Neuroradiology

Neurofibroma, acoustic

(also called acustic schwannoma and acoustic neurinoma), benign tumour deriving from schwann cells of the acoustic nerve. It is also termed neurilemmoma or neurinoma. It is the most frequent intracranial schwannoma. Most commonly the neoplasm originates from the vestibular portion of the eighth cranial nerve; it can be exclusively intracanalicular or may extend as a mass in the cerebellopontine angle. Schwannomas are mainly tumours of adulthood; when they appear earlier they usually occur with NF2 (see neurofibromatosis) and are typically bilateral. Histologically, they are encapsulated masses with an ovoid, tubular or lobulated shape. Two distinct histological types are recognizable: Antoni type A, with a compact texture, and Antoni type B, with looser composition and frequent microcystic areas that can coalesce into big cysts. One quarter of tumours are hypervascular. Haemorrhage and necrosis may occur.

The earliest symptoms are tinnitus and sensorineural hearing loss. Vestibular signs occur later.

Imaging

Plain film radiographs can reveal widening of the internal auditory canal; which, however, is a late manifestation of the tumour. On CT axial images intracanalicular schwannomas are usually missed. Larger tumours both intra- and extracanalicular appear as oval or lobulated masses, they may present as a broad base against the petrous ridge at the level of the internal auditory meatus and behave like extra-axial masses enlarging the cerebellopontine angle (CPA) cistern (Fig.1). They are iso- to slightly hypodense to brain and show strong but variable enhancement owing to cystic degeneration in larger neoplasms. Calcifications are rare. Erosion of the posterior wall of the canal can also be appreciated.

MR images using thin-sections (3 mm) will reveal both intra- and extracanalicular schwannomas.They appear as well demarcated CPA masses, round or oval in shape, separated from the adjacent pontocerebellar tissue by a vascular rim and/or a CSF cleft. The signal is slightly hypointense on T1-weighted images and heterogeneously hyperintense on intermediate and T2-weighted images. The heterogeneous aspect is due to the presence of cysts, areas of different cellular histology, neovascularity and haemorrhage. It should be noted that significant heterogeneity on MRI is more typical of acoustic schwannoma than meningioma, and that meningioma does not show intracanalicular extension. The contrast enhancement of schwannomas is usually marked but frequently inhomogeneous. Contrast medium injection is necessary to detect smaller intracanalicular tumours that may not otherwise be seen (Fig.2)

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Fig.1

a. Skull X-ray, Towne's projection, showing the internal auditory canals. Arrows and arrowheads to the right point at the expanded canal. b. Axial CT, bone window. The internal auditory canal on the right is expanded. c, d. MR, axial and coronal T1-weighted images following gadolinium injection. Enhancement of the neurinoma, that grows within the cerebellopontine angle cistern, compressing and displacing the brainstem.
Neurofibroma, acoustic, Fig.1 (a)
Neurofibroma, acoustic, Fig.1 (b)
Neurofibroma, acoustic, Fig.1 (c)
Neurofibroma, acoustic, Fig.1 (d)
Neurofibroma, acoustic, Fig.2 (a)
Neurofibroma, acoustic, Fig.2 (b)
Neurofibroma, acoustic, Fig.2 (c)
Neurofibroma, acoustic, Fig.2 (d)