Three dimensional disorders of gaze and binocular alignment after brainstem and ocular motor nerve lesions

grantor: University of TorontoThe effects of paralytic strabismus and neural lesions on ocular motor systems were investigated. Patients with sixth, fourth, and third nerve palsy, as well as skew deviation caused by brainstem or cerebellar lesions were studied. The vestibular-ocular reflex (VOR) and saccades in three axes of rotation were investigated using the magnetic search coil technique. The effects of paralytic strabismus and neural lesions on Listing's and Donders' laws, which specify torsional eye position, were also studied. Monocular adaptive changes in the VOR were demonstrated; in sixth nerve palsy, horizontal VOR gains are not only decreased during abduction, but also during adduction of the paretic eye, without a conjugate decrease in gains of the non-paretic eye. Similarly, in fourth nerve palsy, VOR gains not only reduced during incyclotorsion, depression, and abduction, but also during ' excyclotorsion, elevation and adduction' of the paretic eye, while VOR gains in the non paretic eye remain normal. In third nerve palsy, VOR gains are not only decreased during adduction and excyclotorsion, but also during 'abduction' and 'incyclotorsion' of the paretic eye, without a conjugate decrease in gains of the non-paretic eye. This monocular adaptation is attributed to retinal slip differences between the two eyes. Monocular adaption was also identified in the saccadic system. In mild and moderate sixth nerve palsy, saccade peak velocities in the paretic hemifield of motion are normal in the paretic eye, without a conjugate increase in peak velocities in the non-paretic eye. This selective adaptation is also visually driven; it allows both eyes to reach a target in the paretic hemifield of motion rapidly and simultaneously. During saccades and fixation, 'acute' peripheral sixth and fourth nerve palsy violates Listing's law, whereas 'chronic' palsy obeys it, indicating that neural adaptation can restore Listing's law even when the eye muscle remains abnormal. Patients with skew deviation caused by brainstem or cerebellar lesions have abnormal ocular torsion in both eyes, providing evidence that elements of the neural pathway that enforce Listing's law traverse the regions of the brainstem and cerebellum that are damaged in skew deviation.Ph.D