The linear vestibulo-ocular reflex in patients with skew deviation.

PURPOSE

The linear vestibulo-ocular reflex (LVOR) is mediated primarily by the otolith organs in the inner ear. Skew deviation is a vertical strabismus believed to be caused by imbalance of otolithic projections to ocular motor neurons (disynaptically through the medial longitudinal fasciculus in the brain stem or polysynaptically through the cerebellum). The authors postulated that if skew deviation is indeed caused by damage to these projections, patients with skew deviation would show abnormal LVOR responses.

METHODS

Six patients with skew deviation caused by brain stem or cerebellar lesions and 10 healthy subjects were recruited. All subjects underwent brief, sudden, interaural translations of the head (head heaves) using a head-sled device at an average peak acceleration of 0.42g (range, 0.1-1.1g) while continuously viewing an earth-fixed target monocularly at 15 and 20 cm. LVOR sensitivity (peak rotational eye velocity to peak linear head velocity) and velocity gain (peak actual-to-ideal rotational eye velocities) were calculated for the responses within the first 100 ms after onset of head movements.

RESULTS

LVOR sensitivities and velocity gains in patients were decreased by 56% to 62% in both eyes compared with healthy subjects. This binocular reduction in LVOR responses was asymmetric--the magnitude of reduction differed between eyes by 37% to 143% for sensitivities and by 36% to 94% for velocity gains. There were no differences in response between right and left heaves.

CONCLUSIONS

The binocular, asymmetric reduction in LVOR sensitivity and velocity gain provides support that imbalance in the otolith-ocular pathway is a mechanism of skew deviation.