Monocular and Binocular Testing to Improve the Diagnostic Value of Horizontal Optokinetic Nystagmus

This report describes the effects of unilateral peripheral vestibular lesions on the binocular and monocular optokinetic nystagmus (OKN). Six subjects who had undergone surgical removal of an acoustic neuroma were compared to 14 normal subjects. Stimuli consisted of 15° vertical stripes projected onto a flat screen 1 m from the subject. Stimulus velocities were 15, 30, 45, and 60°/sec. Before surgery, contralateral gain of the slow phase velocity was low, approximately 0.6, compared to 0.9 for ipsilateral gain at 60°/sec. In the acute postoperative period (<30 days), the gains of horizontal OKN were low and symmetrical for both stimulus directions. In the intermediate period (30-60 days postoperative), the OKN was asymmetrical (contralateral gain lower than ipsilateral gain), with the greatest discrepancy at 60°/sec. During the chronic phase (>2 months) the OKN was still asymmetrical in 3 of the 6 subjects. These data suggest that monocular and binocular optokinetic testing can be used to monitor the vestibular compensation of subjects following vestibular nerve section.