FP, male 69 yo, was taken on first-aid area for dizzines and hearing-loss on right ear. After a CT scan, the pt. underwent at enhanced MRI evaluation.
Fig. 1: AX TSE T2w (a); COR TSE T2w (b); FLAIR (c). A large mass can be depicted in right ponto-cerebellar angle. The mass in centered over the IAC, and there is a funnel-shaped portion that extends directly into the widened canal. The features that indicate an extraaxial location include the pseudocapsule of
displaced vessels and the widening of the ipsilateral CPA and lateral pontine cisterns,
produced by displacement of the pons toward the left side. The mass is bulging and
convex toward the brain stem and cerebellum. The mass is invaginating and deforming the pons
and cerebellum, and the fourth ventricle is compressed and displaced. The contralateral lateral
brain stem cisterns (pontine, medullary) are effaced.
On the T2-weighted MR image,the mass is markedly heterogeneous. The intracanalicular portion is isointense relative to cortical gray matter. The periphery of the tumor is as the same signal intensity as cerebrospinal fluid on both T2W and FLAIR.
|Fig 1 A|
|Fig 1 B|
|Fig 1 C|
Fig. 2: AX SE T1w. T1-weighted MR image shows an oblong, slightly heterogeneous mass.
Fig. 3: (a) AX FFE T1W C; (b) AX SE T1W C: (c) COR SE T1W C: the mass enhances heterogeneously. Portions of the tumor in and near the IAC show homogeneous enhancement while the extracanalicular portion shows an very inhomogeneous contrast enhancement.
|Fig 3 A|
|Fig 3 B|
|Fig 3 C|
Schwannomas are encapsulated, benign, slowly growing neopiasms arising from the schwann cells of the cranial, spinal, and peripheral nerves. These cells normally produce the myelin around the axons of the peripheral nervous system. In contrast, the myelmn within the substance of the brain is made by the oligodendrocytes. Primary intracranial nerve sheath neoplasms are nearly always schwannomas, and they do not have malignant potential. In the overwhelming majority of patients, a schwannoma will develop as a sporadic neoplasm. However, approximately 5%-20% of patients with solitary intracranial schwannomas have type 2 neurofibromatosis, and the presence of bilateral
“acoustic” schwannomas is sufficient to make a presumptive diagnosis of the disease. Type 2 neurofibromatosis is an inher ited autosomal dominant syndrome characterized by a propensity for developing multiple schwannomas, meningiomas, and gliomas of ependymal derivation. A mnemonic for remembering this is MISME (Multiple Inherited Schwannomas, Meningiomas, and Ependymomas).
The schwannoma should not be confused with a similar, yet significantly different nerve sheath neoplasm, the neurofibroma. In contrast to schwannomas, neurofibromas do have the potential for transformation into malignant nerve sheath tumors. The spinal neurofibroma is rarely a sporadic tumor and is usually a sign ofvon Reckbinghausen disease (type 1 neurofibromatosis). Neurofibromas may arise from the peripheral extracranial portions of the trigeminal (and other) cranial nerves and then secondarily extend back into the calvaria along the nerve. However, this probably does not occur with the eighth cranial nerve. Occasionally, neurofibromas occur in Jacobson (nervus tympanicus) and Arnold (nervus auricularis vagi) nerves. However, the exceptional reports of neurofibromas arising from peripheral (eg, cochlear) portions of the eighth cranial nerve in patients with type 1 neurofibromatosis are sufficiently rare that neurofibroma should not be a consideration in the differential diagnosis of an eighth cranial nerve mass.
Intracranial schwannomas, either without or with type 2 neurofibromatosis, most often arise from sensory nerves, and the eighth (vestibulocochlear) cranial nerve is overwhelmingly the most common site. Other synonyms for this neoplasm include acoustic neuroma, acoustic neurinoma, and neurilemoma. Because they usually develop from the vestibular component (often from the superior division or Scarpa ganglion), rather than the cochlear portion of the eighth cranial nerve, a National
Institutes of Health panel considered changing the name from acoustic schwannoma to vestibular schwannoma. The trigeminal (fifth) cranial nerve is the second most common site of intracranial schwannoma, but it is about five to 10 times less frequent. Both sites relate to the CPAand its cistern; the eighth cranial nerve passes through it, and the fifth nerve forms part of its anteromedial boundary.
Patients with type 2 neurofibromatosis may present with vestibular schwannoma during
the 2nd decade oflife and only exceptionally after 55 years of age. However, patients with the more common sporadic schwannoma usually present in their 30s and 40s. Several reported series have shown no gender predilection, whereas others indicate a female predominance of up to 2 :1 for intracranial schwannomas, especially for large tumors.
Deafness is the most common presenting symptom for patients with vestibular schwannoma who have type 2 neurofibromatosis, whereas vertigo and balance problems are uncommon (occurring in less than 10% of cases). Often, the patient has a long history of slowly progressive hearing loss, usually affecting the perception of high-frequency sounds more severely. In some cases, the patient may suddenly discover a unilateral hearing loss when using headphones or after holding a telephone to the “wrong” ear.
Histologically, the schwannoma is an encapsulated neoplasm composed of proliferating, spindle-shaped (ie, fusiform) schwann cells. The tumor consists of highly cellular dense regions (Antoni A) that have reticulin and collagen and alternating loose areas (Antoni B) with widely separated cells in a reticulated myxoid matrix. These loose areas often mimic the histologic characteristics of a neurofibroma. Degenerative changes are common, including cyst formation, prominent vascular features (similar to those of a primary vascular lesion, with a potential for thrombosis or hemorrhage), and “fatty degeneration” with lipid-laden foam (ie, xanthomatous) cells.
It is well known that small vestibular schwannomas are usually entirely contained within the IAC (ie, the “intracanalicular” schwannoma). The reason for this is that the portion ofthe eighth cranial nerve traversing the cerebrospinal fluid of the CPA cistern does not have schwann cells but rather oligodendrocytes. However, at the opening of the IAC (ie, the porus acusticus), there is a transition zone at which the myelin production switches from the oligodendrocytes to the schwann cells. Thus, most vestibular schwannomas probably arise inside the IAC, or near its mouth, but then secondarily grow into the fluid-filled CPA cistern. The vestibular schwannoma is a rounded mass, centered on the long axis of the IAC. In more than 70%-90% of cases, there are signs of IAC erosion and enlargement. Conversely, erosion of the IAC (as seen on plain radiographs) indicates a schwannoma in up to 93% of cases. The largest portion ofthe tumor is usually in the CPA and is roughly spheric, but almost invariably there is a funnel-shaped component that extends into the IAC.
Small schwannomas (up to 2 cm in diameter) may be homogeneously solid tumors. As they grow (and age), degenerative changes become more frequent and more prominent so that larger ( > 2-3 cm) schwannomas become heterogeneous, with cystic regions and occasional hemorrhages.
On computed tomographic (CT) and MR images, the schwannoma is a spheric mass that forms acute angles with the petrous bone. Compared with the attenuation of the brain on unenhanced CT scans, the schwannoma is usually either the same (isoattenuating) or lower (hypoattenuating). Schwannomas invariably enhance with iodinated contrast material. Intracanalicular and small CPA schwannomas usually enhance homogeneously. Larger tumors show variable and heterogeneous features, including old and new hemorrhagic and cystic changes. On unenhanced Ti-weighted images, vestibular schwannomas can be isointense to slightly hypointense relative to the brain. After gadolinium infusion, they enhance intensely and often appear more homogeneous than on unenhanced Ti-weighted images. It has been suggested that the intensity and homogeneity ofenhancement are produced because of extracellular spaces that are much larger than those of other solid tumors. On T2-weighted images, the vestibular schwannoma usually becomes hyperintense, whereas most meningiomas remain hypo- or isointense. However, these signal intensity changes are not pathognomonic for schwannoma. Overall, the shape (round) and location (centered on the IAC) are more reliable features, suggestive of a vestibular schwannoma. Eighth cranial nerve schwannomas are usually confined entirely to the postenor fossa. The trigeminal schwannoma usu-
ally straddles the boundary between the posterior and middle cranial fossae.
Smirniotopoulos JG, Chang Yue N, Rushing EJ. Cerebellopontine Angle Masses: Radiologic-Pathologic Correlation. RadioGraphics. 1999; Vol 13 n5, 1131-1147.