High-resolution FSE T2-weighted imaging provides excellent demonstration of the intracanalicular segments of the facial and vestibulocochlear nerves and is a useful adjunct to conventional contrast-enhanced studies of the IAC (16–18). Both contrast-enhanced and high-resolution fast spin-echo (FSE) T2-weighted imaging are invaluable in the evaluation of IAC lesions. MR imaging is the examination of choice for depicting of the IAC in patients with a suspected vestibulocochlear schwannoma (13–15). Optimal therapeutic decisions for patients with IAC schwannomas require accurate imaging with precise lesion localization. Therefore, it is imperative to recognize when a lesion of the IAC is not a simple intracanalicular schwannoma. Should the dumbbell lesion extend to the geniculate ganglion, this would indicate a facial nerve neuroma and would influence the approach, as well as preparation, for facial nerve repair. Hearing preservation surgery is not an option when a lesion extends into the labyrinth, as removing a tumor from the labyrinth would be expected to result in profound sensorineural hearing loss (4). When schwannomas leave the confines of the IAC and extend into the inner ear, surgical approaches and prognostic implications are affected.Ī “dumbbell” lesion of the IAC is defined as a mass with two bulbous segments, one in the IAC fundus and the other in the membranous labyrinth of the inner ear or the geniculate ganglion of the facial nerve canal, spanned by an isthmus. Classically, these schwannomas are treated surgically by using one of three approaches (translabyrinthine, middle fossa, or retrosigmoid/suboccipital), depending on the size and location of the mass, as well as level of hearing. Treatment of patients with simple IAC schwannomas involves surgical resection with a goal of preserving hearing and facial nerve function (7–12). Typical IAC schwannomas are intracanalicular without extension into the membranous labyrinth of the inner ear. Schwannomas may arise from any of the cranial nerves within the IAC, however, including the facial nerve (1, 5). This tumor most commonly originates near the vestibular ganglion, at the junction of the central and peripheral myelin near the fundus of the IAC. “Acoustic” schwannomas most often arise from the vestibular division of the vestibulocochlear nerve. Schwannoma is a benign neoplasm of the nerve sheath and is the most common neoplasm of the internal auditory canal (IAC) and cerebellopontine angle (1–6). Temporal bone CT is reserved for presurgical planning in the dumbbell facial nerve schwannoma group. Dumbbell schwannomas of the vestibulocochlear nerve (14/24) included transmodiolar (8/14), which extended into the cochlea, transmacular (2/14), which extended into the vestibule, and combined transmodiolar/transmacular (4/14) types.ĬONCLUSION: Simple intracanalicular schwannomas can be differentiated from transmodiolar, transmacular, and facial nerve schwannomas with postcontrast and high-resolution fast spin-echo T2-weighted MR imaging. Characteristic features included an enhancing “tail” along the labyrinthine segment of the facial nerve and enlargement of the facial nerve canal. RESULTS: Ten of 24 lesions were facial nerve dumbbell lesions. Images were evaluated for contour of the mass and extension into the membranous labyrinth or geniculate ganglion. Twenty-four patients with dumbbell lesions of the IAC had their clinical and imaging data retrospectively reviewed. METHODS: A dumbbell lesion of the IAC is defined as a mass with two bulbous segments, one in the IAC fundus and the other in the membranous labyrinth of the inner ear or the geniculate ganglion of the facial nerve canal, spanned by an isthmus. In this article, the imaging and clinical features of these dumbbell schwannomas are described. It is important to differentiate dumbbell lesions, which include facial and vestibulocochlear schwannomas, from simple intracanalicular schwannomas, as surgical techniques and prognostic implications are affected. BACKGROUND AND PURPOSE: Benign tumors of the internal auditory canal (IAC) may leave the confines of the IAC fundus and extend into inner ear structures, forming a dumbbell-shaped lesion.
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