Primary Orbital Conjunctival Cyst Associated With the Inferior Rectus Muscle

Primary simple conjunctival cysts are congenital, non-neoplastic, fluid-filled lesions lined by non-keratinizing squamous epithelium without dermal appendages. They are usually located in the anterior superonasal or superotemporal orbit and are typically managed with complete surgical excision. A 69-year-old woman presented with a 10-month history of diplopia and a 2-week history of right lower eyelid swelling. There was no history of any ocular trauma or surgery. Slit-lamp examination of the right eye revealed a greyish subconjunctival cyst located inferiorly. A Hess chart showed a limitation of infraduction in the right eye, and the field of binocular single vision presented diplopia in the lower and right-lower directions. Magnetic resonance imaging demonstrated a non-enhancing homogenous lesion above the inferior oblique muscle. The course of the inferior rectus muscle was unclear. Complete surgical excision was performed via the swinging eyelid approach. The tumor was firmly attached to the inferior rectus muscle intraoperatively. Pathological examinations of the excised tumor showed a unilocular cyst lined with non-keratinizing squamous epithelium, and it was devoid of goblet cells, dermal appendages, and muscle fibers. On day one post-surgery, full and equal extraocular movements were observed, with no complaints of diplopia. At the one-year follow-up, the Hess chart confirmed improvement of the limited infraduction, and no tumor recurrence was noted.