A rare case of benign isolated schwannoma in the inferior orbit.

A rare case of unilateral orbital schwannoma arising from the infraorbital nerve is presented. An excision biopsy with complete removal of the mass in the inferior orbit was performed. A definitive diagnosis was made on histopathological examination. The clinical and histological features of schwannoma are discussed. A need for early removal of such tumors is recommended to prevent complications.

Orbital tumors have always been an enigma and offer a diagnostic challenge to the ophthalmologist. Schwannomas of the orbit are rare and account for only 1-6% of all orbital tumors.1 In the orbit, schwannomas are usually unilateral and may arise from the supraorbital, infraorbital, supratrochlear, ciliary, oculomotor, trochlear or abducens nerves. The variable presentation and location of this tumor can certainly make the diagnosis difficult, which, usually is confirmed by histopathological examination. A rare case of solitary, benign schwannoma arising in the inferior orbit, diagnosed on histopathology is being described.

Case Report

A 35-year-old female presented with a painless, slowly growing mass in the right inferior orbit for the past four years. She did not complain of diminution of visual acuity, diplopia or field defect. On examination, visual acuity in both eyes was 20/20 unaided. The right side demonstrated a solid, non-tender, freely mobile mass in the inferior orbit, which became more prominent on applying pressure on the upper part of the globe. The mass measured approximately 20 mm × 15 mm and the size did not vary with Valsalva maneuver, ocular movements or posture. The mass was non-pulsatile and transillumination was negative.

There was no globe displacement or proptosis and ocular motility was full. The pupillary reactions, color vision and visual fields were normal. Supraorbital, infraorbital and corneal sensations were intact. The rest of the cranial nerves and neurological examination were also normal. Slit-lamp and fundus examination of the eye showed normal anterior and posterior segments. The left eye examination and general physical examination were unremarkable.

B-scan ultrasonography revealed an extraconal, homogenous lesion in the inferior part of the right orbit. Computed tomography revealed a non-enhancing well- defined homogenous mass measuring 20 mm × 12 mm in the right inferior orbit which was displacing the inferior rectus and inferior oblique muscle upwards. The mass was separate from the above-mentioned muscles and produced no globe indentation or displacement [Figure 1a].

Figure 1a

Preoperative CT scan showing the mass in the right inferior orbit

A provisional diagnosis of orbital dermoid was made and the patient was taken up for an excision biopsy. A right inferior orbitotomy was performed through an eyelid crease incision. Peroperative findings were of a smooth, well-encapsulated mass with no attachments to the surrounding muscles. The mass was attached to the infraorbital nerve. The mass was removed en bloc with intact capsule. Postoperatively, patient had an uneventful recovery.

On histopathological examination, the gross specimen consisted of a smooth well-encapsulated solid mass measuring 25 mm in the greatest diameter [Figure 1b]. Microscopically, a biphasic pattern of tumor cells was seen with areas of closely packed spindle cells having fusiform nuclei and eosinophilic cytoplasm (Antoni A) admixed with looser myxoid tissue having ovoid cells (Antoni B). At places, these tumors cells were arranged in palisades and this arrangement of cells is referred to as Verocay bodies. There was no evidence of malignant transformation. A final histological diagnosis of benign schwannoma was made [Figure 2].

Figure 1b

Gross appearance of the excised mass

Figure 2

Photomicrograph showing areas of Antoni A and Antoni B pattern, H/E, ×10

Discussion

Tumors of neurogenic origin are rare in the orbit. Orbital schwannoma is a tumor of adulthood usually presenting between 20 to 70 years of age. Schwannomas are usually asymptomatic when small and may produce progressive, painless proptosis on enlargement. A variable combination of signs and symptoms may be present due to the variable origin and location of the tumor in the orbit.2 Displacement of the globe is related to the site of the tumor mass. Most schwannomas arise from branches of either the supraorbital or supratrochlear nerves and hence produce downward displacement of the globe. Less commonly, the tumor may arise from the infraorbital nerve and produce upward displacement of globe. Larger tumors may produce diplopia, particularly when they arise from the orbital portion of the third, fourth or sixth nerve. Clinical presentation of lesions near the orbital apex may simulate retrobulbar neuritis.

Most schwannomas are progressively growing tumors that eventually require treatment. The growth of the tumor may cause compression of the optic nerve with papilledema or optic atrophy. Surgical excision is the treatment of choice and the tumor should be removed intact at the earliest to prevent compression of the optic nerve.3 Incomplete excision can lead to recurrence or even intracranial extension. Highly cellular tumors have greater chance of recurrence and malignant transformation. Therefore an early treatment is indicated to avoid the complications related to progressive growth of the tumor. In our patient, we were successful in achieving the above objectives with timely and complete excision of the tumor.

Orbital schwannoma arising from the infraorbital nerve is rare and clinical diagnosis is often difficult. Extensive literature search revealed only a few case reports of an orbital schwannoma arising from the infraorbital nerve.4-10 Our patient presented with the tumor in this uncommon location. A solitary schwannoma, though rare, should be considered as a preoperative differential diagnosis of a unilateral slow- growing orbital mass in an adult and prompt management is warranted to prevent development of vision-threatening complications.