The Vestibulo-ocular Reflex In Fourth Nerve Palsy: Deficits And Adaptation

Abstract

Abstract: : Purpose: To systematically investigate the vestibulo-ocular reflex (VOR) in patients with fourth nerve palsy. Methods: Thirteen patients with unilateral fourth nerve palsy and 15 normal subjects were studied. Subjects made active sinusoidal 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 performed using magnetic scleral search coils in each eye in darkness and during monocular viewing in light. Results: Dynamic torsional VOR (in darkness) and visually enhanced VOR (VVOR) gains of the paretic eye were decreased during incyclotorsion and excyclotorsion, whereas gains in the non-paretic eye were normal. Vertical VOR gains of the paretic eye in darkness were decreased during elevation and depression; they were normal in the non-paretic eye. In light, vertical VVOR gains were normal in both eyes. Horizontal VOR gains of the paretic eye in darkness were decreased during abduction and adduction, whereas horizontal gains in the non-paretic eye were normal. In light, horizontal VVOR gains were normal in both eyes. Conclusion: During head rotation in darkness, VOR gains are reduced during incyclotorsion, depression and abduction of the paretic eye, as anticipated from paresis of the superior oblique muscle. VOR gains during excyclotorsion, elevation and adduction of the paretic eye are also reduced, whereas VOR gains in the non-paretic eye remain normal, indicating a selective central adjustment of innervation to the paretic eye. In light, torsional VVOR gains in the paretic eye remained reduced. Visual input increases vertical and horizontal VVOR gains to normal in the paretic eye, without a conjugate increase in VVOR gains in the non-paretic eye, providing further evidence of selective adaptation in the paretic eye. Motions of the eyes after fourth nerve palsy exemplifies monocular adaptation of the VOR in three dimensions, in response to peripheral neuromuscular deficits.

Keywords: 618 vestibulo-ocular reflex • 495 ocular motor control • 588 strabismus