Adaptations and deficits in the vestibulo-ocular reflex after sixth nerve palsy

The effects of paralytic strabismus on the vestibulo-ocular reflex (VOR) have not been systematically investigated in humans. The purpose of this study was to analyze the VOR in patients with unilateral peripheral sixth nerve palsy.

Twenty-one patients with unilateral peripheral sixth nerve palsy (6 severe, 7 moderate, 8 mild) and 15 normal subjects were studied. Subjects made sinusoidal +/-10 degrees head-on-body rotations in yaw and pitch at approximately 0.5 and 2 Hz, and in roll at approximately 0.5, 1, and 2 Hz. Eye movement recordings were obtained using magnetic scleral search coils in each eye in darkness and during monocular viewing in light. Static torsional VOR gains, defined as change in torsional eye position divided by change in head position during sustained head roll, were also measured.

In all patients, horizontal VOR gains in darkness were decreased in the paretic eye in both abduction and adduction, but remained normal in the nonparetic eye in both directions. In light, horizontal visually enhanced VOR (VVOR) gains were normal in both eyes in moderate and mild palsy. In severe palsy, horizontal VVOR gains remained low in the paretic eye during viewing with either eye, whereas those in the nonparetic eye were higher than normal when the paretic eye viewed. Vertical VOR and VVOR were normal, but dynamic and static torsional VOR and VVOR gains were reduced in both eyes in all patients.

In darkness, horizontal VOR gains were reduced during abduction of the paretic eye in all patients, as anticipated in sixth nerve palsy. Gains were also reduced during adduction of the paretic eye, suggesting that innervation to the medial rectus has changed. After severe palsy, vision did not increase abducting or adducting horizontal VVOR gains to normal in the paretic eye, but caused secondary increase in VVOR gains to values above unity in the nonparetic eye, when the paretic eye fixated. In mild and moderate palsy, vision enhanced the VOR in the paretic eye but caused no change in the nonparetic eye, suggesting a monocular readjustment of innervation selectively to the paretic eye. Vertical VOR and VVOR gains were normal, indicating that the lateral rectus did not have significant vertical actions through the excursions that we tested (+/-10 degrees ). Reduced torsional VOR gains in the paretic eye can be explained by the esotropia in sixth nerve palsy. Torsional VOR gain normally varies with vergence. We attribute the reduced torsional gains in the paretic eye to the mechanism that normally lowers it during convergence. The low torsional gains in the nonparetic eye may be an adaptation to reduce torsional disparity between the two eyes.