Effects of tenotomy surgery on congenital nystagmus waveforms in adult patients. Part I. Wavelet spectral analysis

Congenital nystagmus (CN) is an aperiodic oscillatory eye movement disorder. Horizontal rectus tenotomy with simple re-attachment has been proposed as a therapy for CN. This therapy might affect vision and/or eye movements. Another paper deals with improvements in visual acuity. This and the companion paper examine changes in eye movements. In this study, we examined the effect of tenotomy on nystagmus waveforms using wavelet spectral analysis. No common effect was found across the patients on the wavelet spectra of the CN beat, suggesting that tenotomy surgery has no effect, or only a quite small effect, on the waveform structure of CN.     

A new surgery for congenital nystagmus: effects of tenotomy on an achiasmatic canine and the role of extraocular proprioception.

PURPOSE: Human eye-movement recordings have documented that surgical treatment of congenital nystagmus (CN) also produces a broadening of the null zone and changes in foveation that allow increased acuity. We used the achiasmatic Belgian sheepdog, a spontaneously occurring animal model of human CN and see-saw nystagmus (SSN), to test the hypothesis that changes induced by surgical interruption of the extraocular muscle afference without a change in muscle-length tension could damp both oscillations. METHODS: An achiasmatic dog with CN and SSN underwent videotaping and infrared oculography in a sling apparatus and head restraints before and after all extraocular muscles (stage 1: 4 horizontal rectus muscles and stage 2 [4 months later]: 4 vertical rectus muscles and 4 oblique muscles) were surgically tenotomized and immediately reattached at their original insertions. RESULTS: The dog had immediate and persistent visible, behavioral, and oculographic changes after each stage of this new procedure. These included damped CN and SSN, increased ability to maintain fixation, and increased periods of maintaining the target image on the area centralis over a broad range of gaze angles. CONCLUSIONS: Severing and reattaching the tendons of the extraocular muscles affect some as-yet-unknown combination of central nervous system processes producing the above results. This new procedure may prove effective in patients with CN with either no null, a null at primary position, or a time-varying null (due to asymmetric, (a)periodic, alternating nystagmus). We infer from our results in an achiasmatic dog that tenotomy is the probable cause of the damping documented in human CN after Anderson-Kestenbaum procedures and should also damp CN and SSN in achiasma in humans. It may also prove useful in acquired nystagmus to reduce oscillopsia. The success of tenotomy in damping nystagmus in this animal suggests that the proprioceptive feedback loop has a more important role in ocular-motor control than has been appreciated. Finally, we propose a modified bimedial recession procedure, on the basis of the damping effects of tenotomy.     

Effects of extraocular muscle tenotomy on congenital nystagmus in macaque monkeys.

INTRODUCTION: Extraocular muscle tenotomy has been reported to damp congenital nystagmus in an achiasmatic sheepdog. We performed extraocular muscle tenotomy to evaluate its effects on congenital nystagmus in primates. METHODS: Magnetic search coil eye movement recordings were used to document the presence of horizontal congenital nystagmus in 2 adult macaque monkeys that also had naturally occurring infantile strabismus. Extraocular muscle tenotomy was performed by operating on all 4 horizontal recti, surgically detaching the muscles from the globe and suturing them back to their original insertions without resection or recession. Eye movement recordings were repeated 4 months after the procedure, comparing the waveform, amplitude, retinal slip velocity, and intensity (frequency x amplitude) of the nystagmus before and after tenotomy. Visual acuity was also measured before and after surgery in 1 animal. RESULTS: Preoperatively, a disconjugate, pendular nystagmus was evident in 1 monkey, and the other had a conjugate pendular-jerk nystagmus damped by convergence. After tenotomy, nystagmus mean amplitude decreased 18% to 52% in 1 monkey but increased 14% in the other (t test, P <.002). Retinal slip velocity and nystagmus intensity increased in both monkeys. After tenotomy, mean velocity increased 22% to 218%, while mean intensity increased 40% to 208% (t test, P <.002). Visual acuity measured after tenotomy decreased an average of 20% ( approximately 2.0 cycles per degree) in each eye. Tenotomy had no noteworthy effects on eye alignment or other aspects of visual behavior other than the congenital nystagmus. CONCLUSION: Nystagmus velocity and intensity increased after extraocular muscle tenotomy in 2 monkeys. Further studies are required to establish the clinical value of this procedure as a treatment for various subtypes of congenital nystagmus in humans.     

上斜肌减弱术对眼球旋转状态的影响

目的探讨上斜肌减弱术对眼球旋转状态的影响。方法对29例(58只眼)上斜肌功能亢进患者行双眼上斜肌减弱术,术前和术后1周拍摄眼底照片,并使用绘图软件测量黄斑-视乳头夹角度数,记录术眼的客观旋转度数。使用同视机检查主观旋转度数和双眼视觉功能。结果 58只眼术前客观旋转度数右眼为内旋转10.48°±6.01°,左眼为内旋转9.37°±5.88°;上斜肌减弱术后1周右眼和左眼的客观旋转度数分别减少了9.11°±7.09°和7.94°±4.76°。双眼手术前、后客观旋转度数比较,差异均有统计学意义(P<0.01)。上斜肌断腱术后内旋转度数的变化与上斜肌部分切除术后比较,差异无统计学意义(P>0.05)。同视机检查结果显示全部患者手术前、后均无正常双眼视觉功能和旋转斜视。结论减弱亢进的上斜肌功能可矫治眼球内旋转斜视;上斜肌断腱术和上斜肌部分切除术改善内旋转斜视的效果基本相同;主、客观旋转状态的变化存在不一致性。(中华眼科杂志.2006,42:883-887)     

Horizontal rectus muscle tenotomy in children with infantile nystagmus syndrome: A pilot study

PURPOSE: We wished to determine the effectiveness of horizontal rectus tenotomy in changing the nystagmus of children with infantile nystagmus syndrome. DESIGN: This was a prospective, noncomparative, interventional case series in five children with varied sensory and oculographic subtypes of congenital nystagmus (including asymmetric (a)periodic alternating nystagmus) and no nystagmus treatment options. Simple tenotomy of all four horizontal recti with reattachment at the original insertion was accomplished. Search-coil and infrared eye movement recordings and clinical examinations were performed before and 1, 6, 26, and 52 weeks after surgery. Outcome measures included masked pre- and postoperative expanded nystagmus acuity function (NAFX), foveation time obtained directly from ocular motility recordings, and masked measures of visual acuity. RESULTS: At 1 year after tenotomy and under binocular conditions, two of the three patients for whom the NAFX could be measured had persistent, significant postoperative increases in the NAFX of their fixating eye. Average foveation times increased in these patients' fixating eyes. Measured binocular visual acuity increased in four patients; the remaining patient had retinal dystrophy. There were no adverse events due to surgery. CONCLUSIONS: In the two children who could fixate the targets for several seconds and for whom we could obtain accurate measurements of their infantile nystagmus, tenotomy resulted in significant improvements in nystagmus foveation measures. In those patients plus two others (four of five), measured visual function improved.     

Surgical outcomes in correction of Brown syndrome

PURPOSE: To evaluate the outcomes of surgery for Brown syndrome. METHODS: We reviewed the charts of 15 patients who underwent surgery for Brown syndrome. The limitation of elevation in adduction (LEA) ranged from -2 to -4 degrees. A superior oblique muscle (SO) tenotomy was performed in 4 patients, a silicone expander was inserted in the SO of 9 patients, and a SO recession was performed in 2 patients. The results of surgery were analyzed with a follow-up period of more than 6 months, 42.3 +/- 48.42 months on average. RESULTS: Nine female patients and 6 male patients with unilateral Brown syndrome were selected for this study. The left eye was the affected eye in 9 patients. The degree of preoperative LEA was -2 to -4 in 4 patients in whom SO tenotomy was performed, -3 to -4 in 9 patients treated with the silicone expander, and -2 to -4 in 2 patients treated with SO recession. The LEA was released after surgery in all patients without postoperative adhesion. However, unilateral overaction of the inferior oblique muscle due to excessive weakening of the SO occurred in 1 patient with tenotomy (25%) and in 1 patient with insertion of a silicone expander (11%). CONCLUSIONS: LEA was released after tenotomy, insertion of a silicone expander and recession of the SO in 13 of 15 patients with Brown syndrome. SO palsy due to overcorrection and under-correction with postoperative adhesion should be avoided.     

Partial and total tenotomy of superior oblique muscle

The results of 33 superior oblique (SO) posterior and total tenotomy procedures were reviewed in 30 years of 17 patients. Two patients required secondary surgery, and the chosen procedure was tenotomy of SO muscle at the nasal side of the superior rectus muscle, whereas the initial tenotomy of SO muscle could be graded based on the dissection of intermuscular membrane plus the tenotomy site.     

Horizontal rectus tenotomy in patients with congenital nystagmus: results in 10 adults

OBJECTIVE: We wished to determine the effectiveness of horizontal rectus tenotomy in changing the nystagmus of patients with congenital nystagmus and, secondarily, how their visual function changed. DESIGN: This was a prospective, noncomparative, interventional case series. PARTICIPANTS: Ten adult patients with varied associated sensory defects and oculographic subtypes of congenital nystagmus (including asymmetric periodic or aperiodic alternating nystagmus) and no nystagmus treatment options. METHODS: By using standard surgical techniques, simple tenotomy of all four horizontal recti with reattachment at the original insertion was accomplished. Search-coil eye movement recordings and clinical examinations were performed before and 1, 6, 24, and 52 weeks after surgery. MAIN OUTCOME MEASURES: The primary outcome measure was the expanded nystagmus acuity function, obtained in "masked" fashion directly from ocular motility recordings. Secondary outcomes included breadth of null zones, preoperative and postoperative masked measures of visual acuity (Early Treatment Diabetic Retinopathy Study [ETDRS] chart), and the National Eye Institute Visual Function Questionnaire (NEI-VFQ-25). RESULTS: At 1 year after tenotomy and under binocular conditions, 9 of 10 patients had persistent, significant postoperative increases in the expanded nystagmus acuity function of their fixing (preferred) eye; 1 remained high, and 1 was not tested under the same conditions. Average foveation times increased in all 9 fixing (preferred) eyes. Binocular visual acuity measured with the ETDRS chart increased in 5 patients and was unaffected in five, whereas the NEI-VFQ-25 showed an improvement in vision-specific mental health in 9 patients. There were no adverse events. Tenotomy also radically changed the periodicity of one patient's asymmetric periodic or aperiodic alternating nystagmus. CONCLUSIONS: In 9 of 10 adult patients with clinical and oculographic variations in their congenital nystagmus, tenotomy resulted in significant improvements in a nystagmus measure and subjective visual functions.     

直肌离断后房水抗坏血酸的测定及其临床意义

通过腹腔注射抗坏血酸（ＡＡ），测定犬眼不同组合的２条和３条直肌离断后房水抗坏血酸的分泌量。发现２条直肌离断组术后１、３、７天ＡＡ分泌量处理眼较对照眼无显著降低（Ｐ＞０．０５）。３条直肌离断眼术后１、３天处理眼分泌量显著低于对照眼（Ｐ＜０．０５）。术后７天变化不显著。提示２条直肌同时离断对眼前节血流无明显影响。３条直肌同时离断早期可使眼前节血流量明显降低，其后血流可逐渐恢复。血流的代偿可能系睫状后长     

Correction of lower eyelid retraction in thyroid eye disease: a randomised controlled trial of retractor tenotomy with adjuvant antimetabolite versus scleral graft

BACKGROUND/AIMS—Lower eyelid retraction in thyroid eye disease contributes to ocular discomfort and an unsightly appearance, especially if asymmetrical. The use of donor scleral grafts is effective in lengthening the lower eyelids but carries a risk of virus transmission. Other techniques, including those which do not use grafts, need to be compared with scleral grafts. Recurrent retraction is a recognised complication of thyroid eyelid surgery; therefore, the authors investigated the use of antimetabolites to reduce postoperative fibrosis.
METHODS—In this prospective randomised controlled trial of 25 patients (35 eyelids), the use of donor sclera in 20 lower eyelids (13 patients) was compared with partial tenotomy of the anterior part of the lower eyelid retractors (ALER) with adjuvant peroperative antimetabolite in 15 lower eyelids (12 patients). A 5 minute peroperative application of either 5-fluorouracil (25 mg/ml) in nine lower eyelids (eight patients) or mitomycin C (0.2 mg/ml) in six lower eyelids (four patients) was used to focally inhibit fibroblasts. Follow up ranged from 3 to 18 months (mean 7.8).
RESULTS—One month after surgery the results of both groups were similar. However, at 3 months after surgery the results of scleral grafting were better than tenotomy with antimetabolites: 3/12 patients (25%) treated with tenotomy and adjuvant antimetabolite required subsequent surgery using grafts for correction of recurrent retraction. There were no significant complications associated with the use of antimetabolites in the eyelid in the doses used in this study.
CONCLUSIONS—This randomised prospective trial shows that donor scleral grafts were more effective in the long term than partial tenotomy with adjuvant antimetabolite in the correction of lower eyelid retraction associated with thyroid eye disease. The use of peroperative antimetabolites in the lower eyelid was safe.

Keywords: thyroid eye disease; lower eyelid retraction; scleral graft; antimetabolites     

Surgical treatment of true Brown's syndrome.

In 1950 H. W. Brown reported the superior oblique tendon sheath syndrome and cut the sheath as treatment for the condition. My results (as well as those of other surgeons) with similar surgery have been disappointing. Consequently, I carried out different operations consisting of Z tenotomy of superior oblique tendon in nine patients, split tendon lengthening in two patients, complete tenotomy in 16 patients, and a tenectomy in two patients. One of the latter was done on a patient after an unsatisfactory tenotomy where I missed the tendon. Tenotomy of the superior oblique tendon gave the best results. Experiments with the superior oblique tendon in cadavers several hours after death showed that after cutting the tendon just medial to the superior rectus muscle, the cut end of the tendon moved medially only about 8 to 10 mm due to restrictions of the capsular attachments to the trochlea. The cause of "true" Brown's syndrome is a tight tendon, and a safe and effective surgical treatment consists of cutting it just medial to the superior rectus muscle.     

Strabismus surgery for large-angle cyclotorsion after macular translocation surgery.

PURPOSE: We report the results of a new strabismus surgical procedure to address the large-angle cyclotorsion induced by macular translocation surgery for severe age-related macular degeneration. METHODS: The strabismus surgery described is a modification of earlier-described surgery(1) and was performed for symptomatic incyclotorsion measuring 20 degrees or more (by Maddox rod testing) after macular translocation. Surgery included superior oblique tenotomy and inferior oblique advancement, with transposition of the lateral and medial recti to the insertions of the superior and inferior recti, respectively, in the affected eyes. Minimum follow-up time was 6 weeks. RESULTS: Fifteen patients (15 eyes) had macular translocation surgery with incyclotorsion of 20 degrees or more. Mean incyclotorsion after macular translocation (36.1 +/- 9.4 degrees; range, 20-55) was reduced to 1.5 +/- 6.4 degrees after strabismus surgery (P <.0001), with 2 overcorrections and no surgical complications. Mean follow-up time was 24.8 +/- 13.4 weeks. All patients showed a hypertropia of the fellow eye. The mean pre-op hypertropia of 22 +/- 8 PD was reduced to 3 +/- 9 PD after strabismus surgery (P <.0001). The mean exotropia was minimally altered by strabismus surgery (21 +/- 10 PD pre vs 17 +/- 8 PD postop). All patients were symptomatic prior to strabismus surgery: 4 of 15 patients with "tilt" (objects appearing subjectively rotated with respect to their true orientation); 2 of 15 patients with diplopia; and 9 of 15 patients with both. Postoperatively, 7 patients had residual milder symptoms: tilt, 2 patients; diplopia, 3 patients; and both, 2 patients. Two patients required additional muscle surgery (on the fellow eye) for persistent symptoms. No patient showed binocular function (stereopsis or motor fusion) after macular translocation, either before or following strabismus surgery for cyclotorsion. CONCLUSION: The strabismus surgery described is effective at reducing the large degree of cyclotorsion (>or= 20 degrees) often resulting from macular translocation surgery, but does not allow reestablishment of binocular function.     

先天性眼球震颤的手术治疗及对比敏感度在疗效评价中的作用

目的研究改良Park术及眼外肌本体感受器切除联合眼外肌缩短术在治疗先天性眼球震颤中的疗效及对比敏感度检查在评价眼球震颤手术疗效中的作用。方法将82例先天性眼球震颤患者分为有中间带型(40例)和无中间带型(42例)两组。有中间带型眼球震颤患者施行改良的Park术,无中间带型眼球震颤患者施行眼外肌本体感受器切除联合眼外肌缩短术。所有患者术前及术后1个月给予双眼矫正视力及对比敏感度检查。结果所有患者术后眼位均为正位,且眼球转动各方向均不受限。40例有中间带型先天性眼球震颤患者术后1个月异常头位消失,第一眼位视力均较术前明显提高(P<0.05),均提高≥2行;对比敏感度在3周/度(cycle/degree,c·d-1)、6c·d-1、12c·d-1、18c·d-1空间频率下均较术前明显提高(均为P<0.05)。42例无中间带型先天性眼球震颤患者中,仅有7例患者双眼视力在术后1个月时提高≥2行(16.7%),但共有38例(90.5%)患者对比敏感度在各个空间频率上均较术前提高(均为P<0.05)。结论在改善先天性眼球震颤患者的异常头位及视功能方面,改良Park术与眼外肌本体感受器联合眼外肌缩短术具有良好的治疗效果;对比敏感度检查可用于客观评价先天性眼球震颤手术的效果。     

Bilateral recession of superior rectus muscles: Its influence on A and V pattern strabismus

PURPOSE: To determine whether bilateral superior rectus recession modifies A and V pattern strabismus. PATIENTS AND METHODS: Three patients with V patterns and eight patients with A patterns underwent bilateral superior rectus recession with neither oblique muscle surgery nor vertical displacement of the horizontal rectus muscles. Another three patients with A patterns underwent simultaneous superior oblique tenotomy and superior oblique recession. RESULTS: Two of the patients with V patterns demonstrated an increase in the V pattern after surgery; one was unchanged. Five of the patients with A patterns converted to V patterns after surgery. In the remaining three patients the pattern was eliminated. The three patients who underwent superior oblique tenotomy along with superior rectus recession had large shifts toward V pattern (mean shift, 40 delta). CONCLUSION: Bilateral superior rectus recession tends to increase V patterns and reduce A patterns. Superior rectus recession may be synergistic with superior oblique tenotomy in collapsing an A pattern.     

The Management of Posterior Polar Cataract: The Role of Patching and Grading

The outcome of bilateral medial rectus posterior fixation sutures +/– central tenotomy was assessed as a secondary procedure for residual convergence excess esotropia in 11 patients. Ten had previously undergone bilateral medial rectus recessions. One had recess/resect surgery on the deviating eye. The average preoperative near angle was 30 prism diopters with a range of 16 to 45 prism diopters. Eight patients underwent bilateral medial rectus posterior fixation sutures with central tenotomy. Two had bilateral medial rectus posterior fixation sutures only, and one had bilateral medial rectus posterior fixation suture, a lateral rectus resection, and an inferior oblique disinsertion. The postoperative near angle ranged from 4-30 prism diopters, with mean of 12 prism diopters. Five patients demonstrated some stereopsis preoperatively, all needing bifocals. Postoperatively, nine patients demonstrated an improvement in stereopsis, none needing bifocals. Two showed smaller near angles and better control without bifocals. Final stereopsis ranged from 30 seconds of arc to 800 seconds of arc. We feel that bilateral medial rectus posterior fixation sutures with or without central tenotomy is a viable secondary procedure for residual convergence excess esotropia.     

高度近视眼负低度数人工晶状体植入的临床观察

目的:观察超声乳化摘除联合负、低度数人工晶状体植入治疗高度近视性白内障的效果。方法:对66例94眼高度近视眼白内障行超声乳化摘除植入负、低度数人工晶状体,随访3mo。结果:术中2眼发生后囊破裂,术后3眼晶状体后囊混浊,无视网膜脱离者。结论:超声乳化摘除高度近视眼白内障是一种较好的手术方式,尤其同时植入负、低度数后房型人工晶状体增加了眼内组织的稳定性,防止了视网膜脱落,同时矫正了屈光不正。     

Surgery for residual convergence excess esotropia

The outcome of bilateral medial rectus posterior fixation sutures +/- central tenotomy was assessed as a secondary procedure for residual convergence excess esotropia in 11 patients. Ten had previously undergone bilateral medial rectus recessions. One had recess/resect surgery on the deviating eye. The average preoperative near angle was 30 prism diopters with a range of 16 to 45 prism diopters. Eight patients underwent bilateral medial rectus posterior fixation sutures with central tenotomy. Two had bilateral medial rectus posterior fixation sutures only, and one had bilateral medial rectus posterior fixation suture, a lateral rectus resection, and an inferior oblique disinsertion. The postoperative near angle ranged from 4-30 prism diopters, with mean of 12 prism diopters. Five patients demonstrated some stereopsis preoperatively, all needing bifocals. Postoperatively, nine patients demonstrated an improvement in stereopsis, none needing bifocals. Two showed smaller near angles and better control without bifocals. Final stereopsis ranged from 30 seconds of arc to 800 seconds of arc. We feel that bilateral medial rectus posterior fixation sutures with or without central tenotomy is a viable secondary procedure for residual convergence excess esotropia.     

Effects of tenotomy on patients with infantile nystagmus syndrome: Foveation improvement over a broadened visual field

PURPOSE: To investigate the effects of four-muscle tenotomy on visual function and gaze angle in patients with infantile nystagmus syndrome (INS). METHODS: Eye movements of nine patients with infantile nystagmus were recorded using infrared reflection or high-speed digital video techniques. Experimental protocols were designed to record the patients' eye-movement waveforms, pre- and post-tenotomy, at different gaze angles. We used the eXpanded Nystagmus Acuity Function (NAFX) to measure tenotomy-induced changes in the nystagmus at primary position and various gaze angles. The longest foveation domains (LFD) were measured from fitted curves. Peak-to-peak nystagmus amplitudes and foveation-period durations were also measured. All measurements were made unmasked. RESULTS: All seven patients with narrow, high-NAFX, gaze-angle regions showed broadening of these regions of higher visual function. Three patients showed moderate NAFX improvement (13.9-32.6%) at primary position, five showed large improvement (39.9-162.4%), and one showed no NAFX change (due to his high pretenotomy NAFX). Primary position measured acuities improved in six patients. All patients had reductions in nystagmus amplitudes ranging from 14.6 to 37%. The duration of the foveation period increased in all nine patients (11.2-200%). The percentage improvements in both the NAFX and the LFD decreased with higher pretenotomy values. CONCLUSIONS: In addition to elevating primary position NAFX, tenotomy also broadens the high-NAFX regions. This broadening effect is more prominent in patients who had sharp pretenotomy NAFX peaks. Four-muscle tenotomy produces higher primary position NAFX increases in infantile nystagmus patients whose pretenotomy values are relatively low, with the improvement decreasing at higher pretenotomy values. The tenotomy procedure improves visual function beyond primary position acuity. This extends the utility of surgical therapy to several different classes of patients with INS for whom other procedures are contraindicated. The pretenotomy NAFX can now be used to predict both primary position acuity improvements and broadening of a patient's high-NAFX range of gaze angles.     

上斜肌腱缝线延长术

目的 为了避免上斜肌断腱术引起的上斜肌麻痹，手术不能定量，术后发生过矫，及因断端丢失造成再次手术困难等缺点，行改良的上斜肌腱缝线延长术，即能减弱上斜肌的作用，又能达到部分、定量及可逆的上斜肌减弱效果。方法 颞上象限打开球结膜及Tenon氏囊，暴露鼻上象限术野，暴露上斜肌肌腱，用小斜视钩沿上直肌腱走行方向，向后轻轻钩起白色的上斜肌肌腱，分离肌鞘，用两根非吸收线，分别在肌腱上缝两根预置缝线，相距约4mm，在两根缝线之间剪断肌腱，根据术前上斜肌亢进程度，决定两断端间缝线延长的距离。结果 20例A型斜视的上斜肌功能亢进均取得明显改善，无1例过矫。结论 上斜肌腱缝线延长术，手术效果较肯定，可预防过矫，操作较硅胶带延长术容易，较可调缝线的上斜肌腱延长术更便利，并发症少。     

Congenital nystagmus: hypotheses for its genesis and complex waveforms within a behavioral ocular motor system model

Attempts to simulate dysfunction within ocular motor system (OMS) models capable of exhibiting known ocular motor behavior have provided valuable insight into the structure of the OMS required for normal visual function. The pendular waveforms of congenital nystagmus (CN) appear to be quite complex, composed of a sustained sinusoidal oscillation punctuated by braking saccades and foveating saccades followed by periods of extended foveation. Previously, we verified that these quick phases are generated by the same mechanism as voluntary saccades. We propose a computer model of the ocular motor system that simulates the responses of individuals with pendular CN (including its variable waveforms) based on the instability exhibited by the normal pursuit subsystem and its interaction with other components of the normal ocular motor control system. Fixation data from subjects with CN using both infrared and magnetic search coil oculography were used as templates for our simulations. Our OMS model simulates data from individuals with CN during fixation and in response to complex stimuli. The use of position and velocity efference copy to suppress oscillopsia is the key element in allowing for normal ocular motor behavior. The model's responses to target steps, pulse-steps, ramps, and step-ramps support the hypothetical explanation for the conditions that result in sustained pendular oscillation and the rules for the corrective saccadic responses that shape this underlying oscillation into the well-known family of pendular CN waveforms: pendular (P), pseudopendular (PP), pendular with foveating saccades (Pfs), and pseudopendular with foveating saccades (PPfs). Position error determined the saccadic amplitudes of foveating saccades, whereas stereotypical braking saccades were not dependent on visual information. Additionally, we propose a structure and method of operation for the fixation subsystem, and use it to prolong the low-velocity intervals immediately following foveating saccades. The model's robustness supports the hypothesis that the pendular nystagmus seen in CN is due to a loss of damping of the normal pursuit-system velocity oscillation (functionally, it is pursuit-system nystagmus--PSN).