Acoustic schwannoma

Incidence

Acoustic schwannoma accounts for 5-8% of all intracranial tumors. They constitute 70-90% of all cerebellopontine angle masses.

Origin

Acoustic neuromas originate from Schwann cells. They usually originate at the transition zone where the glial cells of the nerve sheath change to Schwann cells, usually located within the internal auditory canal. The most common site of origin is the superior vestibular nerve.

Macroscopic appearance and general characteristics (image 2)

Acoustic neuromas are slowly growing, encapsulated tumors. The tumor arises excentrically, with stretching of the axons of the parent nerve over the mass. Intratumoral bleeding and cyst formation is frequent.

The tumor erodes and widens the internal auditory canal and meatus and shows extra axial growth in the cerebellopontine angle cistern. Compression of the cerebellum and brainstem is possible.

Clinical symptoms initially include slowly progressive sensorineural hearing loss and tinnitus. Later on, gait disturbance and dizziness reflect vestibular symptoms. Finally, compression of other cranial nerves or the brain stem is manifested. Schwannomas occur in the adult population.

Acoustic schwannomas can be an expression of von Recklinghausen's neurofibromatosis type 2. In this case they tend to manifest earlier and are predominantly bilateral. Schwannomas correspond histologically to Grade I.

Microscopic appearance (image 3)

Schwannomas are composed of spindle-shaped neoplastic Schwann cells. Most lesions contain two basic histological patterns: the Antoni-A pattern, with densely cellular areas, composed of compact elongated cells often associated with palisading and the Antoni-B pattern, with a less cellular, loosely textured pattern, with lipid containing cells.

Imaging features (see case reports)

- On CT, acoustic schwannomas appear mostly iso- to hypodense, with intense and homogeneous enhancement. Funnel-shaped widening of the internal auditory canal and meatus is evident on bone window settings. Large tumors tend to show cystic components.

- On MRI, the fact that cortical bone is not depicted is rather an advantage than a disadvantage in the study of acoustic schwannomas. Very small intracanalicular acoustic neuromas enhance strongly and therefore stand out clearly against the absent signal from the cortical bone of the internal auditory meatus. In the same way, the funnel-shaped extension of the tumor into the internal auditory canal is easily recognized and better evaluated than with CT (image 4).

On T1 weighted images the tumor is iso- to hypointense. On T2 weighted sequences the tumor can be hypointense to hyperintense. Hyperintense cystic components are common. Peripheral cysts can be caused by entrapment of CSF spaces or associated arachnoid cysts. Enhancement is mostly intense and homogeneous, with the exception of the cystic or the hemorrhagic components.

Note: The differential diagnosis from cerebellopontine angle meningioma includes

- acoustic schwannoma is centered on the internal auditory meatus; meningioma is not

- acoustic schwannoma is infrequently calcified; meningioma is frequently calcified

- in acoustic schwannoma the internal auditory canal and meatus are widened; in meningioma they are normal, but erosion or hyperostosis of the posterior surface of the petrous pyramid is possible.

- in acoustic schwannoma a dural tail is rarely seen; in meningioma it is frequent

- in acoustic schwannoma enhancement within the internal auditory canal is always present; in meningioma this is rare

- other differential diagnoses include: epidermoid, trigeminal or ninth nerve schwannoma, mestastases and paraganglioma. Differentiation from epidermoid or arachnoid cyst is usually straightforward.

Case reports

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Acoustic schwannoma
Schwannoma

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