The differential diagnosis of vestibular schwannomas consists of masses or other processes that can cause a progressive syndrome referable to the cerebellopontine angle and associated cranial nerves (Press and Hesselink 1988; Curtin and Hirsch 1992; Jackler and Pitts 1992; Lalwani 1992; Smirniotopoulos et al 1993; McKenzie 1994; Mafee 1995; Salzman et al 2001). This includes other primary cerebellopontine angle tumors, cysts, vascular malformations, aberrant normal vascular structures, and metastatic lesions. Vestibular schwannomas make up 80% to 90% of the masses found in the cerebellopontine angle (Lalwani 1992; Smirniotopoulos et al 1993; McKenzie 1994). The second most common tumors of the cerebellopontine angle are meningiomas (10% to 15% of cerebellopontine angle lesions); these have a similar clinical presentation to vestibular schwannomas. Meningiomas have several distinguishing characteristics on CT and MRI (Lalwani 1992; Smirniotopoulos et al 1993). Meningiomas typically grow as an oval or hemispheric mass rather than as a sphere, and usually have a broad dural attachment. On unenhanced CT, meningiomas demonstrate increased attenuation compared to the brain. There may be calcification and associated hyperostosis of adjacent bone. The porus acusticus is not enlarged when viewed using bone windows. Meningiomas have vigorous, homogeneous enhancement after iodinated contrast. On MRI, meningiomas and schwannomas are isointense relative to brain on T1-weighted images; however, meningiomas are more likely to remain isointense when using other pulse sequences (Smirniotopoulos et al 1993). After administration of gadolinium, meningiomas diffusely enhance; tumor is not present within the internal auditory canal. In contradistinction to schwannomas, meningiomas are usually centered away from the porus acusticus. Even when they are located near the porus acusticus, they rarely enlarge or disturb the canal. In 60% to 72% of meningiomas, a tail of enhancement ("dural tail" sign) is visible along the dura (Smirniotopoulos et al 1993; McKenzie 1994). This finding is suggestive of a meningioma, but not pathognomonic. Occasionally, schwannomas and other masses can have a dural tail. The third most common mass of the cerebellopontine angle is the epidermoid inclusion cyst, making up 5% to 9% of all lesions (Lalwani 1992; Smirniotopoulos et al 1993). Patients often have a long history of hearing loss and tinnitus. On CT, epidermoid cysts are usually hypodense compared to brain (similar to water or cerebrospinal fluid). Calcification of the cyst rim is noted in 25% of cases. Enhancement is negligible within the cyst, but may occur along the rim. On MRI, epidermoid cysts are usually isointense to cerebrospinal fluid (ie, low signal intensity on T1 images and high signal intensity on T2 images), and may have a lamellated appearance. Similar to CT, the cyst does not enhance with gadolinium, except for the rim. Arachnoid cysts can also occur in the cerebellopontine angle. They have signal intensity and attenuation characteristics identical to cerebrospinal fluid and do not enhance with contrast. Many other lesions can arise in the cerebellopontine angle; each of these accounts for less than 1% of all cases. Tumors that may extend into, but not arise from, the cerebellopontine angle include: (1) exophytic brainstem gliomas; (2) ependymomas; (3) choroid plexus papillomas; (4) schwannomas of cranial nerves V, VII, IX, X, and XI; (5) jugular foramen paragangliomas; (6) lipomas; and (7) metastases (Lalwani 1992; Smirniotopoulos et al 1993; Salzman et al 2001; Preuss et al 2008). Metastatic tumors usually arise in the cerebellopontine angle unilaterally but can be bilateral in approximately 40% of cases (Preuss et al 2008). Melanoma is the most common type of tumor to present in this fashion. Vascular processes such as aneurysm, malformations, and aberrant loops of normal blood vessels can develop in the cerebellopontine angle. Enhancement on CT and a flow void on MRI are diagnostic of a vascular process. Rarely, infectious processes can develop in the cerebellopontine angle, such as tuberculomas and cysticercosis. Benign vestibular conditions (eg, Meniere disease) can suggest vestibular schwannoma with symptoms and signs of hearing loss, tinnitus, nystagmus, and vertigo (Mafee 1995). However, tumor can be ruled out by a careful history (ie, fluctuating symptoms that occur in attacks, bilateral involvement in approximately 20%) and a normal enhanced MRI scan.
Trigeminal schwannomas have a similar differential diagnosis to vestibular tumors (Miller 1988; Yuh et al 1988). Approximately 30% to 35% of tumors in the Meckel cave region are trigeminal schwannomas. The remainder includes meningiomas, epidermoids, chondromas, chordomas, lipomas, and metastatic lesions. The same differential can be applied to schwannomas of the remaining cranial nerves.
The differential diagnosis of spinal schwannomas and neurofibromas consists of other tumors and numerous benign conditions that can cause myelopathy or radicular symptoms (Sanguinetti et al 1993; Newton et al 1995). Tumors to be considered in the differential include meningiomas, exophytic ependymomas and astrocytomas, epidermoid cysts, dermoids, lipomas, and teratomas. Benign conditions to be considered include syringomyelia, multiple sclerosis, transverse myelitis, spondylosis, herniated disc, and infection (eg, Lyme disease, syphilis). The majority of these diseases can be differentiated from a spinal schwannoma or neurofibroma by a careful general and neurologic examination, cerebrospinal fluid analysis, and enhanced MRI scan.
The differential diagnosis of peripheral nerve schwannomas and neurofibromas consists of other neoplasms such as malignant peripheral nerve sheath tumors, lipomas, metastases, desmoid tumors, granular cell tumors, nerve sheath myxomas, lymphangiomas, and myoblastomas (Lusk et al 1987; Ariel 1988; Miller 1988; Skovronsky and Oberholtzer 2004).