Waveform changes with gaze angle in early–onset nystagmus

Abstract

Abstract: : Purpose:The entire range of eye position against time waveforms found in early–onset nystagmus can be synthesized by adding a sinusoidal waveform to a saw tooth waveform, which implies that two separate mechanisms are involved in generating the waveform. We investigated the relative contributions of the two mechanisms by measuring how the waveforms of subjects with early–onset nystagmus changed with gaze angle. Methods:Ten adult subjects with idiopathic early–onset nystagmus were given full orthoptic and ophthalmological investigations. Horizontal eye movements were measured using an infrared limbus tracker while the subjects maintained steady fixation for 1 minute on targets positioned at +15, 0 and –15 degrees horizontally. Eye movements with both monocular and binocular fixation were recorded. The nonlinear dynamics technique of unstable periodic orbit analysis was used to isolate the underlying waveform despite the variability of the nystagmus. Results:The periodic orbit analysis technique was found to be very effective at revealing consistent changes in waveform despite the variability. The amplitudes of the jerk waveforms were all directly proportional to gaze angle. In the cases where the jerk component was found together with a pendular component, the changes in the amplitude of the jerk component were found to be present and the changes in the appearance of the waveform could be modelled without assuming any changes in the pendular component. Conclusions:The strong dependence of the amplitude of the jerk components of the waveforms on gaze angle implies that the neural integrator is involved in generating this waveform, because the neural integrator is responsible for producing the gaze angle dependent signal required to hold the eye in position. Our results give no information about the origin of the pendular component.

Keywords: nystagmus • eye movements: recording techniques