Double-bellied inferior rectus muscles with aplasia of medial rectus muscles

This report describes a rare occurrence of double-bellied inferior rectus muscle with aplasia of the medial rectus muscle in both eyes of a young woman seeking cosmetic correction for her strabismus. The patient presented with a large-angle exotropia and a -4 limitation of adduction in each eye. On attempted adduction, there was minimal downshoot and upper eyelid retraction. Magnetic resonance imaging of the orbit revealed hypoplastic medial rectus muscles; however, intraoperatively the muscle was not found at the insertion or when traced back as far as possible. The inferior rectus muscles were noted to have double bellies converging close to each other at the insertion. The patient underwent a bilateral lateral rectus recession with nasal transposition of the inferior rectus muscles and was cosmetically well aligned, with a residual exotropia of 4^Δ at 3 months' follow-up. This unusual anatomical variation may be the result of deviation of muscle paths.