AIM: To find out the relationship between the pathological changes of medial rectus muscles and the occurrence of concomitant exotropia.

METHODS: Thirty cases of concomitant exotropia patients were collected to conduct the plastic surgery for strabismus correction. Medial rectus amputated intraoperative were fixed with 4% formaldehyde and then sent to histologic examination. The organizational structure of extraocular muscles was observed, Hematoxylin-eosin(HE)and Masson staining were conducted under microscope.

RESULTS: In the 30 cases, medial rectus muscle fiber presented different sizes, abnormal morphology, number reducing, and the muscle showed varying degrees of fibrosis. After Masson staining, it was found that extraocular muscle fiber content of longer strabismus patients were significantly shorter than that of shorter strabismus patients(P<0.05).

CONCLUSION: The occurrence of concomitant exotropia is related with the abnormal structure of extraocular muscles,with higher extent of fibrosis of extra-ocular muscle in the long period of constant concomitant exotropia than in the late-onset concomitant exotropia. That means the fibrosis of medial rectus probably is the reason of concomitant exotropia. The extent of fiber degeneration change of concomitant exotropia increased with the occurrence of it.