False positive MRI in the diagnosis of small intracanalicular vestibular schwannomas.

The current gold standard for diagnosing vestibular schwannomas is MRI with gadolinium-DTPA enhancement. This imaging modality is particularly useful in the detection of small intracanalicular tumours which can be missed by CT scanning. We present a case where MRI with enhancement suggested the presence of a 4 mm intracanalicular vestibular schwannoma. Surgical exploration of the internal auditory canal via a retrosigmoid approach, revealed no tumour, but inflammatory arachnoid matter around the vestibular nerve was found. A review of the audiological test results uncovered some results which did not correlate with the interpretation of the MRI scan. We would therefore caution against immediate surgical intervention in patients where the diagnosis of a small intracanalicular vestibular schwannoma is not totally supported by the audiological findings. In such cases rescanning with gadolinium enhancement after a suitable interval is recommended.