Extraocular proprioception and new treatments for infantile nystagmus syndrome

Patterns and modulations of Pendular nystagmus in a family with hereditary spastic paraplegia

Hereditary spastic paraplegia (HSP) is characterized by progressive spasticity and weakness of the lower extremities. Additional findings include ataxia, extrapyramidal signs, and dementia. Pendular nystagmus (PN) has been reported in some subtypes of HSP caused by PLP1 (SPG2) or paraplegin (SPG7) mutation. To describe the patterns and modulation of PN in HSP, we performed eye movement recording using video-oculography in a Korean family with HSP and PN. The PN was convergent-divergent in the oblique plane with a frequency of 6 to 7Hz and maximum amplitude at about 1.5°. The nystagmus diminished briefly after blinks and horizontal saccades, and decreased during eccentric gaze and convergence. Horizontal saccades shifted the phase of the oscillations. During lateral gazes, the PN increased in the abducting eye, but decreased in the adducting eye. Vibratory stimuli decreased the nystagmus mostly in the left but not in the right eye. No pathogenic mutation was found in the genetic loci known for causing spastic paraplegia by whole-exome sequencing. The unusual features and modulation of PN in our patients with HSP emphasize the role of disconjugate and disjunctive capabilities of the brain in ocular motor control, and exemplify what can go wrong.

Combined recession and resection surgery in the management of convergence excess esotropia with different levels of AC/A ratio

PURPOSE

To study prospectively the effect of extirpating the proprioceptive impulse at the myotendinous junction combined with recession of the medial rectus muscles in patients with convergence excess esotropia.

METHODS

A total of 21 patients with different sizes of AC/A ratios (high, 8; normal, 12; low, 1) underwent a surgical procedure consisting of combining resection of 2.5 mm of the insertional end of the medial rectus muscles with recession from the original insertion, based on the patient's angle of esotropia at 1/3 m while wearing full cycloplegic refraction, with an additional recession of 1 mm for each rectus muscle based on current surgical tables. A satisfactory outcome was defined as orthotropia or esotropia of <10^Δ at near and distance fixations with available correction and reduction of the distance--near disparity to <10^Δ.

RESULTS

All patients, regardless of the size of AC/A ratio and the amount of near-distance disparity, had satisfactory alignments at near and distance fixations, with residual near-distance disparity of <10^Δ. Consecutive distance exotropia did not develop even when there was preoperative distance orthotropia. Outcome measures remained stable for a mean of 4.3 years. None of the 8 patients with high AC/A ratios required bifocal wear or overcorrection prescriptions to maintain alignment postoperatively.

CONCLUSIONS

This technique of combined resection and recession of the medial rectus muscle shows promise in the treatment of convergence excess esotropia. The main advantage is improvement in distance alignment while selectively reducing the near angle in patients with different levels of AC/A ratios.

Infantile nystagmus syndrome: clinical characteristics, current theories of pathogenesis, diagnosis, and management

Infantile nystagmus syndrome (INS) is an important clinical diagnosis because it is a common presenting sign of many ocular, neurologic, and systemic diseases. Although INS has been studied for more than a century, its diagnosis and treatment remains a challenge to clinicians because of its varied manifestations and multiple associations, and its pathogenesis continues to rouse considerable scientific debate. Fueled by these challenges, recent basic research and clinical investigations have provided new insights into INS. New genetic discoveries and technological advances in ocular imaging have refined our understanding of INS subtypes and offer new diagnostic possibilities. Unexpected surgical outcomes have led to new understanding of its pathogenesis based on novel hypothesized pathways of ocular motor control. Comparative studies on nonhuman visual systems have also informed models of the neural substrate of INS in humans. This review brings together the classic profile of this disorder with recent research to provide an update on the clinical features of INS, an overview of the current theories on how and why INS develops, and a practical approach to the diagnosis and management of INS.

Effect of Photorefractive Keratectomy on Nystagmus and Visual Functions in Myopic Patients With Infantile Nystagmus Syndrome

PURPOSE

To determine the effect of photorefractive keratectomy (PRK) on involuntary eye movements, visual acuity, and contrast sensitivity in myopic patients with infantile nystagmus syndrome.

DESIGN

Prospective interventional case series.

METHODS

This study was conducted on patients with infantile nystagmus syndrome and myopia equal to or more than -1 diopter (D), who were referred to our clinic over a 2-year period. Patients older than 18 years of age with a stable refraction for at least 1 year who were good candidates for PRK were included. Complete ophthalmologic examinations including assessment of best-corrected visual acuity (BCVA), contrast sensitivity, and videonystagmography were performed for all patients before and 3 months after surgery.

RESULTS

Twenty-four eyes of 12 patients with mean age of 23 ± 2 years were enrolled in this study. Spherical equivalent refractive error was -2.82 ± 1.65 D and -0.26 ± 0.25 D before and after PRK, respectively (P < .001). Monocular BCVA improved from 0.36 ± 0.21 logMAR to 0.27 ± 0.25 logMAR and binocular BCVA improved from 0.33 ± 0.2 logMAR to 0.17 ± 0.16 logMAR (P < .001). Contrast sensitivity significantly improved at low (P < .001), intermediate (P < .001), and high frequencies (P = .01). The frequency, amplitude, and intensity of nystagmus were significantly decreased after PRK (P < .001). There was no correlation between the degree of myopia correction and improvement in sensory and motor indices of nystagmus (P > .1, Spearman correlation coefficient).

CONCLUSION

PRK in patients with infantile nystagmus syndrome and myopia improved monocular and binocular BCVA and contrast sensitivity. Furthermore, motor indices of nystagmus (frequency, amplitude, and intensity) were significantly improved after surgery in these patients.

A noninvasive electromagnetic perturbation approach to probe extraocular proprioception

BACKGROUND

Extraocular proprioception has been shown to participate in spatial perception and binocular alignment. Yet the physiological approaches used to study this sensory signal are limited because proprioceptive signaling takes place at the same time as visuomotor signaling. It is critical to dissociate this sensory signal from other visuomotor events that accompany eye movements.

METHODS

We present a novel noninvasive and quantifiable method for probing extraocular proprioception independent of other visuomotor processing by attaching a rare-earth magnet to a real-time model eye and placing an electromagnet <20 mm from the eye. An electromagnet can increase or decrease angular displacements and velocities of the model eye.

RESULTS

Electromagnetic activation rapidly affected (<2 ms) the rotation kinematics of the eye, which were correlated linearly with both the current supply and the distance of the electromagnet relative to the eye.

CONCLUSIONS

This method circumvented the constraints of conventional physiological manipulation of extraocular proprioception, such as manually or mechanically tugging on the eye ball. It can be applied to produce the discrepancy between the intended and the executed eye movements, so that proprioceptive reafference signals are dissociated from corollary motor discharges and other visuomotor events.

Origins of strabismus and loss of binocular vision

Strabismus is a frequent ocular disorder that develops early in life in humans. As a general rule, it is characterized by a misalignment of the visual axes which most often appears during the critical period of visual development. However other characteristics of strabismus may vary greatly among subjects, for example, being convergent or divergent, horizontal or vertical, with variable angles of deviation. Binocular vision may also vary greatly. Our main goal here is to develop the idea that such “polymorphy” reflects a wide variety in the possible origins of strabismus. We propose that strabismus must be considered as possibly resulting from abnormal genetic and/or acquired factors, anatomical and/or functional abnormalities, in the sensory and/or the motor systems, both peripherally and/or in the brain itself. We shall particularly develop the possible “central” origins of strabismus. Indeed, we are convinced that it is time now to open this “black box” in order to move forward. All of this will be developed on the basis of both presently available data in literature (including most recent data) and our own experience. Both data in biology and medicine will be referred to. Our conclusions will hopefully help ophthalmologists to better understand strabismus and to develop new therapeutic strategies in the future. Presently, physicians eliminate or limit the negative effects of such pathology both on the development of the visual system and visual perception through the use of optical correction and, in some cases, extraocular muscle surgery. To better circumscribe the problem of the origins of strabismus, including at a cerebral level, may improve its management, in particular with respect to binocular vision, through innovating tools by treating the pathology at the source.

Aetiology of infantile nystagmus

PURPOSE OF REVIEW

Mechanisms underlying infantile nystagmus are unclear. The aim of this review is to outline recent developments in understanding the aetiology of infantile nystagmus.

RECENT FINDINGS

There have been advances in understanding mechanisms underlying idiopathic infantile nystagmus, which has progressed through determining the role of the FRMD7 gene in controlling neurite outgrowth, and albinism, in which recent models have investigated the possibility of retinal miswiring leading to nystagmus. We also briefly review aetiology of infantile nystagmus in afferent visual deficits caused by ocular disease, and PAX6 mutations. Improved phenotypical characterization of all these infantile nystagmus subtypes has been achieved recently through high-resolution retinal imaging using optical coherence tomography. Several new hypotheses proposing common mechanisms that could underlie various infantile nystagmus subtypes are also highlighted.

SUMMARY

Although there is still no consensus of opinion regarding the mechanisms causing infantile nystagmus, identification of new genes and determining their cellular function, phenotypical characterization of genetic subtypes, and improvements in animal models have significantly advanced our understanding of infantile nystagmus. These recent developments pave the way to achieving a much clearer picture of infantile nystagmus aetiology in the future.

Damping of monocular pendular nystagmus with vibration in a patient with multiple sclerosis

Acquired pendular nystagmus (PN) occurs commonly in multiple sclerosis (MS) and results in a highly disabling oscillopsia that impairs vision. It usually consists of pseudo-sinusoidal oscillations at a single frequency (3-5 Hz) that often briefly stop for a few hundred milliseconds after saccades and blinks. The oscillations are thought to arise from instability in the gaze-holding networks ("neural integrator") in the brainstem and cerebellum.(1,2) Here we describe a patient with monocular PN in whom vibration on the skull from a handheld muscle massager strikingly diminished or stopped her nystagmus.

Albinism: particular attention to the ocular motor system

The purpose of this report is to summarize an understanding of the ocular motor system in patients with albinism. Other than the association of vertical eccentric gaze null positions and asymmetric, (a) periodic alternating nystagmus in a large percentage of patients, the ocular motor system in human albinism does not contain unique pathology, rather has "typical" types of infantile ocular oscillations and binocular disorders. Both the ocular motor and afferent visual system are affected to varying degrees in patients with albinism, thus, combined treatment of both systems will maximize visual function.

Therapy for nystagmus

Pathological forms of nystagmus and their visual consequences can be treated using pharmacological, optical, and surgical approaches. Acquired periodic alternating nystagmus improves following treatment with baclofen, and downbeat nystagmus may improve following treatment with aminopyridines. Gabapentin and memantine are helpful in reducing acquired pendular nystagmus due to multiple sclerosis. Ocular oscillations in oculopalatal tremor may also improve following treatment with memantine or gabapentin. The infantile nystagmus syndrome (INS) may have only a minor impact on vision if "foveation periods" are well developed, but symptomatic patients may benefit from treatment with gabapentin, memantine, or base-out prisms to induce convergence. Several surgical therapies are also reported to improve INS, but selection of the optimal treatment depends on careful evaluation of visual acuity and nystagmus intensity in various gaze positions. Electro-optical devices are a promising and novel approach for treating the visual consequences of acquired forms of nystagmus.

Improvement in visual acuity following surgery for correction of head posture in infantile nystagmus syndrome

PURPOSE

To report the effect of the abnormal head posture (AHP) correcting procedures on the visual acuity improvement in patients with infantile nystagmus syndrome (INS) and the visual acuity improvement outcomes in different AHP correcting surgeries in INS.

METHODS

This was a prospective, non-randomized, interventional study. Twenty-eight patients underwent the Anderson-Kestenbaum procedure or the modified Anderson procedure with or without tenotomy of at least one horizontal recti for correction of AHP. Best-corrected binocular null zone acuity and degree of AHP was recorded preoperatively and compared with those done 1 month postoperatively.

RESULTS

The average null zone logarithm of the minimum angle of resolution acuity was 0.42 preoperatively, which improved significantly to 0.33 postoperatively (P = .002). The AHP ranged from 10° to 40° (mean: 20.89°), which improved significantly to a mean of 3.21° (P = .000). No significant difference (P = .65) was found in the visual acuity improvement among patients who underwent the Anderson-Kestenbaum procedure or the modified Anderson procedure with or without tenotomy. No significant difference in the visual acuity improvement was seen in patients who underwent tenotomy of at least one horizontal rectus muscle along with the modified Anderson procedure compared to those who underwent the modified Anderson procedure alone (P = .28).

CONCLUSION

The procedures used mainly for correction of AHP in INS do yield significant improvement in the visual acuity. This improvement is seen in patients undergoing surgery for both horizontal and vertical AHP.

Nystagmus and saccadic intrusions

We review current concepts of nystagmus and saccadic oscillations, applying a pathophysiological approach. We begin by discussing how nystagmus may arise when the mechanisms that normally hold gaze steady are impaired. We then describe the clinical and laboratory evaluation of patients with ocular oscillations. Next, we systematically review the features of nystagmus arising from peripheral and central vestibular disorders, nystagmus due to an abnormal gaze-holding mechanism (neural integrator), and nystagmus occurring when vision is compromised. We then discuss forms of nystagmus for which the pathogenesis is not well understood, including acquired pendular nystagmus and congenital forms of nystagmus. We then summarize the spectrum of saccadic disorders that disrupt steady gaze, from intrusions to flutter and opsoclonus. Finally, we review current treatment options for nystagmus and saccadic oscillations, including drugs, surgery, and optical methods. Examples of each type of nystagmus are provided in the form of figures.

Two hypothetical nystagmus procedures: augmented tenotomy and reattachment and augmented tendon suture (Sans tenotomy)

PURPOSE

To review the hypothetical mechanism and therapeutic benefits of the four-muscle tenotomy and reattachment (T&R) procedure using knowledge accrued over the 10 years since its proposal; to describe an augmented tendon suture (ATS) technique to improve the procedure based on one of the originally suggested alternative methods (mechanical); and to hypothesize a new ATS procedure to achieve the same therapeutic benefits without extraocular muscle tenotomy or reattachment to the globe.

METHODS

Standard surgical methods were used.

RESULTS

The T&R procedure damps and improves infantile nystagmus syndrome (INS) waveforms, improves eXtended Nystagmus Acuity Function (NAFX) values, broadens the NAFX peak versus gaze angle, and damps slow eye movements but not saccades. The T&R procedure also damps acquired pendular and downbeat nystagmus, decreasing the patients' oscillopsia, and lowers the target acquisition time in INS.

CONCLUSION

The T&R procedure directly affects only the enthesis of the tendon; there is idiosyncratic variation in the distribution of afferent fibers in the tendons. The ATS technique consists of placing several additional sutures in the tendon proximal to the tenotomy. Based on the hypothetical proprioceptive mechanism for the beneficial effects of the T&R procedure, the authors hypothesize that the ATS technique will maximize the therapeutic benefits and that an ATS procedure, using only tendon sutures without tenotomy, will duplicate the therapeutic effects of T&R. Eliminating the tenotomy component results in a simpler procedure more suitable for single-session, multi-muscle surgery that may be required for improving the waveforms of multiplanar nystagmus and less prone to cause complications.

Using the NAFX to measure the effectiveness over time of gene therapy in canine LCA

PURPOSE

To use ocular motility recordings to determine the changes over time of infantile nystagmus syndrome (INS) in RPE65-deficient canines with Leber Congenital Amaurosis (LCA) and assess the time course of the recalibration of the ocular motor system (OMS).

METHODS

Nine dogs were treated bilaterally with AAV-RPE65. A second cohort of four dogs was treated with AAV2.RPE65, an optimized vector. Their fixation eye movements were recorded before treatment and at 4-week intervals for 3 months, by using high-speed (500 Hz) digital videography. The dogs were suspended in a sling and encouraged to fixate on distant (57 inches) targets at gaze angles varying between +/-15 degrees horizontally and +/-10 degrees vertically. The records for each eye were examined for qualitative changes in waveform and for quantitative changes in centralisation with the expanded nystagmus acuity function (NAFX) and compared with ERG results for restoration of receptor function.

RESULTS

First group: Before treatment, five of the dogs had clinically apparent INS with jerk, pendular, or both waveforms and with peak-to-peak amplitudes as great as 15 degrees . One dog had intermittent nystagmus. At the 1- and 2-month examinations, no change in nystagmus waveform or NAFX was observed in any of the initial dogs, while at 10 weeks, one dog treated bilaterally with the standard dosage showed reduced nystagmus in only one eye. The other eye did not respond to treatment, as confirmed by ERG. This result was unexpected since it was previously documented that unilateral treatment leads to bilateral reduction of INS. The other dog treated with the standard dosage showed no reduction of its small-amplitude, high-frequency pendular nystagmus despite positive ERG responses. Second group: Only one dog of the four had clinically detectable INS, similar in characteristics to that seen in the affected dogs of the first group. Unlike any previous dog studied, this one showed a damping of the nystagmus within the first 4 weeks after treatment.

CONCLUSIONS

In all but one of the cases in which OMS recalibration occurred, as measured by the clinical appearance of nystagmus and by quantitative measurement using the NAFX, the improvement was apparent no sooner than 10 weeks after treatment. Longer term, dose-related studies are needed to determine the minimum necessary degree of restored receptor functionality, the duration after rescue for recalibration of the OMS, and the conditions under which recalibration information can successfully affect the contralateral eye.