13–20 When the tissue polarimetric properties are properly decoupled with the help of polarization data analysis technique known as polar Mueller matrix decomposition,21,22 a particular parameter known as retardance emerges. Retardance is a measure of tissue anisotropy, which reports on its organized/disorganized nature. It is a product of tissue optical birefringence and sampled tissue thickness. Some tissues are known to be anisotropic (birefringent) in their normal functional state.19,23,24 For example, polarized light imaging has been utilized to determine the micro-structural organization in cardiac muscle, including its alterations due to infarct and post-infarct recovery induced by stem-cell therapies.17 Cardiac anisotropy was shown to decrease with infarct formation (disorganized collagen scar tissue), and recover towards near-normal anisotropic levels with various forms of regenerative stem-cell therapies.17,25,26 Furthermore, birefringence can be induced by mechanical forces (strain) in most materials, including biological tissues.16 In fact, strain induces additional organization, which can be detected optically as birefringence. Birefringence in a distending bladder is a combination of both the birefringence due to intrinsic tissue organization and the strain induced organization.
In this study, we use polarized light imaging to obtain local maps of bladder birefringence (from polarimetry-derived retardance images, normalized by the bladder wall thickness measured with optical coherence tomography). This potential metric of bladder functional status varies spatially in normal bladders and in its regional response to bladder filling pressure, as characterized in this study. We also perform selected nonlinear microscopic examinations of the bladder wall, to help understand the polarimetric findings of bladder anisotropy. We believe our results are the first systematic optical characterization studies of normal ex vivo bladder microstructural/organizational changes as related to distension pressure.
Female Sprague-Dawley rats were anaesthetised under isofluorane, the abdomen was opened and the bladders were exposed.7,27 A simple anatomical representation of the bladder and a photo of the ex vivo harvested bladder are shown in Fig. 1. The ureters were ligated with a 5-0 blue viacryl suture on the right side of the rat and with a black 8-0 silk suture on the left. This simple colour coding enabled proper orientation of the bladder after harvesting. The urethra was catheterized with a 20-0 angiocatheter and the urethra sutured around the catheter with 4-0 silk suture, in four separate locations. The bladder and urethra were removed carefully from the animal, which was then sacrificed under anaesthesia according to an institutionally approved animal use protocol. The catheterized bladders were then placed in cell culture medium, phenol red-free Dulbecco's Modified Eagle' Medium (MEM), prior to distension.27,28
(a) Ventral and dorsal schematics of the bladder anatomy. Urine enters from the kidneys via the ureters, and exits via the urethra. Clinical obstructions often occur at the urethra. (b) photograph of an ex vivo catheterized bladder. Various bladder structures and regions—dome, apex, ureters and urethra—are identified in the figure. The blue and black sutures indicate the ureters, this colour marking helping to determine the proper bladder orientation (dorsal/ventral) after harvesting.
To distend the bladders, we used varying pressure head, by varying the height of the liquid reservoir coupled to the bladder via the urethra [Fig. 2(a)]. To ensure that the polarimetric signal was coming from the illuminated spot on the bladder wall, we had to minimize the reflection and scattering from sidewalls and distal portions of the bladder wall; to do so, the liquid in the reservoir-bladder system was a mixture of crimson blue dye dissolved in ethanol (to prevent contributions from bladder regions other than the illuminated region) and oil (to prevent the dye from penetrating into the bladder wall and thus altering the wall tissue optics). This mixture has been optimized empirically, ensuring ∼1.5 h before the bladder wall begins to absorb the dye. For this reason and also to make sure that we are testing the bladders in their fresh condition, all imaging measurements were performed within 1 h. The density of the liquid mixture was measured to be 0.747 g/cm3. The pressure P at the top of the bladder (at the urethra) was then determined according to P=ρgh, where ρ is the oil-dye density, h is the reservoir height and g is the acceleration due to gravity. Three bladders (referred to as 1, 2, and 3 in the text) were distended at three different reservoir heights of 15, 30, and 45 cm, corresponding to pressures of 1.0, 2.2, and 3.3 kPa; this range of pressures in within the relevant physiologic range.7 With this method, we could image bladder 1 distended at 1.0 kPa, bladder 2 at 2.2 kPa and bladder 3 at 3.3 kPa. This was done instead of distending a single bladder to three different pressures, to avoid any possible hysteresis effects.
(a) Arrangement for ex vivo bladder distension, the bladder was kept in phenol red-free Dulbecco's Modified Eagle' Medium (MEM) during distension. (b) Polarized light imaging set up. P1 and P2 are polarizers, WP1 and WP2 are waveplates, L1, L2, L3, and L4 are lenses. The bladder was placed on a rotational stage. ϕ was set to 25 deg.
To derive the retardance (and from that the intrinsic tissue birefringence, as a measure of its anisotropy) of the distended bladders, we used polarized light imaging in reflection mode [Fig. 2(b)]. A 635 nm diode laser (Thorlabs, Newton, New Jersey) was used as the light source. We generated four different incident polarization states (three linear and one right circular) using a waveplate and a polarizer for the sample illumination. The ex vivo bladders were placed on a rotational mount. The light spot on the bladder was ∼3 mm in diameter. Several spots in each anatomical bladder region (dorsal, ventral, and dome) were imaged by rotating the bladder. The imaging was not performed in the regions close to the sutures, since these likely exhibit strain anisotropy induced by the surgical preparation. The angle between the incident beam and collected backscattered light was 25 deg off the exact backscattering direction. Six different output polarization states (four linear and two circular) were collected using a polarizer and a waveplate. The polarized light images were detected by a CCD camera (CoolSNAPK4, Photometrics, Tuscon, Arizona). From this 24-measurement methodology (four input and six output polarization states), Mueller matrices were calculated at different regions of the bladder at three different distension pressures.17
The measured Mueller matrix contains several sample polarization effects occurring simultaneously intermixed in a complex way in its 16 elements. One way to extract intrinsic constituent components of potential biological/biophysical significance is to use a technique known as polar decomposition.22 In particular, a Lu-Chipman decomposition approach has been used to decompose the Mueller matrix into its constituent basis matrices and extract individual polarization effects from these.22,25,29 We have adopted this method for biophotonic applications, validated it with phantoms and Monte-Carlo simulations, and successfully used it in biological tissues.25,29 The experimentally measured bladder Mueller matrix was therefore decomposed into the product of three constituent 'basis' matrices: a diattenuator matrix MD, a retarder matrix MR, and a depolarizer matrix MΔ. Mathematically, this can be written as: Display Formula
Although other multiplication orders are possible, the results for biological tissues are essentially unaltered.30 From MR, one can obtain the sample linear retardance as: Display Formula
The average pathlength d (mm) that the polarized photons (detected at 25 deg backscattering direction relative to the illumination direction) have traveled in the bladder wall as function of the bladder wall thickness L (mm). Symbols are results of polarization sensitive Monte Carlo simulations; line is a guide for the eye.
Note that the derived birefringence values may be smaller than the true tissue birefringence; we are, in fact, measuring its projection onto the plane perpendicular to the imaging axis. Thus the apparent birefringence Δn¯app will be different at different detection angles ϕ.37 Here we keep the measurement angle ϕ fixed at 25 deg reflection mode, and report differences in apparent tissue birefringence from different bladder regions and under different distension pressures. However, we have developed methods to derive the true Δn that require additional angular projections,37 and these will be applied to the bladder distension studies in future publications.
The linear retardance driven from Eq. (2) is the net retardance of the tissue. However, biological tissues are heterogeneous and the fiber alignments usually vary in spatial micro-domains, often as a function of depth (i.e., different anisotropy orientations in different bladder wall layers). Therefore, the detected net retardance might be smaller than the real retardance of each layer. Yet it is well known that a stack of retarders with different fast axis orientations results in a total nonzero circular retardance; giving an indication if a derived low net retardance value is due to true tissue isotropy or instead caused by this anisotropic heterogeneity effect. Retardance in general can be written as a combination of linear and circular contributions and can be described by the retardance vector as:22,38Display Formula
To measure the regional thickness of the bladder wall, we used our recently developed Fourier-domain OCT system.39,40 The system is based on Mach-Zehnder interferometer; the source is a frequency domain mode locked fiber-ring laser with a center wavelength at 1310 nm and a sweep rate of 43 to 67 kHz. A polygon based filter is used to sweep a 112 nm wavelength range. The coherence length is 12 mm, the axial resolution is 9 μm in biological tissues, and the average power is 48 mW. We acquired the B-mode cross-sectional OCT images the bladder wall. The ex vivo distended bladders were placed under the OCT probe and several images from different regions were acquired by changing the bladder position. The catheter was inside the bladder in all the experiments and the bladder was filled with the oil-dye mixture to the predetermined distension level.
Multiphoton microscopy has been used in previous studies to investigate the morphology of the bladder wall.12,41,42 Similarly, we have used mulitphoton microscopy to shed light on the underlying causes of the obtained polarimetric anisotropy signals and gain some information on interrogated tissue composition.43 A Zeiss LSM 510 microscope (META NLO, Oberkochen, Germany) with a 63× water immersion objective lens was used to focus a mode locked Ti:sapphire laser at 800 nm. Depth of field was approximately 2 μm and lateral field of view was 140×140 μm2. The second harmonic generation (SHG) signal, purported to originate primarily from collagen, was collected at 400 nm, in a wavelength window of ±10 nm. The two photon excited fluorescence (TPEF) signal was collected from a spectral window of 430 to 570 nm, and likely represents both smooth muscle and elastin. To flatten the tissue for microscopic examinations, the distended bladder was cut along the apex to dome (see Fig. 1), and placed on a glass coverslip (no top coverslip to flatten the tissue was used, to avoid mechanical artefacts). In order to inhibit the nerve fiber response to slicing and consequently prevent smooth muscle contraction, Oxybutynin was added to the bladder prior to cutting.44 The images were taken in reflection mode, at a depth of about 50 μm below the outer surface of the bladder wall.
To quantify the distension pressure/wall thickness relationship, we performed OCT imaging of bladder 1 at 1 kPa, bladder 2 at 2.2 kPa and bladder 3 at 3.3 kPa. Figure 4 shows representative B-mode OCT images, from the dome region of the three bladders. The effect of decreasing wall thickness with increasing distension pressure is evident in these images.
Representative OCT B-mode structral images of the dome region in the three bladders distended to three different pressures to enable thickness determination.
The bladder wall thickness is not as uniform as may appear from the OCT images in Fig. 4. Moving several millimetres to a different spot but still remaining within the same anatomical bladder 'zone' (dorsal/ventral/dome) gave rise to ∼20% wall thickness variation. The mean values of regional wall thicknesses averaged over ∼3×3 mm2 area of each bladder are shown in Fig. 5. Note the varying magnitude of this effect in different bladder regions: dome exhibit larger thickness decreases than the dorsal and the ventral zones.
Average wall thickness of different regions at three different distension pressures obtained from the structural OCT images. Error bars represent the standard deviation of the mean value (averaged over ∼3×3 mm2 region).
As seen in Fig. 5, increasing distension pressure decreases dome thicknesses significantly, but has only minimal effect on the ventral and dorsal wall; specifically, the ventral wall becomes the thickest bladder wall structure at 3.3 kPa. We have examined additional distended bladders (results not shown); although actual numbers vary, the above trends are consistent. Now with this bladder thickness information in hand, we can look up the average pathlength that the photons travel in each bladder wall from Fig. 3 and use it to calculate birefringence from the measured retardance [Eq. (4)].
It is important to note the possible variations in choosing the location of the spots in each zone on different bladders and therefore the necessity for our future studies to evaluate the whole bladder wall anisotropy for larger number of samples to reach statistically significant and clinically relevant results. In this paper, we have chosen illustrative spots with the highest retardance from each region of the distended bladders. The measured retardances varied from 15 deg to 160 deg for the dome, from 10 deg to 78 deg for the ventral and from 8 deg to 114 deg for the dorsal regions. We did not observe phase-wrapping in any of the examined spots. The anisotropy was then calculated using Eq. (4) for each spot. Note that usually anisotropy is gauged from the thickness change or displacement with the approximate assumption of spherical or ellipsoid shape of the examined structure.10,11 In contrast, our approach enables the characterization of the bladder with respect to its optical anisotropy (birefringence), independent of its thickness and 'regular geometry' assumptions. Resulting images of regional anisotropy Δn¯app are shown in Fig. 6. The images present 2 mm-diam circular fields of view, chosen from the highest signal to noise ratio (SNR) regions of each 3 mm-diam examined spot.
Regional anisotropy (birefreingence) images of bladders 1, 2, and 3 (distended to 1.0, 2.2, and 3.3 kPa, respectively). The rows show the birefreingence images from the anatomical region indicated on the ex vivo bladder figure in the left column. The color represents the value of the anisotropy (color bar on right), the arrows indicate the anisotropy direction. The field of view is 2 mm in diameter.
Maximum anisotropy appears in the dome region distended at 3.3 kPa. The dome is not the most anisotropic region at low and mid pressures (1.0 and 2.2 kPa), but becomes more anisotropic as the pressure increases. Conversely, the ventral wall is nearly isotropic at low pressures, and remains so as the distension is increased. On the other hand, the dorsal wall anisotropy is significant at mid and high pressures. It can thus be concluded that the pressure-anisotropy behavior in dorsal and ventral regions differs from that in the dome region. Physiologically this seems reasonable, since the dome (the far region from the urethra) experiences high pressures when the bladder is almost full and probably shows its maximum compliance at the final filling stages. Also, the dome is known to be the most physiologically vulnerable area of the bladder to acute pressure insults, and is most prone to rupture.45 Further, note that the regional anisotropy–pressure relationship does not follow the regional pressure-thickness correlation depicted in Fig. 4. The polarimetry data hence implies that mechanical strain which results in distension and thickness change is not the only factor leading to change in anisotropy. Most likely, other factors such as organizational changes that are occurring in the different layers and regions of the bladder wall are also responsible for the observed anisotropy (birefringence) changes.
The net birefringence orientation (represented by arrows in Fig. 6) also shows interesting trends. Dorsal and ventral regions both exhibit anisotropic longitudinal alignment along the dome to the urethra. The longitudinal alignment is known to be the dominant direction of the smooth muscle layer as shown previously.3 Therefore, we surmise that the dominant anisotropy in the bladder wall during distension is due to the stretching of the smooth muscle fibers. The orientation of the anisotropy in the dome follows the general orientation of the fibers in dorsal and ventral regions (note that the dome was examined in a horizontal position as shown in Fig. 2). However, at maximum pressure (∼pathological level) the anisotropy orientations in the dome deviate from the rest of the dominant orientations in bladder wall. In order to explore the reason we evaluated the retardance ellipticity at this spot; recall that this is a metric of depth-dependent birefringence heterogeneity. As shown in Fig. 7, the retardance ellipticity is high at those regions with abnormal orientations. This indicates existence of spatial micro-domain retarders with different fast axis orientations throughout the depth, resulting in small values of net retardance and random like orientations. The retardance ellipticity angle for all other spots in Fig. 6 is lower than ∼0.2 rad (about 20% of the ellipticity in Fig. 7), suggesting more uniform anisotropy orientation throughout those regions of the bladder wall.
Birefrience and retardance ellipticity angle images at a representative spot (2-mm-diam) from dome of the bladder distened to 3.3 kPa. (a) The birefringence image; scale bar shows the value of the birefringence and the arrows indicate the orientation of the fast axis, (b) the ratardance ellipticity angle E image, the scale bar indicating the value of E.
Another interesting point is that although the average thicknesses of dorsal and ventral regions are similar during different distension pressures (Fig. 5) and they both exhibit dominant longitudinal orientations, their anisotropy values are very different at mid and high pressure (Fig. 6). One possible reason might be the differences in the extracellular matrix organization in these two regions; because as previously has been reported, the extracellular matrix organization and orientation alter during the bladder filling.12,46 Thus, to elucidate the nature of the anisotropy–extracellular matrix micro-organization connection, we performed two-photon confocal microscopy at selected depths in the dorsal and ventral regions at 3.3 kPa (high pressure). We were not able to keep the curvature of the dome (when distended) with Oxybutynin after cutting the fresh bladder, so our microscopy results are limited to looking at the differences between dorsal and ventral regions. A normal rat bladder was distended up to 3.3 kPa and depth resolved two-photon microscopy images with 2 μm thick slices (depth of field) were acquired from representative spots of its dorsal and ventral regions. To get some idea of the depth dependence of the tissue structures, the Z-stacks of depth resolved slices were acquired around a central plane at 50 μm depth. 3-D rendering of these slices are shown in Fig. 8, where the green colour codes the SHG signal (likely emanating from the collagen fibers), while red shows the TPEF signal (likely from the elastin and smooth muscle compartments—see Sec. 2 for details). The collagen content is not very different in the two regions of the bladder wall (at least at the examined depths); however, as seen in the 2-D (maximum intensity projection) top views (1st column of Fig. 8), it appears more densely packed in the dorsal compared to the ventral regions. On the other hand, the elastin/smooth muscle network in the dorsal area is highly organized compared to the ventral zone (2nd column of Fig. 8). Comparison of Figs. 6 and 8 thus suggests that the organized and compact nature of extracellular matrix in the dorsal wall enhances its birefringence (anisotropy) compared to the ventral structures. Note that deriving a dominant orientation for the extracellular matrix is not straightforward due to its complex three dimensional organization.
Nonlinear microscopy images from representative spots from dorsal and ventral regions of a bladder distended at 3.3 kPa; about 25 adjacent slices, each 2 μm thick, were acquired above and below the central plane at 50 μm, (a) and (d) the green pseudocolor represent SHG signal indicating collagen fibers, (b) and (e) the red pseudo-color TPEF signal represents smooth muscle and elastin (see text for identification and spectral detection details), (c) and (f) show 3D rendering of the combined TPEF and SHG signals.
To fully explain the measured birefringence trends with nonlinear microscopy images would require that the latter be acquired throughout the entire polarimetry imaging depth (∼entire bladder wall thickness), which is not technically possible. However, even these observed micro-structural differences support our speculation about the role of extracellular matrix organization in determining tissue anisotropy properties. From Figs. 67–8, we can note a regional correlation between distension pressure in the bladder and structural organization (morphology) in its wall. This relation may prove to be useful in improving tissue engineered bladders function. For example, Helse et al., have shown that mechanical stimulation of the in vitro tissue engineered bladders triggers elastin production and improves bladder compliance.9 Knowing the regions of the bladder which possess high content of elastin (e.g., dorsal zone according to our results) and their organization, tissue engineers can apply the optimum mechanical stimulation. In addition, our results demonstrate that the different regions of ex vivo distended bladders exhibit very different (thickness independent) anisotropy. Achieving similar regional anisotropy/distension pressure trends in artificial bladders may serve to improve their functionality.
Using polarized light imaging for regional birefringence mapping and OCT for local thickness measurements, we examined organizational anisotropy in ex vivo distended rat bladders as a function of distension pressure. The dome region was seen to become most anisotropic with distension, whereas the ventral side remained roughly isotropic. The dorsal wall showed significant anisotropy at mid and high distension pressures. The dominant average direction of anisotropy in the bladders was longitudinal, along the dome-apex axis. As expected, depth resolved multiphoton microscopy of the ex vivo distended bladder revealed the difference in organization of the extracellular matrix in the dorsal and ventral regions of the bladder wall. Overall, the thickness—pressure relation, birefringence—pressure relation and microscopy data, all together suggest that anisotropy changes with micro-structural remodelling in the bladder wall as well as with mechanical strain (which is proportional to thickness change and distension). Since the bladder function (distension/contraction) is supported by the specific extracellular matrix organization of its wall, these initial results may prove useful in guiding the design and enhancing the functionality of tissue engineered bladders in order to assist bladder failure patients.
A.S. thanks Deutsche Forschungsgemeinschaft for funding support.
Sanaz Alali ; Karen J. Aitken ; Annette Schröder ; Darius J. Bagli and I. Alex Vitkin"Optical assessment of tissue anisotropy in ex vivo distended rat bladders", J. Biomed. Opt. 17(8), 086010 (Aug 17, 2012). ; http://dx.doi.org/10.1117/1.JBO.17.8.086010
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Products/Services for Aerospace Insulation Blanket
Ceramic Insulation and Textiles - (164 companies)...and hydrophobic ceramic insulation and textiles are commonly available. Products with a UL 94 rating from conform to Flame Class 94V-0 standards from Underwriters Laboratories (UL). Applications. Some products are designed for: Aerospace. Automotive. Marine...Show MoreTypeWoven ProductFelt / Nonwoven Product
Insulation Fabrication Services - (121 companies)Insulation fabrication services convert, cut, laminate, sew, and/or assemble fiberglass, foam, mineral wool, ceramic, or other insulating materials into custom fabricated products such as thermal covers, blankets, and pipe wraps. Typically...Show More
Fabric and Fiber Insulation - (88 companies)Fabric and fiber insulation materials comprise textiles which limit thermal transfer. They are frequently used to insulate structures, and are critical components of building sustainability. Fabric and fiber insulations are textile materials which...Show More
Specialty Fibers, Fabrics, and Textiles - (491 companies)...that are processed into a knitted structure with either two or three dimensions. Braided structures - Used for tubular composite structures, thermal insulation, and in other applications. Blankets or batts - Made of thick layers of fabric sheets. Textiles...Show More
Encapsulants and Potting Compounds - (242 companies)...insulation and other specialized characteristics. Basic Information. Encapsulants and potting compounds are designed to insulate and protect electrical and electronic components. They typically provide environmental protection, electrical insulation...Show More
Mineral Wool and Glass Wool - (50 companies)...differences in exact fiber types, the wool types above share common applications based on the characteristics listed below. These qualities are often combined into a single product; for example, mineral wool batt insulation may be installed...Show More
Industrial Fabrics - (665 companies)...insulation, or as the key component in woven fabrics, braids, knits, rope roving, or other specialty fabrics. Roving- Roving is made of parallel filaments. Graphite rovings are referred to as tows. Rovings are marked by the number of filaments...Show More
Aramid Fibers and Fabrics - (103 companies)...including protective apparel, insulation, and more. Types. Today, tens of thousands of tons of meta-aramid and para-aramid fibers are manufactured each year by multiple manufacturers under several brand names. By chemistry, there are numerous types...Show More
Glass Fibers and Fiberglass Cloth - (256 companies)...insulation or as the key component in woven fabrics, braids, knits, rope roving or other specialty fabrics. Strands / multi-filament - Thin, continuous, multi-fiber filaments are used in composite reinforcement applications, or as the key components...Show More
Coated Fabrics - (209 companies)...properties. features. Applications. Coated fabrics also differ in terms of applications. Some products are used in: aerospace. automotive. chemical processing. electronic. geotextile. filtration. HVAC applications. Others are used in carpeting, clothing...Show More
Product News
Mid-Mountain Materials, Inc.
Blankets and Seals for Emissions Control Fabricated Seals, Curtains and Blankets. THERMOPAK®Fabricated Seals, Curtains and Blankets are manufactured for use as thermal insulation barriers for high-temperature resistant applications, emissions reducing applications, and applications requiring resistance to chemical attack or abrasion resistance. Mid-Mountain has years of experience manufacturing fabricated parts for a variety of industries, including Automotive, Aluminum, Aerospace, Transportation, Petrochemical, Welding, Power... (read more)Browse Gaskets and Gasketing Datasheets for Mid-Mountain Materials, Inc.
Mid-Mountain Materials, Inc.
Fabricated Seals, Curtains and Blankets THERMOPAK (R)Fabricated Seals, Curtains and Blankets are manufactured for use as thermal insulation barriers for high-temperature resistant applications, emissions reducing applications, and applications requiring resistance to chemical attack or abrasion resistance. Mid-Mountain has years of experience manufacturing fabricated parts for a variety of industries, including Automotive, Aluminum, Aerospace, Transportation, Petrochemical, Welding, Power Generation, and many others. Our vertical... (read more)Browse Coated Fabrics Datasheets for Mid-Mountain Materials, Inc.
Mid-Mountain Materials, Inc.
Advanced, High Performance Insulation Products adaptive insulation materials offered by Mid-Mountain is its ARMATEX® Coated Fabrics and Textiles. These coated fabrics are resistant to high temperatures, chemicals, and abrasion. The ability to cut and sew these coated fabrics makes them incredibly versatile for use as blankets, custom seals, removable insulation covers, curtains, and pads. ARMATEX® SF 17 Premium, SF 17 Industrial and SF 32 are widely used for the fabrication of insulation pads and blankets. For applications in higher... (read more)Browse Specialty Fibers, Fabrics, and Textiles Datasheets for Mid-Mountain Materials, Inc.
Mid-Mountain Materials, Inc.
Performance and Safety in the Aerospace Industry gas does not penetrate into the cabin. Cable covers and sleeves are often fabricated from coated fabrics to provide properties such as fire resistance. Coated fabrics used for process applications in aircrafts include sound insulation, back panels for luggage bins, aircraft loading walkways and insulation blankets. Mid-Mountain 's Range of Coated Fabrics for the Aerospace Industry. Mid-Mountain Materials, Inc. supplies a large range of coated fabrics that are suitable for use in the aerospace... (read more)Browse Coated Fabrics Datasheets for Mid-Mountain Materials, Inc.
Mid-Mountain Materials, Inc.
Textiles for the Aerospace Industry to ensure our customers get a product specifically suited to their application. Our materials can withstand temperatures ranging from 400ºF * 204ºC to over 5000ºF * 2760ºC (no oxygen present), and include: Silica fabrics and textiles. Coated fabrics. Fabricated thermal protection seals, blankets and curtains. Thermal insulation blankets and papers. High performance industrial textiles. Refractory cements, coatings and moldables. Since 1976, Mid-Mountain has been servicing the global market... (read more)Browse Insulation Jacketing Datasheets for Mid-Mountain Materials, Inc.
Firwin Corp.
Removable Insulation Blankets Removable Insulation Blankets are used to insulate engine parts, exhaust piping and components, and industrial process piping and machinery. In particular, applications which use diesel engines, such as power generation, off highway & on-highway vehicles, and marine often require insulation blankets to manage the heat that these engines generate. Removable insulation blankets are typically constructed in 3 layers. The table and photos below illustrate the construction and materials used... (read more)
Firwin Corp.
Insulation Blankets for Engines & Exhausts Insulation Blankets for Engines & Exhausts - "We've Got You Covered" Firwin offers an extensive range of insulation solutions for engines. Their insulation products are installed on manifolds, turbo chargers, exhaust piping, silencers, and purifiers. And they offer industrial sound insulation solutions in partnership with industry leading engine manufacturers and consultants. It's no wonder Firwin Corp. is considered an industry authority in complex hot environment insulation solutions. Custom... (read more)
Firwin Corp.
Vehical Insulation We manufacture removable insulation blankets for vehicle of all kinds - trucks, lift trucks, buses, fire trucks, mining, construction, agriculture equipment, and forestry machinery - at both the OEM and end user levels. If you have diesel or gas powered machinery we can manufacture removable. Firwin Corporation is a leading manufacturer of custom insulation products for diesel and gas engines & exhaust systems, and industrial applications. Our 2 main product lines are removable and reusable... (read more)
Mars Metal Company
SILFLEX SHIELDING BLANKETS Marshield now offers an excellent alternative to lead wool shielding blankets with our SILFLEX (R) product. SILFLEX (R) shielding blankets are fabricated from a proprietary blend of bismuth and iron powders optimized to provide equivalent attenuation to either a 10 lb/ft or 15 lb/ft lead wool products at approximately 50% of the weight. SILFLEX (R) lead replacement sheets are designed to provide a flexible and durable industrial grade product that will provide 20 years of service... (read more)Browse Radiation Shielding Datasheets for Mars Metal Company
Mid-Mountain Materials, Inc.
CERMEX ® Thermal Insulation Our line of CERMEX® Thermal Insulation Materials encompasses a wide variety of technical thermal insulations including blankets, mats, and papers. These special high temperature resistant insulations are manufactured from varying chemical composition fibers to accommodate a range in service temperature from 1000ºF/540ºC to over 2000ºF/1093ºC. Consisting of fiberglass, mineral wool, silica and biosoluble fiber, these products can solve a variety of thermal management problems. (read more)Browse Nonwovens Datasheets for Mid-Mountain Materials, Inc.
Spartan infrared camera: high-resolution imaging for the SOAR Telescope
We use a multilayer insulating blanket fabricated by Aerospace Fabrication and Materials of Lakeville, MN.
The Huygens probe's back cover insulation is installed in the PHSF
A Daimler-Benz Aerospace staff member installs thermal blanket insulation on the back cover of the Huygens probe in the Payload Hazardous Servicing Facility at KSC in July.
Accelerated weathering of wood surfaces coated with multifunctional alkoxysilanes by sol-gel deposition
iron.l:~ 15 Methyldimethoxysilane, a multifunctional alkoxysilane, has been shown to be effective for rapid ,~,~aterpr'oofing of ceramic tiles and blankets used as themlal insulation on aerospace vehicles.16 The sol-gel process allows deposition of inorganic-or- ganic polymeric…
Surface chemistry and moisture sorption properties of wood coated with multifunctional alkoxysilanes by sol‐gel process
iron.11–13 Methyldimethoxysilane, a multifunctional alkoxysilane, has also been shown to be effective for rapid waterproofing of ceramic tiles and blankets used as thermal insulation on aerospace vehicles.14 The sol-gel process allows the deposition of inorga- nic–organic polymeric networks on…
Facile fabrication of polyimide foam sheets with millimeter thickness: Processing, morphology, and properties
For many aerospace applications, such as multilayer insulation (MLI) blankets of BepiColombo,8 it is most desirable to have higher temperature stability, flexibility, low thermal conductivity.
A Novel Approach to CAFO Odor Assessment: Odorant Field Collection by Sorbent Tube / Thermal Reconstitution in the Laboratory
An interesting alternative was enabled through the discovery of an obscure source of a specialized film from the aerospace industry; metalized, heat wieldable FEP films utilized in insulation blankets and mirror surfaces.
The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres
The Multi-Layer Insulation blanket (MLI) has been manufactured and installed by Austrian Aerospace .
The Dawn Framing Camera
The Multi-Layer Insulation blanket (MLI) has been manufactured and installed by Austrian Aerospace .
Thermal Performance Testing of Cryogenic Multilayer Insulation with Silk Net Spacers
R. A. Mohling, H. L. Geir, et al.,Multilayer Insulation Thich Blanket Performance Demonstration, AFAL TR-87-037, Ball Aerospace Systems Group, Boulder, CO, July 1987.
Fibers, Specialty Inorganic
Aerospace applications include ligHTIWeight fire protection for PMCs and ligHTIWeight burn-through resistant insulation blankets for jet engine nacelles.


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Michelle Rodriguez wants to sort things out with her ex-girlfriend Cara Delevingne.
The 'Fast & Furious 7' actress reportedly wants to win the supermodel back after seeing her at the LACMA Art and Film Gala in Los Angeles on Saturday.
A source told the Daily Star newspaper: "Michelle was telling her friends that seeing Cara made her miss her and she desperately wants to sort things out."
The insider also revealed the 36-year-old actress even went as far as to send Cara a note via a waitress who was working at the showbiz event.The source said: "Michelle asked a waitress to give Cara a note asking her to come over and talk to her.
"But Cara ignored it. She's still hurt about everything that happened. She was really nervous about seeing Michelle and clung to her pal Selena Gomez all night. Selena was telling friends on the night that Cara seemed nervous and anxious which is very unusual for her.
"And Selena also said there was some unfinished business between them. Everyone could sense the tension."


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|Alexander Klimetschek <alexander.klimetsc...@mindquarry.com>
|Refactored RequestProcessor swallows all exceptions in BlockServlet case
|Fri, 17 Nov 2006 00:37:28 GMT
Hi Daniel, while updating to the latest Cocoon I stepped into the problem that you cannot see the exceptions thrown in a BlockServlet called by another one, since the new refactored RequestProcessor swallows all exceptions. The generated error page is fed into the response output stream which is eventually read by anything in the calling pipeline, which mostly cannot handle that html error page (in my case it gets some xml) and will throw another exception like SAXParseException. IMHO the RequestProcessor should throw the exceptions in the blocks case instead of generating error pages. I am trying to fix that. Alex -- Alexander Klimetschek http://www.mindquarry.com
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Does power corrupt? Absolutely! Does the hunger and lust for absolute power course through your icy blue veins? Do you feel the need to rule the world with an iron fist? To subjugate the masses with a whimsical snap of your fingers? To rise above the insolent and unworthy with your superior intelligence and unlimited supply of cold hard cash? Have you ever wondered if there was a simple step-by-step guide for following your impulses and instincts toward world domination? Youve come to the right place. Here is the opportunity of a lifetimethe chance to turn your sociopathic needs into bloodthirsty greed! Now you can learn at the feet of the Master of Megalomania . . . Charles Montgomery Burns! Dont just stand there, you sniveling slugabed. Purchase this portable periodical posthaste and leave your conscience at the cash register!
- Author: Matt Groening
- Publisher: Harper Design Intl
Read MoreLess
More Details
- Dimensions: 10.25 H x 0.50 D x 6.75 inches
- Format: Paperback
- Number of Pages: 176 Pages
- Release Date: July 22, 2014
Availability: In Stock
Ready to ship in 5-7 Business Days


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Warrington, Challenge Cup winners in 2009 and 2010, are within one match of a return to Wembley after a 32-22 victory over the Catalan Dragons in Perpignan.
The Wolves looked well in control when Brett Hodgson guided them to a 20-6 half-time lead, but tries from Michael Simon, Ben Fisher and Vincent Duport brought the French within range. Chris Bridge's interception try, however, ended their fightback.
The cup holders, Wigan, beat their great rivals, St Helens, with surprising comfort on Saturday. Saints took the lead with a try from Paul Wellens, but after that it was the Wigan pack, all six of whom came through their youth-development programme, who were firmly in charge.
They won 18-4, with tries from Sam Tomkins, Sean O'Loughlin and a brilliant solo effort from their man of the match Brett Finch. It was the first time in 12 years that Saints have failed to reach the cup semi-finals.
Last year's beaten finalists, Leeds, had a resounding 60-12 win at Leigh, swiftly banishing any thoughts of the famous Wembley upset of 1971.
The Rhinos' captain Kevin Sinfield led the way with 28 points, from two tries and 10 goals from 10 attempts.Reuse content