Abstract
Purpose: :
While cyclovertical strabismus in humans is frequently attributed to SO palsy, anatomic effects of SO denervation are largely unstudied. Magnetic resonance imaging (MRI) in human SO palsy suggests a variable SO muscle atrophy, and loss of contractility. We employed MRI and whole orbital histology to study effects of acute trochlear (CN4) denervation on the monkey SO.
Methods: :
Adult monkeys M1 and M2 had undergone detailed ocular motor evaluations before and after unilateral 10 mm intracranial trochlear neurectomy, and inferior oblique extirpation (Shan et al, Tian et al, IOVS 2007; 48: 2602 - 2625). Animals were perfused with formalin 12-15 mos after denervation. Denervated and fellow orbits were imaged by MRI, embedded whole in paraffin, serially sectioned at 10 micron thickness, and stained with Masson trichrome. We quantitatively compared whole muscle and individual fiber cross sections in denervated and fellow SO muscles throughout the orbit. Controlling for shrinkage, data were also normalized by whole orbital cross section. Eye position was computed from optic nerve position relative to the orbital centroid.
Results: :
Both animals had exhibited ipsilateral hypertropia consistent with SO palsy in vivo, but M1’s palsied eye was 20 deg hypotropic postmortem. MRI demonstrated reduced cross section of the denervated SO in mid-orbit. Denervated CN4 and its branches were fibrotic, but some small, unmyelinated nerves persisted in denervated SO muscles. In M1 at mid-orbit 2 mm posterior to the globe-optic nerve junction, there was 60% reduction in overall denervated SO cross section. However, due to paradoxical anterior shift of denervated SO cross-section in infraduction, total denervated volume was similar to the fellow. While orbital layer (OL) fiber cross sections were identical in denervated and control SO muscles, denervated global layer (GL) fibers had only 20% of control cross section. Scattered GL fibers near small, unmyelinated nerves were relatively preserved, and in M2 there were scattered clusters of markedly hypertrophic GL fibers.
Conclusions: :
CN4 denervation produces SO GL but not OL atrophy, with overall mid orbital SO atrophy evident by MRI. GL fibers near small, presumably autonomic nerves may be spared. Occasional GL fiber hypertrophy might reflect sparse reinnervation. OL fibers may constitutively resist denervation atrophy.
Keywords: extraocular muscles: structure • strabismus: etiology • eye movements