Unexpected extraocular muscle hypoplasia during strabismus surgery: case series

Reports of congenital isolated medial rectus muscle abnormalities are relatively uncommon and are seldom seen. According to our clinical experience, some rare cases of abnormalities could not be detected by clinical examination and imaging before surgical treatment, which brought difficulties to diagnosis and surgery.

In order to provide clinical guidance, we summarized 4 cases with congenital hypoplasia of the medial rectus muscle in our hospital recently. All the patients exhibited exotropia in the primary position. Only one patient (25.0%) exhibited clinically significant limitations of ocular movements. All the patients were identified with congenital hypoplasia of the medial rectus muscle during strabismus surgery; one patient also had hypoplasia of the lateral rectus muscle. However, abnormalities of the rectus muscles were not identified by MRI in three patients (75.0%). In terms of treatment, we enhanced the surgery amount in three patients. Good correction of exotropia was achieved in all patients.

Congenital hypoplasia of the medial rectus muscle is extremely rare and some cases are difficult to be detected by clinical examination or imaging. Surgeons should be aware of this condition and should actively but cautiously adjust the surgical parameters based on the patients' intraoperative status.