To report a case of granular cell tumor (GCT), that developed in superior rectus (SR) muscle.

A 30-year old woman presented with complaint of diplopia since three months ago. History was unremarkable for medical, ophthalmic, surgical or familial diseases. In primary position, 10 prism diopters of right hypertropia was observed, but motility exams, including elevation of right eye, were normal. External exam revealed 2 millimeters of right proptosis. Refraction showed +2.00 diopters hyperopia OD and plano OS. Best corrected visual acuity was 20/20 OU. In funduscopic exam, right eye choroidal folds and normal left side fundus were observed. Orbital MRI demonstrated a fusiform mass, inseparable from SR muscle. It was hypointense to extraocular muscles on T1W and T2W. The mass extended into the orbital apex and induced mass effect on the right globe. Post contrast images demonstrated significant tumor enhancement. Differential diagnoses included lymphoma, peripheral nerve sheath tumors, metastatic disease, or an inflammatory etiology such as orbital pseudotumor. Incisional biopsy was performed through superior lid crease incision.

Histopathologic analysis demonstrated cellular elements having hyperchromatic nuclei with abundant PAS-positive cytoplasm. The neoplastic cells extended between surrounding muscle fibers. Immunohistochemistry staining was positive for S-100, but negative for Desmin and Myogenin, compatible with diagnosis of GCT. The tumor was resected via a cranial approach. Intraoperatively, the mass was successfully separated from SR muscle. Proptosis resolved after surgery and MRI showed no signs of presence of GCT. After one year, the vision was restored to 20/20 without correction and there was no diplopia.

GCT should be considered in the differential diagnosis of well-demarcated solitary slow-growing orbital tumors. Its radiologic appearance, being hypointense on T2W MRI images, is helpful in differentiation from most other orbital masses, which are typically hyperintense on T2W MRI sequence.