Foveation dynamics in congenital nystagmus III: Vestibulo-ocular reflex

Abstract

It has been shown that, during fixation of a stationary target with a fixed head, an individual with congenital nystagmus (CN) can repeatedly (beat-to-beat) foveate (within 13 minarc) and maintain low retinal slip velocities (less than 4 degrees/sec). With the head in motion, vestibuloocular reflex (VOR) data showed eye velocities during these foveation periods that approximation head velocity. Despite some claims that the VOR of CN subjects was deficient or absent, individuals with CN hardly ever complain of oscillopsia or exhibit any of the symptoms that would accompany such deficits in the VOR, whether during simple walking and running or while skiing down a mogul field. We developed and describe several different and unrelated methods to accurately assess the function of the VOR in an individual with typical idiopathic CN. We investigated the dynamics of CN foveation periods during head rotation to test the hypothesis that eye velocities would match head velocities during these periods. At about 1 Hz, horizontal VOR instantaneous (beat-to-beat) gains were 0.96 in the light and 0.94 in the dark while imaging a stationary target. Vertical VOR gains were 1.00 and 0.99 for these two conditions at the same frequency; the CN was horizontal. Also, during the VOR there is a CN neutral-zone shift comparable to that found during smooth pursuit. Our methods demonstrated that gaze velocity was held constant during foveation periods and we conclude that the VOR in this subject is functioning normally in the presence of the CN oscillation. Based on our findings in this and previous studies, we hypothesize that CN may be due to a peripheral instability.

Single-plane compensatory phase shift of head and eye oscillations in infantile nystagmus syndrome

“…This led to the conclusion, that apart from few exceptions, head movements do not serve a clear purpose and may represent an associated pathological head tremor [6]. The single study with magnetic search coil showed similar results, but was confined in the horizontal plane [4]. An analogous, although different, phenomenon has been described in children with spasmus nutans: in contrast to most INS patients, children with spasmus nutans show spontaneous nystagmus that changes dramatically during periods of head nodding: eye movements alter their phase and frequency and become a mirror to the head movements.…”

“…Moreover, nodding was diminished or absent in periods of looking without having an attentional focus and became prominent when the subject was asked to read a simple text presented 1 m away. Previous work on patients with nystagmus and concomitant head movements yielded various results, but all studies had in common that the vast majority of cases showed no clear relation between head and eye movements [1,[3][4][5][6]. This led to the conclusion, that apart from few exceptions, head movements do not serve a clear purpose and may represent an associated pathological head tremor [6].…”

“…Thirdly, only recently have objective recordings of the complex three-dimensional nystagmic waveforms been available in clinical neuroophthalmology. It thus remains uncertain whether nodding is compensatory to control the nystagmus or represents pathological oscillations (head tremor) [4]. Here, we tried to find out if nodding is compensatory in a patient with INS, and, more importantly if it compensates equally for all three degrees of freedom of eye movements.…”

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

“…[7][8][9] These "foveation periods" have been shown to enhance visual acuity. The accuracy and duration of foveation has been directly linked to visual acuity.…”

“…The accuracy and duration of foveation has been directly linked to visual acuity. 5,[7][8][9][10] If these periods can be lengthened or increased by the patient (adaptation) or by therapeutic interventions, the patient's visual acuity may be increased.…”