Abstract
Purpose: :
To investigate static and dynamic torsion after induced IV nerve palsy in rhesus monkeys.
Methods: :
The IV nerve was sectioned intracranially in 2 rhesus monkeys (M1 and M2). Post–lesion the paretic eye (PE) was patched for 6 (M1) and 9 (M2) days before binocular viewing (BEV) was allowed. 3D eye movements were recorded pre and post–lesion during fixation (±20° horizontal and/or vertical) and vertical saccades. Dynamic or ‘blip’ torsion during vertical saccades was calculated from the amount of torsion above that expected from the change in static torsion during fixation before and after the saccade.
Results: :
Following surgery, monkeys developed extorsion of the PE that increased with down gaze and abduction. Post–lesion, but before BEV was allowed, the PE, viewing at straight ahead, was extorted by 5.8° in M1 and 5.5° in M2. With down gaze and abduction, extorsion was 15.7º (M1) and 15.3° (M2). Once BEV was allowed, the extorsion of the PE for M1 changed little but for M2 increased to ∼9º. Post–lesion, during down vertical saccades (10º to –10º), both monkeys developed increased extorsional blips in the PE, the change was largest in adduction (M1, 1º; M2, 0.4º). During upward saccades (–10 º to 10 º) both monkeys also developed an increased extorsional blip in the PE, but the change was largest in abduction (M1, 2.1º; M2, 1.6º). There were oppositely–directed changes in the torsional drift (first 80ms) following vertical saccades. Both monkeys also showed a temporal displacement (∼25–30º) of Listing’s primary position for the PE eye, but no change for the NE.
Conclusions: :
Rhesus monkeys with induced IV nerve palsy showed changes in ocular torsion similar to human acquired IV nerve palsy. The changes in torsion, however, were not tightly correlated with changes in vertical phoria. There were also changes in ‘blip’ torsion during vertical saccades and in torsional drift following vertical saccades that were direction and orbital position dependent. The orientation of Listing’s plane was also changed, consistent with the important role of the SO muscle in determining Listing’s plane.
Keywords: eye movements: saccades and pursuits • strabismus • ocular motor control