Biologically Relevant Models of Infantile Nystagmus Syndrome: The Requirement for Behavioral Ocular Motor System Models

Analysis of anomalous head posturing in patients with infantile nystagmus syndrome

PURPOSE

To investigate anomalous head posturing in patients with INS.

METHODS

This was a prospective, cohort analysis of clinical and anomalous head posture (AHP) data in 34 patients with INS and an AHP. Particular outcome measures included measurement of AHP in three dimensions of pitch (anterior posterior flexion/extension), yaw (lateral rotation), and roll (lateral flexion) during best-corrected binocular acuity testing and during their subjective sense of straight. Patients were also queried as to their subjective sense of head posture in forced straight position and in their preferred AHP. The paired t test was used to determine significance in differences between measures.

RESULTS

A total of 34 patients (19 males [56%]) 9-56 years of age (mean, 16.5 ± 6) were included. Associated systemic or ocular system deficits were present in 30 patients (88%). AHP during best-corrected visual acuity testing averaged 16.5° ± 8.20° (range, 10°-51°), which was significantly different from the mean voluntary "comfortable" position only in the pitch and roll directions (P < 0.001). There was a significant noncongruous response during subjective response to head posturing with most sensing their head as "crooked" (76.5%) when manually straightened (P = 0.001).

CONCLUSIONS

The clinical AHP of patients with INS exists in all three spatial dimensions of pitch, yaw, and roll. Although the visual system may be causally related to the onset, amount, and direction of a compensatory AHP in patients with INS, its persistence over time or after surgical intervention is likely due to a combination of visual system (eg, nystagmus, strabismus) and nonvisual system (egocentric and musculo-skeletal) factors.

Clinical and Electrophysiological Outcomes After Eye Muscle Surgery in 81 Adults With Infantile Nystagmus Syndrome

PURPOSE

To characterize the effects of eye muscle surgery on patients older than 18 years with infantile nystagmus syndrome (INS) who have had only optical treatment.

METHODS

This was a prospective, single-center, interventional case series analysis of clinical and electro-phyisological data before and after surgery. Outcome measures included: clinical characteristics, surgical procedure, and preoperative and postoperative binocular best corrected visual acuity (BCVA) in the null position, anomalous head posture (AHP), contrast sensitivity, strabismic deviation, and nystagmus acuity function (NAFX). Postoperative data used were collected for a minimum of 12 months after surgery. Parametric and non-parametric statistical analysis of the outcome measures was performed.

RESULTS

Ages ranged from 18 to 72 years (average: 36 years) and follow-up from 12 to 74 months (average: 26 months). A surgical algorithm of nine separate procedures involving at least two recti muscles on each eye was used for each patient. Most patients had associated systemic and/or ocular diagnoses, including albinism (35%), amblyopia (23%), optic nerve or retinal disorders (48%), refractive error (80%), AHP (44%), aperiodicity (27%), and strabismus (69%). There were no serious surgical complications, with a reoperation rate of 12%. There were significant group mean increases in BVCA, AHP, contrast sensitivity, strabismic deviation, and NAFX after surgery. Sixty percent of patients who were legally ineligible for driving prior to surgery became eligible after eye muscle surgery.

CONCLUSIONS

Adult patients with INS showed sustained improvement in many afferent and efferent measures of visual function after eye muscle surgery. [J Pediatr Ophthalmol Strabismus. 2021;58(2):93-104.].

Congenital Nystagmus and Its Congeners

In recent years, we have made enormous strides in elucidating the phenomenology of congenital nystagmus. The purpose of this review is to briefly summarize our current understanding of congenital nystagmus in terms of its clinical symptomatology, pathophysiology, differential diagnosis, and ancillary testing, and clinical management. Finally, this discussion provides the reader with an armamentarium of clinical pearls to facilitate diagnosis of the numerous sensory visual disorders that can underlie congenital nystagmus.

Quick phases of infantile nystagmus show the saccadic inhibition effect

PURPOSE

Infantile nystagmus (IN) is a pathological, involuntary oscillation of the eyes consisting of slow, drifting eye movements interspersed with rapid reorienting quick phases. The extent to which quick phases of IN are programmed similarly to saccadic eye movements remains unknown. We investigated whether IN quick phases exhibit 'saccadic inhibition,' a phenomenon typically related to normal targeting saccades, in which the initiation of the eye movement is systematically delayed by task-irrelevant visual distractors.

METHODS

We recorded eye position from 10 observers with early-onset idiopathic nystagmus while task-irrelevant distractor stimuli were flashed along the top and bottom of a large screen at ±10° eccentricity. The latency distributions of quick phases were measured with respect to these distractor flashes. Two additional participants, one with possible albinism and one with fusion maldevelopment nystagmus syndrome, were also tested.

RESULTS

All observers showed that a distractor flash delayed the execution of quick phases that would otherwise have occurred approximately 100 ms later, exactly as in the standard saccadic inhibition effect. The delay did not appear to differ between the two main nystagmus types under investigation (idiopathic IN with unidirectional and bidirectional jerk).

CONCLUSIONS

The presence of the saccadic inhibition effect in IN quick phases is consistent with the idea that quick phases and saccades share a common programming pathway. This could allow quick phases to take on flexible, goal-directed behavior, at odds with the view that IN quick phases are stereotyped, involuntary eye movements.

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.

Severity of infantile nystagmus syndrome-like ocular motor phenotype is linked to the extent of the underlying optic nerve projection defect in zebrafish belladonna mutant

Infantile nystagmus syndrome (INS), formerly known as congenital nystagmus, is an ocular motor disorder in humans characterized by spontaneous eye oscillations (SOs) and, in several cases, reversed optokinetic response (OKR). Its etiology and pathomechanism is largely unknown, but misrouting of the optic nerve has been observed in some patients. Likewise, optic nerve misrouting, a reversed OKR and SOs with INS-like waveforms are observed in zebrafish belladonna (bel) mutants. We aimed to investigate whether and how misrouting of the optic nerve correlates with the ocular motor behaviors in bel larvae. OKR and SOs were quantified and subsequently the optic nerve fibers were stained with fluorescent lipophilic dyes. Eye velocity during OKR was reduced in larvae with few misprojecting optic nerve fibers and reversed in larvae with a substantial fraction of misprojecting fibers. All larvae with reversed OKR also displayed SOs. A stronger reversed OKR correlated with more frequent SOs. Since we did not find a correlation between additional retinal defects and ocular motor behavior, we suggest that axon misrouting is in fact origin of INS in the zebrafish animal model. Depending on the ratio between misprojecting ipsilateral and correctly projecting contralateral fibers, the negative feedback loop normally regulating OKR can turn into a positive loop, resulting in an increase in retinal slip. Our data not only give new insights into the etiology of INS but may also be of interest for studies on how the brain deals with and adapts to conflicting inputs.

Fixation control and eye alignment in children treated for dense congenital or developmental cataracts

BACKGROUND

Many children treated for cataracts develop strabismus and nystagmus; however, little is known about the critical period for adverse ocular motor outcomes with respect to age of onset and duration.

METHODS

Children who had undergone extraction of dense cataracts by the age of 5 years were enrolled postoperatively. Ocular alignment was assessed regularly throughout follow-up. Fixation stability and associated ocular oscillations were determined from eye movement recordings at ≥5 years old. Multivariate logistic regression was used to evaluate whether laterality (unilateral vs bilateral), age at onset, and/or duration of visual deprivation were associated with adverse ocular motor outcomes and to determine multivariate odds ratios (ORs).

RESULTS

A total of 41 children were included. Of these, 27 (66%) developed strabismus; 29 (71%) developed nystagmus. Congenital onset was associated with significant risk for strabismus (OR, 5.3; 95% CI, 1.1-34.1); infantile onset was associated with significant risk for nystagmus (OR, 13.6; 95% CI, 1.6-302). Duration >6 weeks was associated with significant risk for both strabismus (OR, 9.1; 95% CI, 1.9-54.2) and nystagmus (OR, 46.2; 95% CI, 6.0-1005). Congenital onset was associated with significant risk for interocular asymmetry in severity of nystagmus (OR, 25.0; 95% CI, 2.6-649), as was unilateral cataract (OR, 58.9; 95% CI, 5.1-2318).

CONCLUSIONS

Laterality (unilateral vs bilateral) and age at onset were significant nonmodifiable risk factors for adverse ocular motor outcomes. Duration of deprivation was a significant modifiable risk factor for adverse ocular motor outcomes. The current study demonstrated that reduced risk for nystagmus and strabismus was associated with deprivation ≤6 weeks.

Comparison of infantile nystagmus syndrome in achiasmatic zebrafish and humans

Infantile nystagmus syndrome (INS; formerly called congenital nystagmus) is an ocular motor disorder characterized by several typical nystagmus waveforms. To date, restrictions inherent to human research and the absence of a handy animal model have impeded efforts to identify the underlying mechanism of INS. Displaying INS-like spontaneous eye oscillations, achiasmatic zebrafish belladonna (bel) mutants may provide new insights into the mystery of INS. In this study, we demonstrate that these spontaneous eye oscillations match the diagnostic waveforms of INS. As a result, zebrafish bel mutants can be used as an animal model for the study of INS. In zebrafish bel mutants, visual pathway abnormalities may contribute to the spontaneous nystagmus via an inverted signal to the pretectal area. We hypothesized that human INS may also be linked to visual pathway abnormalities (possibly underdiagnosed in INS patients) in a similar way.

Vestibular deficits do not underlie looping behavior in achiasmatic fish

Zebrafish belladonna (bel) mutants carry a mutation in the lhx2 gene that encodes a Lim domain homeobox transcription factor, leading to a defect in the retinotectal axon pathfinding. As a result, a large fraction of homozygous bel mutants is achiasmatic. Achiasmatic bel mutants display ocular motor instabilities, both reserved optokinetic response (OKR) and spontaneous eye oscillations, and an unstable swimming behavior, described as looping. All these unstable behaviors have been linked to the underlying optic nerve projection defect. Looping has been investigated under different visual stimuli and shown to be vision dependent and contrast sensitive. In addition, looping correlates perfectly with reversed OKR and the spontaneous oscillations of the eyes. Hence, it has been hypothesized that looping is a compensatory response to the perception of self-motion induced by the spontaneous eye oscillations. However, both ocular and postural instabilities could also be caused by a yet unidentified vestibular deficit. Here, we performed a preliminary test of the vestibular function in achiasmatic bel larval mutants in order to clarify the potential role of a vestibular deficit in looping. We found that the vestibular ocular reflex (VOR) is normally directed in both bel mutants and wild types and therefore exclude the possibility that nystagmus and looping in reverse to the rotating optokinetic drum can be attributed to an underlying vestibular deficit.

Effects of acetazolamide on infantile nystagmus syndrome waveforms: comparisons to contact lenses and convergence in a well-studied subject

AIM

To determine if acetazolamide, an effective treatment for certain inherited channelopathies, has therapeutic effects on infantile nystagmus syndrome (INS) in a well-studied subject, compare them to other therapies in the same subject and to tenotomy and reattachment (T&R) in other subjects.

METHODS

Eye-movement data were taken using a high-speed digital video recording system. Nystagmus waveforms were analyzed by applying an eXpanded Nystagmus Acuity Function (NAFX) at different gaze angles and determining the Longest Foveation Domain (LFD).

RESULTS

Acetazolamide improved foveation by both a 59.7% increase in the peak value of the NAFX function (from 0.395 to 0.580) and a 70% broadening of the NAFX vs Gaze Angle curve (the LFD increased from 20° to 34°). The resulting U-shaped improvement in the percent NAFX vs Gaze Angle curve, varied from ~60% near the NAFX peak to over 1000% laterally. The therapeutic improvements in NAFX from acetazolamide (similar to T&R) were intermediate between those of soft contact lenses and convergence, the latter was best; for LFD improvements, acetazolamide and contact lenses were equivalent and less effective than convergence. Computer simulations suggested that damping the central oscillation driving INS was insufficient to produce the foveation improvements and increased NAFX values.

CONCLUSION

Acetazolamide resulted in improved-foveation INS waveforms over a broadened range of gaze angles, probably acting at more than one site. This raises the question of whether hereditary INS involves an inherited channelopathy, and whether other agents with known effects on ion channels should be investigated as therapy for this condition.