Structural alterations in overacting inferior oblique muscles.

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.