Acute superior oblique palsy in monkeys: I. Changes in static eye alignment.

PURPOSE To investigate immediate and long-term changes in static ocular alignment with acute acquired superior oblique palsy (SOP) in monkeys. METHODS The trochlear nerve was severed intracranially in two rhesus monkeys. After the surgery, the paretic eye was patched for 6 to 9 days, and then binocular viewing was allowed. Three-axis eye movements (horizontal, vertical, and torsional) were measured with binocular, dual search coils. Eye movements were recorded over a +/-20 degrees horizontal and vertical range of fixations before the lesion and then, beginning the first day after surgery. Changes in alignment with +/-30 degrees head tilt were also studied. RESULTS The main findings were (1) misalignment (10-12 degrees vertical in adduction, down; 10-12 degrees torsional in abduction, down); (2) changes in vertical deviation (VD) with head tilt (Delta 2-6 degrees with left versus right 30 degrees tilt); and (3) changes in comitance and VD over time. During the early postlesion period, before binocular viewing was allowed, VD decreased and comitance improved. Once binocular viewing was allowed, VD increased and comitance worsened. CONCLUSIONS Rhesus monkeys with induced SOP show a characteristic pattern of misalignment that helps define the ocular motor signature of acute denervation of the superior oblique muscle. The animals also showed striking changes over time in the amount and comitance of the vertical misalignment that depended on whether viewing was monocular or binocular, suggesting a role for proprioception in adaptation to misalignment with habitual monocular viewing.