Nerve Sheath Myxoma Involving the Medial Rectus Muscle: A Case Report

A 63-year-old man reported a 2-year history of painless OS proptosis. The first MRI, performed in the institution of origin, resulted in a diagnosis of hemangioma versus orbital pseudotumor. Examination revealed proptosis and mild chemosis. Ancillary tests were normal, and oral prednisone was initiated without remarkable changes. Later, an incisional biopsy identified the lesion as an intramuscular myxoma of the left rectus, and a new MRI was ordered. It described a fusiform thickening of the medial rectus with and exophytic growth below the inferior rectus and upward toward the superior oblique. Due to the localization of the lesion, a combined transcaruncular and swinging eyelid approach with a lateral orbitotomy was performed. Histopathological examination revealed a proliferation of stromal origin composed of myxoid matrix with fusocellular and stellate-shaped cells. Cells were S-100, CD34, and CD56 positive and negative for epithelial membrane antigen, CD68, CD10, actin, and desmin. Results were consistent with a left medial rectus nerve sheath myxoma.