Abstract
The aim of this study was to propose a new pathophysiological hypothesis for involuntary eye oscillation in infantile nystagmus (IN): patients with IN exhibit impaired gaze fixation, horizontal smooth pursuit and optokinetic nystagmus (OKN) and use saccadic eye movements for these underlying impairments. In order to induce saccades, they make enough angle between gaze and target by precedent exponential slow eye movements. IN consists of the alternate appearance of the saccade and the slow eye movements. Unlike most previous theories, IN is therefore considered a necessary strategy allowing for better vision and not an obstacle to clear vision. In five patients with IN, eye movements were analyzed during the smooth pursuit test, saccadic eye movement test, OKN test and vestibulo-ocular reflex (VOR) test. Their gaze fixation, horizontal smooth pursuit, OKN and the last half of the slow phase of VOR were impaired. The lines obtained by connection of the end eye positions of fast phase of nystagmus coincided with the trajectories of targets. The findings indicate that patients followed the target by the fast but not the slow phase of nystagmus, which supports our hypothesis. By setting the direction of slow phase of nystagmus opposite to the direction of the OKN stimulation, enough angle can be effectively made between the gaze and target for the induction of saccade. This is the mechanism of reversed OKN response. In darkness and when eyes are closed, IN weakens because there is no visual target and neither the saccade for catching up the target or slow phase for induction of the saccade is needed.
Highlights
Patients with congenital ocular oscillations show three distinct syndromes: infantile nystagmus (IN), fusion maldevelopment nystagmus syndrome, and spasmus nutans syndrome [1]
It rarely happens that the nystagmus is unchanged when the eye position changes across the null zone as shown in part B of Figure 1A in patient 1
We proposed a new theory on the cause of IN with a horizontal jerk waveform
Summary
Patients with congenital ocular oscillations show three distinct syndromes: infantile nystagmus (IN), fusion maldevelopment nystagmus syndrome, and spasmus nutans syndrome [1]. IN is characterized by waveforms, the most common of which is the horizontal jerk type where horizontal slow phases exhibit increasing velocity exponentials [2]. This type of nystagmus exhibits two unusual characteristics that appear to Viewing Targets in Infantile Nystagmus impair visual function. Patients may exhibit reversed optokinetic nystagmus (OKN) response, where the slow phase direction opposes that of the stimulus movement [5]. Our novel theory explains how IN patients with horizontal jerk waveform view visual targets, and why [1] attempts to fixate upon an object accentuate nystagmus that is suppressed in darkness. We demonstrate how their visuo-vestibular system performs during rotation in light
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