Abstract
Purpose: :
To investigate vertical misalignment following 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 immediately patched for 6 (M1) and 9 (M2) days to prevent binocular viewing (BEV). Eye movements were recorded pre and post–lesion during straight ahead or eccentric fixation (±20°, horizontal and/or vertical) with the head upright or tilted (±30º).
Results: :
Monkeys developed a vertical deviation (VD) that increased with down gaze and adduction. Post–lesion, the VD peaked in 2 to 3 days, 5.7° in straight and 9.3° in adduction/down gaze for M1, and 9.4° and 12.8° for M2. Then, while still patched, the VD decreased by ∼30% for both animals. Once BEV was allowed the VD increased gradually over days for both monkeys; the maximum VD was 5.2° in straight and 11.0° in adduction/down gaze for M1, and 14.4° and 17.5° for M2. Neither monkey could overcome the VD with both eyes viewing. To assess comitancy, a gradient (Δ VD/ Δ eye position) was calculated (larger gradient = less comitant deviation). Post–lesion, before BEV, the max vertical gradient (VG) along the midline was 0.11 for M1 and 0.16 for M2. The max horizontal gradient (HG) along the horizontal meridian was 0.16 for M1 and 0.17 for M2. After allowing BEV the VG gradually increased to 0.2 for M1 and 0.26 for M2; but was least in adduction (0.06 for M1 and 0.2 for M2). The HG first decreased to 0.03 for both monkeys, but then increased to a max of 0.21 for M1 and 0.17 for M2. The HG was least in down gaze. For head tilt, post–lesion while still patched, the difference in VD in straight ahead gaze between ±30º tilt (ΔVDtilt) was 0.8° for M1 and 6.3° for M2. Over the next 6 weeks, ΔVDtilt increased to a maximum of 3.9° for M1 and 7.6° for M2.
Conclusions: :
Rhesus monkeys with induced IV nerve palsy show changes in vertical alignment similar to human acquired IV nerve palsy. Striking, also, were intersubject variability and changes over time in both the amount and comitancy of the misalignment. Some changes occurred before binocular viewing, suggesting a proprioceptive effect. Other changes occurred later, raising questions of mechanical vs. central adaptive responses.
Keywords: strabismus • eye movements • ocular motor control