Abstract
Purpose: :
To investigate the effects of congenital, peripheral and central fourth nerve palsies on vertical saccades.
Methods: :
Seven patients with unilateral congenital fourth nerve palsy, one with unilateral chronic peripheral fourth nerve palsy caused by a skull base clivus chordoma, and one with unilateral central palsy caused by a lesion in the midbrain were studied. Ten normal subjects served as controls. All patients underwent MR imaging. Palsy under 1 month in duration was designated as acute and those of longer duration were chronic. Subjects made 20 deg vertical saccades while wearing search coils. The vertical saccades were elicited from three different horizontal positions (left 20°, midline, and right 20°). The patient with acute central palsy was retested after 3 and 6 months. Amplitude gains, peak velocities and durations of vertical saccades were analyzed.
Results: :
Centrifugal downward saccadic amplitude gains, velocities and durations in the paretic eye were in the normal range (for the tested range of excursion) for patients with congenital and peripheral chronic palsy. In the patient with acute central palsy, downward saccadic amplitude gains and velocities were reduced, and durations were prolonged. When this patient was tested again in the chronic stage, peak velocities and amplitude gains remained reduced, and durations remained prolonged.
Conclusions: :
For acute central palsy, saccade amplitude gains and peak velocities are reduced and their durations are prolonged in the field of action of the palsied muscle. Saccadic velocity remains reduced in chronic central palsy, consistent with limited regeneration within the brain. Saccade velocities are restored in chronic peripheral palsies, probably by remyelination and axonal regeneration. There may also be central monocular adaptation of innervation selectively to the paretic eye, in order to match the saccade dynamics of the two eyes when moving into the paretic field of motion.
Keywords: eye movements: saccades and pursuits • strabismus • adaptation: motion