Human ocular motor neural integrator failure

Abstract

Eye movements in the light and the dark were recorded from two young patients: the first aged five years had been diagnosed with cerebral palsy secondary to perinatal hypoxia, and the second aged six years had atypical congenital idiopathic nystagmus but was otherwise healthy. In the dark sustained head rotation elicited abnormal tonic eye deviations instead of vestibular nystagmus. In both patients, some quick phases were present but they were always followed by decaying exponential drifts back to the current level of tonic deviation with time-constants of 0.34 and 0.24 s. These waveforms were very similar to those recorded from animals who have had neurotoxins injected into the region of the nucleus prepositus hypoglossi and the medial vestibular nucleus, where the brainstem neural integrator is thought to reside. In the light, the first patient had gaze-paretic nystagmus, no optokinetic nystagmus (although quick phases were present), and virtually no smooth pursuit. In marked contrast, the second patient exhibited optokinetic nystagmus and pursuit around primary position; gaze-paretic nystagmus was not evident but saccades and quick phases of nystagmus exhibited dynamic overshoot and post-saccadic oscillations. In lateral gaze congenital nystagmus was evident. It is concluded that (a) severe loss of neural integrator function exists in man and is compatible with life; (b) it may only be detected in the dark; (c) it may not always be due to brainstem pathology.

Key Words:

• paediatric eye movements
• neural integrator
• saccades
• congenital nystagmus
• gaze-paretic nystagmus