Correction: Intramuscular Orbital Schwannoma: Case Report and Review of the Literature

Hammam Alotaibi^1*Firas Madani²Rawan N Althaqib³Hamad Alsulaiman^3*

• ¹Prince Sultan Military Medical City, Riyadh, Saudi Arabia
• ²King Abdulaziz University, Jeddah, Saudi Arabia
• ³King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia

Introduction:Schwannomas, as the name suggests, are benign tumors that originate from Schwann cells in the peripheral nerves. They are rare findings, accounting for less than 1% of all orbital tumors. Given their slow growth, they are observed in patients between the ages of 20 and 70 years and rarely before. Pinpointing the origin of schwannoma is difficult, given the complex nature of orbital structures. However, they most commonly originate from branches of oculomotor, trochlear, trigeminal, and abducens nerves (1,2).When small, these tumors are asymptomatic and do not cause any noticeable effect on globe position, extraocular motion, or nerve function. However, as they grow, such symptoms develop and progressively worsen over time, mainly when located along the orbital apex (3).Diagnosis of orbital schwannoma is made through imaging techniques such as Computerized tomography (CT) scans and magnetic resonance imaging (MRI) scans with or without contrast. However, the definitive diagnostic tests are biopsy (incisional vs. excisional) and immunohistochemistry.The principal value of a CT scan is to identify the location/extension of the lesion, whether contained entirely in the orbit or causing bony remodeling/erosion and affecting structures beyond the orbit into the sinuses, for example, and by doing so, assists in differentiating lesions with clinically similar presentation (4). MRI is the method of choice in diagnosing schwannomas, given its high sensitivity. The lesion typically has low intensity on T-1 weighted images and high intensity on T-2 weighted images (5).Surgical excision is curative and has a low recurrence rate. On histological examination, schwannoma will have one of two characteristic cellular morphological patterns denoted Antoni type-a and Antoni type-b. Immunohistochemistry shows a sensitivity of schwannomas to S-100 and vimentin stains, which are proteins prevalent in Schwann cells (6).Herein, we describe a case of an 8-year-old with proptosis due to an orbital schwannoma found within an unusual structure, the inferior rectus muscle.Case description: An 8-year-old boy presented to the emergency department complaining of headache, progressive proptosis, and intermittent hyperdeviation of the right eye for one year. Upon presentation, his visual acuity was 20/20; there was no headache nor any signs of elevated intracranial pressure, such as nausea and vomiting. Examination revealed hypertropia and proptosis of the right eye with limited extraocular movement of -1 in supraduction and -2 in infraduction and a 2 mm difference between eyes on exophthalmometer. There was no afferent pupillary defect and no visual field defect on confrontation test. Examination of the right eye shows a grossly obvious elevation of the right lower lid. Upon palpation, a firm mass too deep for size assessment was noted. However, it was non-tender and did not bruit on auscultation, as seen in (Figure 1).The anterior and posterior segment examinations were unremarkable, showing quiet phakic eyes with clear corneas, a deep anterior chamber, a flat retina with good foveal reflex, and normal, healthy discs in both eyes.Diagnostic assessment:A computerized tomography scan (CT scan) is indicated for this case due to a suspicious area of bone erosion. Moreover, a biopsy was scheduled along with the CT scan.The CT scan shows a solid-appearing inferior orbital mass lesion that is seen at the anatomical location of the right inferior rectus muscle belly, causing significant scalloping and thinning of the adjacent part of the right inferior orbital wall. The muscle cannot be separated from the adjacent muscle along its insertion. No other extraocular muscle group involvement of the same pattern. No fat density. As seen in (Figure 2A).Magnetic resonance imaging (MRI) was ordered for the patient. An intraconal mass with a spindle shape configuration was noted with a well-circumscribed outline and measuring 37.5 x 20.4 mm. The central part of the mass shows a pericystic component. The lesion is in line with the inferior rectus muscle and cannot be separated from the adjacent part of the muscle. Other extraocular muscles and orbital structures, including the optic nerve, were clear of the lesion. there was no sinus extension (Figure 2B, 2C).Upon excision, complete resection was achieved by blunt dissection, preserving the muscle and removing the mass whole. The orbital mass consisted of round, spongy, whitish-tanned tissues. Upon dissection, the cut surface was yellowing white and smooth at the periphery and had central hemorrhagic cavities.Histologically, the section contained a soft tissue tumor surrounded by a Perineural fibrous capsule. The tumor consists of spindle cell tumor palisading forming Antoni A and Antoni B arranged with a pattern with numerous Verocay bodies. Findings were consistent with right orbital schwannoma (Figure 3). No genetic testing was done for this patient given that it is not available in our institution.At last visit, 2 years following the surgery, the patient was stable, had quiet eyes, minimal scarring of the palpebral conjunctiva of the right lower lid, clear cornea, deep anterior chamber and normal healthy disc. The patient had no diplopia in primary gaze and lower gaze but complained of minimal diplopia in upper gaze. An MRI of the orbit following the surgical resection (Figure 2D, E) and at last follow up two years later (Figure 2F) demonstrating no recurrence and return of normal orbital anatomy.