Changes in the human vestibulo-ocular reflex after loss of peripheral sensitivity

Quantitative rotational testing was used to study changes in the vestibulo-ocular reflex of patients with unilateral and bilateral peripheral vestibular lesions. Compared with normal subjects, the patients exhibited a characteristic pattern of decreased gain and increased phase lead at low frequencies of sinusoidal stimulation and decreased time constants on impulsive stimulation. By contrast, gain and phase measurements on high-frequency-low-amplitude sinusoidal stimulation were often normal. In the patients with bilateral lesions, the results of caloric testing correlated with the results of low-frequency rotational testing but not with the results of high-frequency testing. There are two main clinical implications of these findings. First, patients with absent response to caloric stimulation (unilateral or bilateral) may have a normal response to high-frequency sinusoidal rotation (i.e., the frequencies that constitute most natural head movements). This probably explains why such patients do not report oscillopsia. Second, low-frequency sinusoidal rotational testing and caloric testing are more sensitive than high-frequency sinusoidal or impulsive rotational testing for detecting early loss of vestibular sensitivity due, for example, to ototoxic drugs.