Expression of Myosin Heavy Chain Isoforms in the Inferior Oblique Muscles in Patients with Inferior Oblique Overaction

Abstract

Abstract: : Purpose: Inferior oblique overaction (IOOA), an elevation of an eye as it moves toward adduction, is a common oculomotor disease. Primary IOOA in which there is no evidence for a past or present ipsilateral superior oblique paralysis or paresis is difficult to explain. Secondary IOOA is caused by paresis or paralysis of the ipsilateral superior oblique muscle or by paresis or paralysis of the contralateral superior rectus muscle. Secondary IOOA is different from primary IOOA in that the vertical deviation is large in the primary position with the accompanied torsional deviation. Although clinical distinction between primary and secondary IOOA is not so difficult, the explanations given for apparent primary IOOA in the old literature are vague. In this study, the expression of myosin heavy chain (MyHC) isoforms in inferior oblique muscle (IO) were investigated in primary and secondary IOOA, and the distribution of MyHC isoforms in IOOA is compared with that in normal control so as to further understand the pathogenesis of IOOA. Methods: The IOs of the patients(4 years of age) who were diagnosed as primary or secondary IOOA, were obtained through a weakening procedure (myectomy) under general anesthesia. Additional IOs, as normal control, were obtained from two 4–year–old patients (2 eyes) who underwent enucleation due to retinoblastoma. Small amount of the skeletal muscle, rectus femoris, of a preschool child with femur fracture was obtained during the surgery. Immunohistochemical assay, confocal microscopy, gel electrophoretic analysis and western blotting analysis were performed. Results were described as a mean of eight independent experiments with the specimen. The statistical analysis was done with Kruskal–Wallis test. Results: The most abundant fast isoform had identical staining pattern in OL and in GL. The expression of fast MyHC isoform was decreased significantly in primary IOOA. Slow MyHC isoform was scattered through both layers, but it was detected predominantly in the fibers of OL. There was increasing tendency in expression of the slow MyHC isoform among normal, primary and secondary IOOA. Developmental isoform was detected only in the fibers of OL. The expression of developmental MyHC isoform was increased significantly in secondary IOOA. Conclusions: It was shown that there is a significant difference in the expression of MyHC isoforms among different IOOAs.

Keywords: extraocular muscles: structure • strabismus: etiology