Introduction
Background
Prior to the advent of MRI, imaging of the cranial nerves (CNs) was difficult, and mass lesions arising from these nerves was often indirectly detected only by looking at bony changes in the skull base foramen or by using invasive techniques such as cisternography and angiography. Current imaging techniques provide noninvasive highly detailed imaging of most of the CNs and the lesions (eg, schwannomas) that affect them.1,2,3,4
Patients with CN schwannomas can present with loss of function of the affected nerve, but they can also be asymptomatic. In these latter patients, the lesion may be incidentally discovered on CT scans or MRIs obtained for reasons other than the evaluation of a schwannoma.
CN schwannomas are usually isolated lesions, except when they are associated with neurofibromatosis type 2 (NF2), a rare autosomal dominant disorder occurring in approximately 1 live birth in 50,000. NF2 is also called the multiple inherited schwannomas, meningiomas, and ependymomas (MISME) syndrome.
NF2 is characterized by bilateral vestibular schwannomas. Schwannomas of the other CNs occur more frequently in NF2, and the presence of one of the rare CN schwannomas should suggest the possibility of NF2. Meningiomas and intramedullary ependymomas of the spinal cord also occur in NF2.
(Also see the eMedicine articles Neurofibromatosis Type 1 and Neurofibromatosis Type 2.)
Pathophysiology
Schwannomas arise from the nerve sheath and consist of Schwann cells in a collagenous matrix. Histologically, the terms Antoni type A neurilemoma and type B neurilemoma are used to describe varying growth patterns in schwannomas. Type A tissue has elongated spindle cells arranged in irregular streams and is compact in nature. Type B tissue has a looser organization, often with cystic spaces intermixed within the tissue. The cystic spaces can result in high signal intensity on T2-weighted MRIs. Tumors originating in Schwann cells can be detected at immunohistochemical examination by virtue of their positive results with S-100 antigen tests.5
Frequency
United States
Schwannomas account for 6-8% of intracranial neoplasms. Autopsy studies have shown that the incidence rates of occult vestibular schwannomas are as high as 2.7%. A study of patients undergoing MRI for indications other than the evaluation of schwannoma revealed an estimated prevalence of 0.07%.
Vestibular schwannomas (see Images 1-4, 6) are the most common CN schwannomas, followed by trigeminal and facial schwannomas and then glossopharyngeal, vagus, and spinal accessory nerve schwannomas. Schwannomas involving the oculomotor, trochlear, abducens, and hypoglossal nerves are rare.
Mortality/Morbidity
Morbidity resulting from schwannomas includes nerve dysfunction and brainstem compression. Mortality can result from mass effect with brainstem compression.
Race
No racial predilection has been described in schwannomas.
Sex
No sex predilection has been described in schwannomas.
Anatomy
CNs III-XII are peripheral nerves that arise from the brainstem and exit the skull base through their respective foramina.
Motor neurons of the oculomotor nerve (ie, CN III) leave the midbrain at the level of the tegmentum and emerge in the interpeduncular cistern. Here, it passes between the posterior cerebral artery (PCA) above) and the superior cerebellar artery (SCA) below, and it turns anteriorly to enter the cavernous sinus. In the cavernous sinus, the oculomotor nerve courses along the lateral wall; it is the most superior of all the nerves in the sinus. The nerve enters the orbit via the superior orbital fissure and then splits into superior and inferior divisions.
From the trochlear nucleus in the midbrain, fibers of the trochlear nerve (ie, CN IV) cross the midline dorsal to the cerebral aqueduct and exit the midbrain dorsally. From here, the fibers run around the midbrain to the ventral surface. Like the oculomotor nerve, the trochlear nerve also courses between the PCA and SCA and along the lateral wall of the cavernous sinus. It enters the orbit at the superior orbital fissure.
The trigeminal nerve (ie, CN V) exits the brainstem at the level of the mid pons, and its 3 divisions—the ophthalmic (CN V1), maxillary (CN V2), and mandibular (CN V3) branches—together proceed anteriorly toward the trigeminal ganglion in the Meckel cave (see Image 16). From here, the mandibular division exits inferiorly via the foramen ovale. The maxillary and ophthalmic divisions continue anteriorly along the lateral aspect of the cavernous sinus. Eventually, the ophthalmic division enters the orbit via the superior orbital fissure, while the maxillary division exits the cranial vault through the foramen rotundum.
The abducens nerve (ie, CN VI) exits the brainstem ventrally at the level of the junction of the pons and medullary pyramid and courses anterolaterally toward the dorsum sellae, passing over the petrous apex where it makes a sharp turn to enter the cavernous sinus. In the sinus, the abducens nerve is medial to CN IV, CN V1, and CN V2. Along with the oculomotor and trochlear nerves, the abducens nerve also enters the orbit via the superior orbital fissure, then enters the deep surface of the lateral rectus muscle.
The paths of the facial nerve (ie, CN VII) and vestibular nerve (ie, CN VIII) are intimately associated. They exit the brainstem at the pontomedullary junction, with the facial nerve slightly medial to the vestibular nerve. From there, they enter the internal auditory canal (IAC). Once in the IAC, the facial nerve courses in the superior-anterior quadrant of the canal, while the vestibular division of the vestibular nerve courses in the posterior superior and inferior quadrants, and the cochlear division courses in the inferior-posterior quadrant. CN VIII then enters the labyrinth. (The mnemonic for this arrangement is "Seven-Up and Coke [ie, cochlear] down.")
The facial nerve enters the labyrinth (labyrinthine segment), courses anteriorly in the temporal bone to the geniculate ganglion, turns posteriorly to pass beneath the lateral semicircular canal (tympanic segment) and then inferiorly to course through the mastoid (vertical segment), and exits the temporal bone via the stylomastoid foramen. Finally, the facial nerve courses within the parotid gland (parotid segment) before branching.
The glossopharyngeal (ie, CN IX), vagus (ie, CN X), and accessory (ie, CN XI) nerves emerge cranial to caudal, in that order, from the ventral medulla, lateral to the medullary olive. From there, they course toward the jugular foramen and exit the skull base at the jugular foramen. The glossopharyngeal nerve is located in the pars nervosa of the jugular foramen, and the vagus and accessory nerves are located within the more posterior pars vascularis.
The hypoglossal nerve (CN XII) is formed by the fusion of multiple rootlets that emerge from the ventrolateral sulcus between the medullary olive and pyramid. The nerve exits the cranial vault via the hypoglossal canal, then lies medial to CN IX, CN X, and CN XI.
Presentation
Typically, presenting symptoms of schwannomas are based on the affected nerve.
Patients with vestibular schwannomas present with sensorineural hearing loss, tinnitus, and disequilibrium. Rarely, patients present with acute sensorineural hearing loss secondary to hemorrhage into a vestibular schwannoma. The lesions arise from the inferior or superior division of the vestibular nerve, but they typically cause symptoms due to mass effect on the adjacent cochlear nerve. Some authors report that these lesions more commonly arise from the inferior division of the nerve, but others report equal frequency for lesions from the superior and inferior divisions.
Trigeminal schwannomas can present with facial pain and/or atrophy of the muscles of mastication.
The presentation of facial schwannomas is variable and depends on the segment of the facial nerve from which the tumor arises. Symptoms can range from facial palsy to compressive hearing loss resulting from ossicular interference and sensorineural hearing loss due to effects on cochlear nerve in the internal auditory canal (less common). Tumors in the IAC can present with vestibular symptoms resulting from compression of the intimately associated vestibular nerve. Facial schwannomas in the middle cranial fossa or those distal to the stylomastoid foramen can be several centimeters in size at the time of presentation because no immediately adjacent sensitive structures exist, and symptoms may not be evident until the lesions are large.
Schwannomas in the jugular foramen that arise from the glossopharyngeal, vagus, or accessory nerves, can present with variable cerebellar and acoustic symptoms, depending on the extent of the intracranial growth of the mass. They also can cause glossopharyngeal dysfunction (eg, hoarseness, difficulty swallowing) and/or spinal accessory symptoms (eg, trapezius atrophy).
Schwannomas involving the oculomotor, trochlear, and abducens nerves are rare. Presenting symptoms can include palsy of the affected muscle and ipsilateral cavernous sinus symptoms if the mass is in the cavernous sinus.
Hypoglossal schwannomas are rare lesions that can present with ipsilateral deviation of the tongue, possibly with associated ipsilateral hemiatrophy.6
Preferred Examination
MRI with the use of gadolinium-based contrast medium is the technique of choice for imaging the CNs.1,7,4 MRI provides the highest degree of soft tissue resolution, it can provide images in multiple planes, and it is not encumbered by bone artifact from the skull base. CT is ideal for evaluating the secondary effects on the neural foramen.2
Limitations of Techniques
CT evaluation is limited primarily to the assessment of bony changes in the skull base. Artifact from the skull base limits the soft tissue resolution of CT, particularly in small lesions.2 Plain radiography has no role in the evaluation of the lesions.
Aside from a patient's claustrophobia or incompatible hardware, the only significant imaging drawback of MRI is that CT can be more sensitive in depicting adjacent bone destruction.2
Differential Diagnoses
Brain, Lymphoma
Brain, Metastases
Glomus Tumor (Head and Neck)
Meningioma, Brain
Neurofibromatosis Type 1
Neurofibromatosis Type 2
Other Problems to Be Considered
The differential diagnosis varies with the location, but meningiomas can occur in similar regions and have similar imaging appearances (see the eMedicine articles Meningioma, Brain and Meningioma, Spine). The differing growth patterns, as well as the dural tail and associated hyperostosis that can be seen with meningiomas, are often helpful differentiating factors (see Image 5).
Cerebrospinal fluid (CSF) spread of metastatic disease or lymphoma can appear as a focal CN mass (see Image 16, Image 18). In patients with carcinomatous meningitis and lymphoma, focal metastatic masses can involve the cranial nerves and mimic a schwannoma.
Neuritis (ie, inflammation of a nerve) can be confused with a mass (see Images 10-11).
NF2 is one of the phacomatoses characterized by multiple intracranial schwannomas, meningiomas, and ependymomas. Bilateral vestibular schwannomas are diagnostic of this entity, but patients can have schwannomas involving any CN (CN III-XIII) (see Image 6).
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References
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Gilmer-Hill HS, Kline DG. Neurogenic tumors of the cervical vagus nerve: report of four cases and review of the literature. Neurosurgery. Jun 2000;46(6):1498-503. [Medline].
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Katsumata Y, Maehara T, Noda M, Shirouzu I. Neurinoma of the oculomotor nerve: CT and MR features. J Comput Assist Tomogr. Jul-Aug 1990;14(4):658-61. [Medline].
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Further Reading
Keywords
neuroma; CN schwannomas; Schwann cells; vestibular schwannomas; trigeminal schwannomas; facial schwannomas; glossopharyngeal schwannomas; vagus schwannomas; spinal accessory schwannomas; oculomotor schwannomas; hypoglossal schwannomas; abducens schwannomas; trochlear schwannomas; neurofibromatosis type 2; NF2; multiple inherited schwannomas, meningiomas, and ependymomas syndrome; MISME syndrome
