LOCALIZING VALUE OF OPTOKINETIC AFTERNYSTAGMUS

Previous reports have indicated that optokinetic afternystagmus (OKAN) becomes asymmetric after the occurrence of unilateral peripheral vest ibular lesions, and suggested that OKAN may be used for localizing the side of the lesion. These studies did not take into account spontaneo us nystagmus. We compared OKAN in 12 subjects with unilateral vestibul ar loss after resection of acoustic neuroma to OKAN in 30 normal subje cts. After offsetting the data for spontaneous nystagmus, we calculate d the initial amplitude, the time constant, and the slow-phase cumulat ive eye position (SCEP) parameters of OKAN. The directional asymmetry of parameters to rightward and leftward stimulation were also calculat ed. The mean SCEP, initial amplitude, and time constant parameters wer e reduced significantly in the patients, and each also showed a direct ional asymmetry, such that they were greater for stimulation toward th e side of the lesion. The directional preponderance of the SCEP parame ter had the highest sensitivity for the side of the lesion, being abno rmally elevated in 58.3% of patients with unilateral loss. We conclude that OKAN might be useful in combination with other subtests of a bat tery, but that by itself OKAN is only moderately sensitive to unilater al peripheral vestibular loss.