Neurology India Medknow Publications on behalf of the Neurological Society of India ISSN: 0028-3886 EISSN: 1998-4022 Vol. 59, Num. 3, 2011, pp. 471-473

Neurology India, Vol. 59, No. 3, May-June, 2011, pp. 471-473

Letter to Editor

Preoperative clinico-radiological diagnosis of schwannoma arising from cavernous segment of abducens nerve

Justin E Moses¹, Nittin Vermani¹, Sanjay K Bansal²

¹ Department of Radiology, Arora Neuro Centre, Civil Lines, Ludhiana, Punjab, India
² Department of Neurosurgery, Arora Neuro Centre, Civil Lines, Ludhiana, Punjab, India

Correspondence Address: Justin E Moses Department of Radiology, Arora Neuro Centre, Civil Lines, Ludhiana, Punjab India drnv23@gmail.com

Date of Submission: 08-Feb-2011
Date of Decision: 08-Mar-2011
Date of Acceptance: 28-Mar-2011

Code Number: ni11136

PMID: 21743189

DOI: 10.4103/0028-3886.82743

Sir,

Schwannoma of the cavernous or cisternal segments of the abducens nerve are extremely rare and only 20 cases have been reported till date, and in most cases correct preoperative diagnosis was seldom made. ^[1],[2],[3] Intra-operative visualization of tumor attachment to the abducens nerve and histological confirmation often led to the diagnosis in all instances. ^[3] We report a case of schwannoma arising from the cavernous portion of right abducens nerve diagnosed preoperatively on the basis of clinical and radiological features.

A 65-year-old male patient presented with the complaint of headache associated with progressive diplopia on the right lateral gaze of five months duration. On neurological examination, there was complete right sixth nerve palsy associated with ipsilateral facial hypoaesthesia in mandibular nerve distribution. Magnetic resonance imaging (MRI) showed a large extra-axial mass lesion in the right middle cranial fossa, mainly involving the cavernous sinus and parasellar regions. Extension of the lesion into sella and suprasellar cisterns and encasement of the cavernous segment of right internal carotid artery was noted [Figure - 1] and [Figure - 2]. The lesion showed a uniformly bright T2-signal and was found to be distinctly separate from ipsilateral fifth nerve ganglion in the Meckel's cave and the cisternal portion of the trigeminal nerve. The multiplanar imaging evaluation, however, showed a small posterior extension of the lesion into the anatomical location of Dorello's canal [Figure - 3] and [Figure - 4]. Scalloping of bony margins of the Dorello's canal in the region of petrosphenoidal suture made this extension more conspicuous; this was a valuable imaging finding that led us to consider that the lesion origin was right sixth nerve. The signal pattern was more typical of schwannomas rather than highly cellular neoplasms like meningioma. A significant atrophy of the right lateral rectus muscle on MR images was a corroborative evidence. A contrast study was not done for the possible risk of nephrogenic systemic fibrosis as the patient had diabetic retinopathy and high creatinine levels.

The clinico-radiological diagnosis of abducens nerve schwannoma was confirmed at operation. He had subtotal microsurgical resection of the tumor through a fronto-temporal approach. As the tumor removal proceeded, a small posterior extension of the lesion causing enlargement of the abducens nerve around the Dorello's canal in clivus was identified, thereby confirming its origin from the sixth nerve. The more distal intracavernous portion of abducens nerve, however, was not separately visualized. The portion of tumor encasing the right internal carotid artery could not be removed. Histological examination of the resected specimen revealed a schwannoma.

Our patient is probably the first description of a sixth nerve schwannoma to be correctly diagnosed preoperatively on the basis of its clinical and imaging features. It has been suggested that most of the schwannoma arising from ocular cranial nerves have, as the initial presenting symptom, deficit of the nerve of origin. ^[4],[5] Typical sixth nerve palsy was the presenting feature in all reported cases except in two cases. ^[1],[6] Thus, in patients with isolated sixth nerve palsy and a mass lesion in the region of cavernous sinus or prepontine area, a diagnosis of schwannoma originating from abducens nerve needs to be considered. However, in patients with fifth nerve schwannoma, sixth cranial nerve palsy can also be the presenting feature. ^[7] Fifth nerve schwannoma originates from the nerve root in the cerebellopontine angle or from the trigeminal ganglion in Meckel's cave and not from the cavernous sinus. ^[8] MR demonstration of normal appearing cisternal segment and ganglion of trigeminal nerve separate from the mass lesion may, therefore, help exclude the possibility of fifth nerve schwannoma, as was the case in our patient. Also, extension of the lesion along the course of abducens nerve in the region of Dorello's canal has not been demonstrated by preoperative imaging in earlier descriptions of abducens schwannoma. This finding was, however, a valuable clue in our patient. We, therefore, recommend a comprehensive imaging evaluation to look for schwannoma growth along the expected anatomical course of a cranial nerve. By virtue of their submillimeter spatial resolution, three-dimensional Steady-State Free Precession MR sequences, such as constructive interference steady state, may be valuable in this regard. However, these were not available with the MRI scanner used. Nonetheless, a careful analysis of the characteristic clinical and radiological features may provide an accurate preoperative diagnosis.

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