chronic subdural hematoma ( csdh ) is one of the most common types of intracranial hemorrhages encountered in neurosurgical practice . the overall incidence of bilateral csdh varies from 14% to 25%.1,5,7,11,12,15 ) compared to patients with unilateral csdh , those with bilateral csdh tend to be elderly ( older than 75 years ) , have coagulation abnormalities , and experience a more aggressive clinical course.7,10,13 ) however , the clinical differences between bilateral and unilateral csdh remain unclear . the purpose of this study was to investigate the clinical features , clinical course , and postoperative outcomes for bilateral and unilateral csdh . we compared with bilateral and unilateral csdh and evaluated the factors influencing differences between the two patient groups . a retrospective analysis was performed on 120 consecutive patients diagnosed with csdh who were surgically treated at our institute during january 2008 to december 2012 . their medical records including clinical features , brain ct findings , surgical results , and postoperative status were reviewed . the subdural hematoma ( sdh ) was evacuated and/or washed out by irrigation with warm physiological saline solution . closed system drainage of the sdh cavity using a soft silicon drain was performed in all cases for 1 to 3 days . for bilateral csdh , burr holes were trephined on both sides of the head at the same time . subdural drainage catheters were removed following confirmation of near total removal of hematoma on postoperative brain ct scan and definite improvement of symptoms . after discharge from the hospital , brain ct scans were performed on a monthly basis until the sdh regressed and patients recovered to a premorbid functional status . recurrence was defined as the occurrence of symptoms and signs attributable to an ipsilateral hematoma seen on a brain ct scan within 3 months of the first drainage procedure . functional status on hospital admission was assessed using the glasgow coma scale ( gcs ) and modified rankin score ( mrs).14,17 ) the glasgow outcome scale and mrs were measured at discharge.6 ) all patients were followed postoperatively for more than 3 months . the following variables were compared between the two groups : sex , age , preceding head trauma , hypertension , diabetes mellitus , heart disease , cerebrovascular disease , liver disease , renal disease , antiplatelet therapy , anticoagulant therapy , seizure , presenting symptom ( headache with nausea or vomiting , hemiparesis , aphasia , mental change , and seizure ) , preoperative functional status ( mean gcs score , mean mrs score ) , and preoperative ct findings ( degree of midline shift , hematoma thickness , and hematoma density ) . statistical analyses were conducted by performing fisher 's exact test and student 's t - test using the statistical package for the social sciences ( spss ) software ( version 12.0 ; spss inc . , chicago , il , usa ) . in all analyses , the 120 patients ( 86 men and 34 wo - men ) diagnosed with csdh had a mean age of 62.7 years ( range 26 - 86 years ) . with regard to the initial neurological status of the patients , the mean gcs score on hospital admission was 13.7 and the mean mrs score on admission was 2.5 . hypertension was the most common underlying disease and was present in 47 patients ( 39.2% ) . among the 120 patients , 11 were treated with bilateral trephination and drainage ; in our patient sample , the incidence of bilateral csdh was 10.9% . we compared the clinical features of the unilateral and bilateral csdh groups ( table 1 ) . differences in gender ( p= 0.073 ) , mean age ( p=0.799 ) , and advanced age group ( 60 years ; p=1.000 ) were not statistically significant . the mean gcs on admission was 13.7 and 14.2 in the unilateral and bilateral csdh patients , respectively ( p=0.779 ) . the mean mrs on admission was 2.5 and 2.5 in the unilateral and bilateral hematoma groups , respectively , and there was no significant difference between the groups ( p=0.876 ) . a history of head injury was found in 65 of 109 ( 59.6% ) patients with unilateral csdh and in four of 11 ( 36.4% ) patients with bilateral csdh . there were no statistically significant differences in the presentation of head injury history between unilateral and bilateral csdh patients ( p=0.201 ) . the prevalence of underlying diseases , including hypertension , diabetes mellitus , heart disease , cerebrovascular disease , or others , was similar between the unilateral and bilateral csdh groups . statistical analyses revealed no significant intergroup differences in the main symptom of presentation ( p= 0.337 ) . the average degree of a midline shift on preoperative brain ct was 9.6 mm for unilateral csdh and 3.3 mm for bilateral csdh . patients with unilateral csdh had a significantly higher degree of midline shift on ct scans than those with bilateral csdh ( p=0.010 ) . hematomas thickness was 16.9 mm and 18.1 mm in patients with unilateral and bilateral hematoma , respectively ( p=0.870 ) , but hematoma density showed no significant differences between groups ( p=0.552 ) . there were two postoperative complications in our patients ( 1.6% ) , which included postoperative epidural hematoma and wound infection . most patients with csdh had a good postoperative outcome after surgery , except for one patient who died ( 0.8% ) . the death was related to peritonitis caused by renal disease . within 3 months after the operation thought the six recurrences of csdh were found only in the unilateral csdh group , there was no significant difference in recurrence between the groups ( p=1.000 ) . although recurrence is possible , postsurgical prognosis has been satisfactory.4 ) bilateral csdh is rare compared to unilateral csdh , but was occasionally present in our clinical practice . the incidence of bilateral csdh accounts for 14 - 25% of the cases of csdh reported previously.1,5,7,11,12,15 ) compared to unilateral csdh , bilateral csdh has an increased tendency for coagulopathy and has a more aggressive clinical course.7,16 ) the incidence of bilateral csdh in our patient group was 10.9% , which is slightly lower than the previously reported value . although the pathophysiology of bilateral csdh is not well understood , like unilateral csdh , bilateral csdh is postulated to be caused primarily due to traumatic injury to the bridging veins . head trauma is an important precipitating factor for patients with csdh . in our cohort , a history of head injury was present in 36.4% of patients with bilateral csdh and 59.6% of patients with unilateral csdh . these results show that bilateral csdh has a lower rate of head injury , but there was no statistical difference between the groups . several risk factors for bilateral csdh have been reported ; these include age ( more than 75 years ) , coagulopathy , use of antiplatelet or anticoagulation medications , and hemodialysis.7,13 ) however , our analyses did not identify the abovementioned risk factors to predispose one to development of bilateral csdh . in addition to the abovementioned factors , factors such as intracranial hypotension and skull shape have been identified as predisposing factors of bilateral csdh in some studies.2,8 ) however , intracranial hypotension and skull shape were not evaluated in present study . clinical presentation of bilateral csdh varies ; there are no symptoms specific to patients with bilateral csdh . it has been suggested that bilateral csdh has an aggressive clinical course , and is affected by coagulofibrinolytic abnormalities , which somehow contribute to the enlargement of the sdh.7 ) it has also been suggested that bilateral csdh has a lower incidence of hemiparesis compared to unilateral csdh , possibly because there is less opportunity for the central brain structures to deviate owing to counterbalance of the mass effect on both hemispheres.5 ) this mass counterbalance effect is likely why patients with bilateral csdh have a lower incidence of hemiparesis . in our patient group , the most common presentation was headache and hemiparesis in both bilateral and unilateral csdh ; yet , no significant difference was found between the groups . however , a marked midline shift in the preoperative ct was found to be significantly more frequent in unilateral csdh than in bilateral csdh ( p=0.010 ) . the unilateral csdh group had an increased rate of midline shift compared to bilateral group , but the hematoma thickness was not different . hematoma thickness in the bilateral csdh group ( mean , 18.1 mm ) was larger than the unilateral csdh group ( mean , 16.9 mm ) , which might contribute to the lack of significant differences in clinical symptoms , especially hemiparesis , between our patient groups . most postoperative complications are related to procedural errors occurring during hematoma removal and subsequent drainage . in one recent meta - analysis of csdh , the postoperative complication rate was estimated at 2.5 - 9.3%.3 ) in our study , two patients with unilateral csdh experienced postoperative complications . one had an epidural hematoma and the other had a wound infection , requiring further treatment with surgery and antibiotics , respectively . fortunately , the functional outcome at discharge of these two patients was not worse than their initial status . finally , no significant difference was found between bilateral and unilateral csdh groups regarding the postoperative complications we analyzed . most of our patients had a good postoperative outcome , consistent with previous reports.3,9 ) previous work reports the mortality rate in csdh varying between 2 - 12.2%.3,9 ) because of the advanced age and multiple medical problems of patients , csdh can be frequently associated with complications and mortalities . in our study , one patient ( 0.8% ) died from peritonitis caused by renal disease . bilateral csdh has been reported to be independent predictor for the recurrence of csdh.5,12,15 ) patients with bilateral csdh tend to have previous brain atrophy , which may lead to poor brain re - expansion after surgery . poor brain re - expansion correlates with recurrence and is thought to facilitate hematoma reaccumulation.9,12 ) in addition , some authors reported that bilateral csdh occurs more frequently in patients with prolonged coagulation time.10 ) in our cohort , the six recurrences of csdh were found only in the unilateral csdh group , but the difference in the number of recurrences was not statistically significant ( p=1.000 ) . midline shift on ct scans was the only statistically significant difference between the bilateral and unilateral csdh groups . this may arise due to limitations of this study , such as the retrospective nature of analysis , small sample size , and lack of existing research on contributing factors like intracranial hypotension , skull shape , and preoperative brain magnetic resonance imaging . we expect further analyses with more refined data could clarify the clinical significance of bilateral csdh . in our patient group , we showed that bilateral csdh constituted 10.9% of csdh , particularly in patients older than 60 years . patients with bilateral csdh had a lower rate of head injury history ( 36.4% ) , but we found no statistically significant difference in clinical features and precipitating factors between unilateral and bilateral csdh . compared with patients with unilateral csdh , patients with bilateral csdh had a significantly lower incidence of midline shift on ct scans .
objectivechronic subdural hematoma ( csdh ) is a common intracranial hemorrhage , encountered in neurosurgical practice . 
 most csdhs are unilateral , but some show bilateral involvement . 
 however , the clinical characteristics of bilateral csdh remain unclear . in this study , we investigated the clinical differences between bilateral and unilateral csdh.methodsa retrospective study was performed on 120 patients with csdh surgically treated at our institute from january 2008 to december 2012 . 
 patients were divided into two groups : the bilateral csdh and the unilateral csdh groups . 
 clinical presentations , precipitating factors , computed tomography ( ct ) findings , postoperative complications , and outcomes of patients were analyzed.resultsbilateral csdh was identified in 11 of 120 ( 10.9% ) patients with csdh . 
 patients with bilateral csdh tended to have a lower rate of head injury compared to patients with unilateral csdh ( 36.4% vs. 59.6% ) , but it had no statistical significance ( p=0.201 ) . 
 the frequency of marked midline shift on ct scans was significantly greater in unilateral csdh than in bilateral csdh ( p=0.010 ) . 
 presenting symptoms , coexisting systemic diseases , postoperative complications , and clinical outcomes were not significantly different between the two groups.conclusionbilateral csdh has comparatively similar clinical features and precipitating factors as unilateral csdh . 
 patients with bilateral csdh have significantly lower incidences of midline shift on ct scans , and most patients with either bilateral or unilateral csdh have good postoperative outcomes .