Structural alterations in overacting inferior oblique muscles
R. F. Spencer and K. W. McNeer
Primary and secondary overacting inferior oblique muscles were examined by
light and electron microscopy and compared to normal inferior oblique
muscles. The most conspicuous light microscopic difference between primary
overacting and normal muscles was the presence of granular fibers in
overacting muscles that contained sarcoplasmic masses. Some primary and
secondary overacting muscles also contained fibers displaying increased
vacuolization and atrophy. Other secondary overacting muscles showed only
numerous fibers at various stages of atrophy. The affected fibers were
distributed throughout the central and global regions of the posterior
portion of the muscle. Electron microscopic examination showed aggregations
of mitochondria and degenerating mitochondrial profiles. Longitudinal fiber
splitting and activated satellite cells were associated with fibers
displaying increased vacuolization. The results suggest that the primary
overacting inferior oblique muscle is the result of a bilateral paresis of
the superior oblique muscle.
