Slow Volitional Saccades and Poor Fast Phases to an Optokinetic Stimulus, with Preserved Head Impulse Testing

This is a 67-year-old woman presenting with imbalance and binocular horizontal diplopia at near. On examination there were frequent square wave jerks, limited supraduction OU and convergence insufficiency, which explained her diplopia. Pursuit and suppression of the vestibulo-ocular reflex were saccadic. There was gaze-evoked nystagmus bilaterally, with rebound nystagmus. There were long tract signs, bradykinesia and gait unsteadiness without tremor. Saccades were slow (and slightly hypometric) in all directions. With an optokinetic flag, slow phases (pursuit) are seen in the same direction as the flag while the fast phases (saccades) were either very weak or absent entirely. With the vestibulo-ocular reflex (VOR), the range of supraduction improved. With head impulse testing (HIT, assesses the high frequency VOR, seen in the video), eye movements were of normal speed horizontally and vertically, although only the horizontal HIT is shown in the video. If saccades are slow because of nuclear (e.g., 3rd or 6th nucleus) or infranuclear (e.g., 3rd or 6th fascicle or nerve) pathology, the eye movements will be equally slow whether they are initiated volitionally, with an optokinetic flag, or with the VOR/HIT. Therefore, the burst cells which initiate saccades (paramedian pontine reticular formation, PPRF for horizontal saccades; rostral interstitial medial longitudinal fasciculus, riMLF for vertical saccades) were thought to be involved. Motility should also be affected when nuclear/infranuclear processes cause slow saccades. In this case, supraduction was affected, but this was demonstrated to be ‘supranuclear' in origin since the vertical VOR could overcome this deficit. Since there were also signs suggestive of cerebellar (or its connections) pathology including gaze-evoked and rebound nystagmus, and signs suggestive of basal ganglia pathology including mild parkinsonism, convergence insufficiency. There were also long tract signs; therefore, diffuse or multifocal central processes such as spinocerebellar ataxia, multiple system atrophy, or a PSP variant were all considered. The impaired smooth pursuit and square wave jerks could localize to either the posterior fossa or to the basal ganglia.