Ultrastructural pathological alterations in ocular muscles in a cat model with exotropia

To study the ultrastructural alterations of the ocular muscles in a cat model with exotropia. A total of 18 cats (4-6 weeks old) were included in the study and randomly divided into experimental group (n=12) and sham group (n=6). The experimental group underwent surgery to generate the exotropia model. 4 and 2 cats of each group were euthanatized respectively, at the end of 1^st month, 2^nd month, and 3^rd month after strabismus surgery, and the right medial rectus muscles were harvested, followed by staining and examination of ultrastructural changes under electron microscope. The proportion of irregularly arranged myofibrils and abnormal Z lines, and the number of mitochondria per unit area were recorded. In experimental group myofibril irregular arrangement percentage was shown to be markedly higher than that of the sham group (t=2.882, 6.085, 8.641, P<0.05). There was significant difference of abnormal rate of Z line in the medical rectus muscles between the experimental and sham group at the 2^nd and 3^rd month after surgery (t=8.230, 34.272, P<0.01). No significant difference of abnormal rate of Z line was observed between experimental and sham group at 1^st month after surgery, but the number of mitochondria per unit area was found to be significantly higher in experimental group than that of sham group at 1^st month after surgery (t=4.90; P<0.01). At 2^nd month and 3^rd month after surgery, the number of mitochondria tends to be lower in experimental group than that of sham group; however, statistically significant difference can be observed only at 3^rd month (t=-5.49; P<0.01). There was also significant positive correlation between the duration of strabismus and proportion of irregularly arranged myofibrils and proportion of abnormal Z lines (r=0.997 and r=0.987, respectively; P<0.05); and negative correlation between duration of strabismus and number of mitochondria (r=-0.953; P<0.05). High proportion of irregular arrangement of myofibrils and Z line in the medical rectus muscle were present in the ocular muscles of cat model with exotropia, the abnormal number of mitochondria per unit area was also noticed. In addition, the ultrastructural alterations tend to be enhanced with the extention of strabismus.