eMedicine Specialties > Radiology > Brain/Spine

Schwannoma, Cranial Nerve: Imaging

Author: Mahesh Jayaraman, MD, Interventional Neuroradiology Fellow, Department of Radiology, Stanford University Medical Center
Coauthor(s): Lawrence M Davis, MD, Assistant Professor of Diagnostic Imaging (Clinical), Department of Diagnostic Imaging, Brown Medical School
Contributor Information and Disclosures

Updated: Oct 18, 2007

Radiography

Findings

No role exists for plain radiographic evaluation of schwannomas. Findings on conventional radiographs are nonspecific, and typically, lesions are visualized on plain radiographs only when they are large.

Computed Tomography

Findings

On nonenhanced CT scans, most schwannomas are isoattenuating relative to brain parenchyma. Calcification or areas of hemorrhage are rare. On contrast-enhanced CT scans, the enhancement pattern is typically homogeneous.²

Bone-window images can demonstrate remodeling of the adjacent skull base, such as expansion of the IAC by vestibular schwannomas and expansion of the facial canal by facial schwannomas (see Image 7). Expansion of the jugular foramen by CN IX, CN X, or CN XI schwannomas can also be seen.

Thin-collimation CT imaging of the skull base can be helpful in evaluating bone destruction. This finding is useful in differentiating jugular foramen schwannomas from paragangliomas (see Image 23).

Degree of Confidence

With CT, large lesions can be diagnosed with a high degree of confidence. Distinguishing a schwannoma from a meningioma may not be possible by using CT. A small lesion that affects a CN cannot be confidently excluded.

False Positives/Negatives

CT findings can be false-negative in small lesions. Occasionally, a false-positive diagnosis occurs because a streak artifact in the cerebellopontine angle cistern mimics a lesion.

Magnetic Resonance Imaging

Findings

Similar to CT imaging, MRI tends to depict schwannomas as homogeneous masses.^1,7 Schwannomas are typically isointense or slightly hypointense relative to gray matter on T1-weighted images and slightly hypointense to CSF on T2-weighted images. Gadolinium enhancement is typically homogeneous, although larger schwannomas can show areas of cystic degeneration and heterogeneous signal intensity; these findings are based on increased numbers of areas with Antoni type B histologic features.

High-resolution thin-section heavily T2-weighted 3-dimensional sequences have been used to look for acoustic neuromas. On images obtained with these sequences, individual nerves in the cistern and IAC can be visualized as linear filling defects in the bright CSF. Small masses can be identified without the use of an intravenously administered contrast agent (see Image 3).

Vestibular schwannomas are the most common CN schwannomas. Typically, the masses are located in the cerebellopontine angle (CPA) and centered at the porus, with extension into the IAC. Their appearance has been described as that of a comet tail or ice cream cone, with the cone as the intracanalicular extension and the ice cream as the cisternal component. The long axis of the tumors is parallel to the petrous surface.

Occasionally, tumors can be entirely intra-canicular, in which case the primary differential diagnosis is a meningioma of the CPA. Unlike vestibular schwannomas, meningiomas tend to form obtuse angles with the adjacent petrous bone, are typically hemispheric, and often extend into the middle fossa as a result of herniation (see Images 1-5). Meningiomas can be differentiated by their broad base of attachment along the petrous bone and by the presence of a dural tail. Meningiomas uncommonly extend into the internal auditory canal.⁷

Schwannomas of the facial nerve can occur along any segment, but they frequently involve the geniculate ganglion and extend proximally or distally from there. MRI and CT imaging characteristics are similar to those of vestibular schwannomas. The location of the mass results in variable growth patterns. In the IAC, facial schwannomas are indistinguishable from vestibular lesions. When facial schwannomas cross the petrous bone to involve both the middle and posterior fossa, they cross in the mid portion of the petrous bone. In contrast, trigeminal schwannomas cross near the petrous apex. Lesions in the geniculate ganglion can be mistaken for temporal lobe lesions, and imaging in the coronal plane is useful in evaluating the lesions (see Images 8-9).

Trigeminal schwannomas can arise in the Meckel cave or in the cistern along the course of the nerve.⁸ Extension and expansion of the foramen rotundum or ovale is common, and the masses can have a bilobed appearance. Tumors can also grow posteriorly to involve the posterior fossa, or they can grow anteriorly into the cavernous sinus. Trigeminal schwannomas tend to have a more cystic component than other schwannomas (see Images 12-15).

Glossopharyngeal, vagus, or accessory nerve schwannomas are rare and difficult to distinguish from one another. The tumors are classified on the basis of their growth patterns: Type A lesions grow predominantly intracranially, type B lesions grow predominantly at the jugular foramen, and type C lesions grow predominantly extracranially. CT and MRI characteristics are similar to those of other schwannomas. In contrast to the more common paragangliomas in this region, schwannomas expand but do not infiltrate the adjacent bone (see Images 19-20). Unlike paragangliomas, which infiltrate and erode adjacent bone, schwannomas smoothly expand the bone and leave an intact cortical margin.

Hypoglossal schwannomas have growth patterns and imaging characteristics similar to those of jugular foramen schwannomas. When large enough, the tumors can erode the hypoglossal canal to such an extent that their differentiation from jugular foramen schwannomas can be difficult.⁶

Schwannomas of abducens nerve (ie, CN VI) are rare. They are reported to occur in the prepontine cistern, with a heterogeneous appearance on CT scans and MRIs and extension into the adjacent cavernous sinus. As with other schwannomas, meningioma is the primary differential diagnosis, and the presence of areas of cystic change (which have high signal intensity on T2-weighted images) can suggest the likelihood of schwannoma instead of meningioma. Similarly, CN IV schwannomas are rare (see Image 17).

Schwannomas of the oculomotor nerve are reported in the literature, but they are exceedingly rare. The tumors can present as masses in the suprasellar cistern, and they can be difficult to distinguish from meningiomas in this region.

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint  stiffness with trouble  moving  or  straightening  the  arms,  hands,  legs,  or  feet;  pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.

Degree of Confidence

A diagnosis of a CN mass can be made with a high degree of confidence by using MRI.

False Positives/Negatives

False-positive findings occur primarily in the jugular fossa, where slow flow in the jugular bulb can mimic a mass. A false-negative diagnosis can occur if imaging is inadequate — for example, if the image sections are too thick or if fat suppression is not used in evaluating the skull base.

Ultrasonography

Findings

No role exists for ultrasonography in the evaluation of CNs.

Nuclear Imaging

Findings

No role exists for nuclear medicine studies in the evaluation of CNs.

Angiography

Findings

Angiography is not used as a diagnostic modality for schwannomas. When studied with angiography, schwannomas typically appear hypovascular; this finding distinguishes them from paragangliomas when the lesion is in the jugular fossa (see Image 24).

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