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Linda McLoon, Joost Felius, David Stager; A Morphometric Analysis of the Inferior Oblique Muscle in Patients with Inferior Oblique Overaction. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1303. doi: https://doi.org/.
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Over-elevation in adduction has traditionally been termed “inferior oblique overaction.” This diagnostic term implies hyper-contractility of the muscle as the etiologic mechanism for the motility disturbance. However, little is known about the muscle anatomy among the various diagnoses associated with the overaction. Here, inferior oblique muscle histology is compared across two diagnostic groups and between those who have or did not have prior surgery.
Inferior oblique muscle specimens were collected from 18 patients with primary inferior oblique overaction (IOOA) with or without prior inferior oblique surgery, 7 with craniofacial dysostosis (CFD) (all scheduled for nasal myectomy), and 9 control subjects without strabismus. Total myofiber number, mean cross-sectional area and the percent of myofibers with central myonuclei were determined. Nerve density was determined for all subject groups.
Clear differences were observed between patient and control muscles, and amongst the two patient populations. Total myofiber number in normal muscles was 12,776 ± 784 but larger in patients with inferior oblique overaction and significantly lower in patients with craniofacial dysostosis. Mean myofiber area was significantly smaller in the orbital layers from all patient cohorts, averaging between 23 and 34% smaller for the IOOA groups and 57% smaller for the CFD group. The global layer fibers of CFD muscles were 28% smaller than controls. In the IOOA patients with no prior surgery, nerve density was increased 6-fold over control muscles. Interestingly, prior surgery reduced this although not to normal levels. CFD muscles did not show increased innervation compared to controls.
Despite a similar strabismus pattern, inferior oblique muscles from patients with inferior oblique overaction and craniofacial dysostosis presented with very different histological findings. There were significant differences in total myofiber counts and fiber areas, the number of centrally nucleated myofibers, as well as nerve density. Prior surgery modulated innervation towards normal levels. These adaptations of the inferior oblique in different patient populations compared to each other and to controls help to explain the difficulties in patient diagnosis and treatment as well as suggest new treatment options.
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