periodic alternating nystagmus ( pan ) is a spontaneous horizontal nystagmus observed in disorders of the central nervous system , such as a cerebellar disorder , and is characterized by a fast phase beating in one direction for a period of approximately 23 minutes at the primary position.1 recently , the ratio of pan prevalence was estimated to be between 9%2 and 39%1 in congenital nystagmus . although pan can occur during infancy or later in life , patients with congenital pan complain of oscillating vision because of their null points in comparison to patients with other congenital nystagmus.3,4 in western countries , the first - choice treatment for pan is medication - based , such as use of baclofen5 and memantine.6 however , in japan , the first choice is prismatic glass and the second is surgery , because medication for pan is not covered by health insurance . furthermore , previous studies reported that these pharmaceutical interventions have limited effects in congenital pan.7,8 therefore , we examined the efficacy of strabismus surgery ; ie , recession of four horizontal rectus muscle ( r - fhr ) . r - fhr has demonstrated levels of efficacy as a good surgical procedure for patients with null point nystagmus.9,10 it is controversial whether r - fhr is effective for treating pan9 or not,4 and currently , there are no reports showing r - fhr efficacy in patients with pan . here , we show ten patients who received r - fhr for oscillating vision caused by congenital pan and demonstrate that the procedure is effective , especially in treating some variants of pan ; the patients complained of oscillating vision mainly . the medical records of ten patients ( seven males and three females ; 24.410.9 years ) with congenital pan who underwent r - fhr between november 2007 and july 2012 at the hyogo college of medicine were analyzed retrospectively during a follow - up period of at least 6 months ( average period 27.8 months ) . patients with congenital pan were extracted for the present study ; however , patients with other types of congenital nystagmus including albinism , fusion maldevelopment nystagmus , nystagmus blockade syndrome , convergence dampened nystagmus , and other congenital nystagmus were excluded . the clinical status for each patient is shown in table 1 . all patients underwent ophthalmologic examinations including best - corrected visual acuity using a snellen chart ( then converted into logarithm of the minimum angle of resolution [ logmar ] ) , major amblyoscope for deviation angle measurements , face photographs for abnormal head posture , titmus stereo test for stereopsis , and electro - oculograms ( eogs ) at the pre- and post - operative periods . eogs were performed to record for nystagmus amplitude and direction ; time constant was 3.0 seconds for original waveforms and 0.01 seconds for the speed waveforms . then , the eye movement in the primary position was recorded at 2 mm / second of the paper feed speed . we recorded eogs until at least three alternations of the nystagmus direction were detected or more than 8 minutes . the therapeutic effects of r - fhr were determined by both subjective complaints , eg , whether the patient could drive without difficulty and/or read the paper without a loupe , and objective complaints such as changes of the nystagmus amplitudes and the abnormal head posture . subjects were fully informed about the nature of the procedures and gave their written consent at the beginning of the treatments . all surgical procedures were performed under general anesthesia by a single surgeon ( om ) . all patients received r - fhr , equal recession for both the medial and lateral rectus muscles ( mr and lr , respectively ) . firstly , the maximum amount of mr was removed ( 9 mm [ 14.5 mm from limbus ] to 11 mm [ 16.5 mm from limbus ] ) , and then the same amount of lr was removed from the muscle attachment site ( 16 mm to 18 mm from limbus ) . the study protocol and data collection methods were approved by the institutional review board of the hyogo college of medicine . written informed consent was obtained from all patients including consent for any accompanying images , including the picture of case number 1 ( figure 1 ) . case number 2 and 9 patients were under the age of 18 years and we therefore obtained permission from their parent(s ) for the surgical consent . 1)pre - operative clinical signs : nine patients complained of an abnormal head posture ( nine face turn , one head tilt , and two chin elevations [ figure 1 ] ; some features overlapped [ table 1 ] ) . the face turn fluctuated over time to various degrees in these patients and the orientation reversed in case number 1 ( figure 1 ) and case number 10 . aside from the face turn cases , there were two esotropia ( et ) and one congenital superior oblique palsy cases ( ie , case number 6 ) . also , the other strabismus or eye movement disorder including dissociated vertical deviation and inferior oblique overaction was not observed in these ten cases . 2)recession amounts : the recession amounts for 40 muscles in all ten patients were as follows : 9 mm in two patients ( 20.0% ) , 10 mm in seven patients ( 70.0% ) and 11 mm in one patient ( 10.0% ; table 1 ) . 3)post - operative course : all ten patients showed a reversal in severity and fluctuation ranges in both nystagmus and abnormal head posture on the day after the surgery ; the nystagmus improved markedly in four patients ( case numbers 1 , 4 , 9 , and 10 ) , and the abnormal head posture disappeared completely in three patients ( case numbers 2 , 5 , and 6 ) . to enhance our manuscript , we uploaded the video file of nystagmus changes before ( supplementary video s1 ) and after surgery ( supplementary video s2 ) in case number 1 . figure 2 shows the eog results for case number 10 , showing a marked reduction of amplitudes after r - fhr . in two patients ( case numbers 5 and 7 ) , we observed recovered visual acuity . furthermore , the recovery in case 7 was 0.4 ( logmar ; table 1 ) . 4)post - operative complications : three patients ( case numbers 5 , 8 , and 9 ) complained of et with double vision . they needed to undergo an additional strabismus surgery for hemi advancement of the lr , which led to improvement of the double vision ; ie , 24 mm lr advancement for et . treatments for correcting nystagmus in western countries rely mainly on medicine - based interventions.11 however , in japan the main avenue for correcting nystagmus is a prism glass due to the limitations of primary eye care and health insurance . thus we conducted a research study on r - fhr as the procedure for pan . briefly , the rationale for this surgical procedure to correct nystagmus leads to separation of patients into two groups based on correction of abnormal head posture ; ie , anderson-,12 kestenbaum-,13 goto-14 and parks - method15 and reduction of nystagmus amplitude via r - fhr . the majority of patients with pan have an abnormal head posture such as a face turn . however , the abnormal head posture has a temporal variation ; therefore , the surgery for correcting abnormal head posture is an unapproved procedure in patients with pan . in 1999 , dellosso et al first reported the effectiveness of tenotomy and re - attachment of all horizontal rectus muscles for congenital nystagmus,16 and the tenotomy resulted in significant improvements in a nystagmus measure and subjective visual functions for ten patients with congenital nystagmus.17 they subsequently introduced modifications.18 in the past , we also performed the tenotomy ; however , unfortunately , we did not experience effectiveness such as described by the dellosso group . we then performed r - fhr to examine its efficacy . here , we describe the result of a retrospective study of ten patients with congenital pan who received r - fhr . r - fhr was first reported by bietti19 in 1956 , but went largely unnoticed until von noorden and sprunger ; and helveston et al revaluated r - fhr as a surgical procedure for null point nystagmus.9,10 von noorden and sprunger performed r - fhr on three patients with congenital nystagmus , including one patient with pan and abnormal head posture that was corrected , but without vision improvement.9 subsequently , gradstein et al reported eight patients with pan who received various surgical procedures in 1997.2 the gradstein team utilized the kestenbaum method for three patients and r - fhr for five patients . there were no beneficial effects in the former , but correction of abnormal head posture and vision improvement were observed in the latter . in 1996 , thomas et al performed r - fhr for a patient with congenital pan after a botulin injection to the lr and concluded that r - fhr was not effective based on the patient s subjective improvement with minimal correction of oscillating vision.4 castillo et al performed r - fhr in a patient with secondary pan who did not respond to baclofen treatment and reported that the nystagmus was reduced , but the patient complained of postoperative exophoria - tropia.7 boyle et al performed r - fhr in ten patients with congenital idiopathic nystagmus and suggested that the visual improvement was only slight.20 however , their reports have a few problems with diagnosis of congenital nystagmus.21 these observations have led us to be uncertain about r - fhr effects as a surgical procedure for pan . in this study , we demonstrated that eight of ten patients with congenital pan who underwent r - fhr showed an improvement including nystagmus amplitude which was significantly reduced in four patients , abnormal head posture that completely disappeared in three patients , and recovered visual acuity in two patients . some of these outcome features overlapped . this is especially true for case number 1 , which involved a patient who had refrained from driving due to the oscillating vision , but after the r - fhr procedure , she found herself routinely driving a car , which was clearly indicative of an improvement in the quality of life . however , r - fhr could decrease the nystagmus amplitude and extend the null point over a large area , resulting in the improvement of the abnormal head posture . previous postoperative complications have indicated the prevalence of exotropia.7 however , in our study , the rate of postoperative et was 30.0% , which had not been reported previously . when we performed the additional strabismus surgery , the removed rectus muscles were attached at the planned site . post - operative et may only occur in people who possess a small orbital volume such as populations of asian descent . an important clinical consideration can be observed when more than the typical 8 mm of rectus muscle recession is performed . bagheri et al suggested that an additional 1 mm recession had the effect of correcting 6 prism diopters in mr and 11 prism diopters in lr when the recession amount was over the typical 8 mm.22 this previous report supports our hypothesis that more than 8 mm rectus muscle recession in asian descent may induce post - operative et . r - fhr is effective for patients with pan who complain of periodic oscillating vision with a regular pattern of periodic nystagmus . although post - operative strabismus may occur , due consideration is warranted for reduction of the recession amount . in addition , the preferable age of receiving surgery is over 10 years of age because of the need for topical anesthesia to perform the additional surgery . as a challenge in the future , we need to increase the sample size to further ascertain the detail of r - fhr that is effective for pan , and to use nafx , foveation time , and gaze - dependent visual acuity as valid scientific evidence.21
purposeperiodic alternating nystagmus ( pan ) is a spontaneous horizontal nystagmus observed in disorders of the central nervous system . 
 patients with congenital pan complain of oscillating vision at high rates . 
 medication is the first - choice treatment for pan ; however , clinicians still seek better therapy . 
 the aim of this study was to evaluate outcomes of recession of four horizontal rectus muscle ( r - fhr ) in patients with congenital pan.patients and methodsthis study reports a retrospective case series of ten patients ( seven males and three females ; mean age 24.410.9 ) with congenital pan who underwent r - fhr between 2007 and 2012 , which was performed by the same surgeon at the hyogo college of medicine . 
 patients were evaluated for complications , recession amount , deviation angle , eye movements including a nystagmus amplitude , and visual acuity during pre- and post - operative periods.resultspre-operatively , patients complained of oscillating vision , abnormal head posture , esotropia , and congenital superior oblique palsy . 
 post - operatively , changes from the previous observations of nystagmus amplitudes and abnormal head posture demonstrated a complete reversal in all patients . 
 in addition , visual acuity determined with a snellen chart improved in two patients . however , esotropia occurred in three patients who underwent additional strabismus surgery 2 days after r - fhr . 
 r - fhr was particularly effective in eight patients who pre - operatively had periodic oscillating vision with a regular pattern of periodic nystagmus.conclusionwe demonstrated that ten patients with congenital pan had improved vision following r - fhr , indicating that r - fhr was an effective procedure , especially in patients suffering pan with periodic oscillating vision .