Extracanalicular Vestibular Schwannoma

This 79 year-old male presents with right sensorineural hearing loss. The MRI CISS 3D sequence shows a rounded heterogeneous lesion in the right cerebellopontine angle which does not enter the internal acoustic meatus. Gadolinium-enhanced sequences showed heterogeneous enhancement.
The most likely diagnosis is extracanalicular vestibular schwannoma. Vestibular schwannoma accounts for 75% of all cerebellopontine angle masses, the remainder being meningiomas (10%), epidermoid tumours, other schwannomas, vascular lesions, metastases, paragangliomas and ependymomas. 10-20% of vestibular schwannomas are entirely extracanalicular, with no extension into the internal auditory canal. The lesion is unlikely to be a meningioma as it appears separate to the dura and there are no “dural tails”. It does not have the typical invaginating appearance of an epidermoid cyst, and the signal would be closer to CSF signal. It may be a schwannoma of a different cranial nerve, but the location at the IAM and the presentation with hearing loss suggests the VIIIth nerve.
References:
Grossman RI, Yousem DM. Neuroradiology: The Requisites 2nd edition. Mosby 2003.
Osborn AG. Diagnostic Neuroradiology 1st edition. Mosby 1994.
Credit: Dr Laughlin Dawes
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