Isolated schwannoma involving extraocular muscles.

BACKGROUND: Progressive strabismus initially considered idiopathic may be caused by isolated schwannomas of motor nerves to extraocular muscles, detectable only on careful imaging. This study reviewed clinical experience of a referral practice in identifying schwannomas on magnetic resonance imaging (MRI).

METHODS: We reviewed 647 cases imaged for strabismus to identify presumed cranial nerve schwannomas, identified by gadodiamide-enhanced, high-resolution surface coil orbital MRI and thin-section cranial MRI. Clinical features and management were correlated with MRI.

RESULTS: Schwannomas were identified as fusiform intraneural enlargements in 8 cases: 1 affecting the trochlear nerve; 2, the abducens nerve; and 5 the oculomotor nerve. Involved muscles were atrophic. Both abducens schwannomas, 1 superior oblique, and 1 oculomotor schwannoma were subarachnoid; 3 were intraorbital, and bilateral oculomotor lesions of 1 case extended from cavernous sinus to orbit. Associated strabismus progressed for 3-17 years. Abducens schwannoma caused esotropia; trochlear schwannoma caused hypertropia and cyclotropia. Intracranial oculomotor schwannoma caused mydriasis and exotropia. Intraorbital schwannoma caused exotropia with or without hypertropia. Since lesion diameters were 3-9 mm, 6 had been previously missed on routine MRI.

CONCLUSIONS: Progressive, acquired strabismus may be caused by isolated cranial nerve schwannomas, representing about 1% of strabismus cases in this study, involving the oculomotor more than abducens nerve. Because most schwannomas are small and deep in the orbit, findings could be readily missed by routine imaging, leading to a possible diagnosis of idiopathic strabismus. Schwannomas should be suspected when extraocular muscles are atrophic, but the causative lesions themselves are identifiable only using targeted, high resolution MRI.