Localizing value of optokinetic afternystagmus

Previous reports have indicated that optokinetic afternystagmus (OKAN) becomes asymmetric after the occurrence of unilateral peripheral vestibular lesions, and suggested that OKAN may be used for localizing the side of the lesion. These studies did not take into account spontaneous nystagmus. We compared OKAN in 12 subjects with unilateral vestibular loss after resection of acoustic neuroma to OKAN in 30 normal subjects. After offsetting the data for spontaneous nystagmus, we calculated the initial amplitude, the time constant, and the slow-phase cumulative eye position (SCEP) parameters of OKAN. The directional asymmetry of parameters to rightward and leftward stimulation were also calculated. The mean SCEP, initial amplitude, and time constant parameters were reduced significantly in the patients, and each also showed a directional asymmetry, such that they were greater for stimulation toward the side of the lesion. The directional preponderance of the SCEP parameter had the highest sensitivity for the side of the lesion, being abnormally elevated in 58.3% of patients with unilateral loss. We conclude that OKAN might be useful in combination with other subtests of abattery, but that by itself OKAN is only moderately sensitive to unilateral peripheral vestibular loss.