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(23-41) CN XII exits medulla at preolivary sulcus. CN IX is in pars nervosa of jugular foramen. CNs X and XI are in the pars vascularis.

(23-42) Axial bone CT shows hypoglossal canals. Foramen ovale (CN V ), foramen spinosum(middle meningeal artery) are also seen.

(23-43) Hypoglossal canal (CN XII exit), jugular foramen, jugular spine . CN IX exits pars nervosa , CNs X, XI exit pars vascularis .

forms the medial wall of the short, obliquely oriented hypoglossal canal (2342).

In the coronal plane, the jugular tubercle and occipital condyle form a visual construct that resembles the head, beak, and body of an eagle. The hypoglossal canal and nerve are located between its "head" and "beak" (2337) (23-43).

A hypoglossal lesion causes atrophy of the ipsilateral tongue muscles. Look for fatty infiltration and volume loss.

Schwannomas

Neuropathologists now recognize four histologic subtypes of schwannoma: conventional, cellular, plexiform, and melanocytic. The vast majority of schwannomas that involve cranial nerves are the conventional type. With the exception of melanotic schwannoma, imaging findings do not distinguish between histologic subtypes. Rather, conventional intracranial schwannomas are distinguished—and discussed here—according to their cranial nerve of origin.

Because the pathology of intracranial schwannomas is similar, we discuss it and other shared features before delving into specific schwannomas.

Schwannoma Overview

Terminology

Schwannomas are benign slow-growing encapsulated tumors that are composed entirely of well-differentiated Schwann cells. Less common terms are neurinoma and neurilemmoma.

Etiology

General Concepts. Schwannomas originate from Schwann cells, which are derived from precursor cells in the embryonic neural crest. Schwannomas may arise along the course of any peripheral nerve or cranial nerves III-XII.

Because the olfactory and optic nerves do not contain Schwann cells, schwannomas do not arise from CNs I and II. The rare reported cases of "olfactory groove schwannomas" are probably tumors that arise from olfactory ensheathing cells.

Schwann cells are also not a component of normal brain parenchyma. The exceptionally rare intraparenchymal schwannoma is thought to arise from neural crest remnants that later express aberrant Schwann cell differentiation. Intramedullary (spinal cord) schwannomas are more common than intraparenchymal brain schwannomas.

Genetics. The neurofibromatosis type 2 (NF2) tumor suppressor Merlin is a protein whose loss results in defective morphogenesis and tumorigenesis in multiple tissues. Genetic studies have linked both sporadic and NF2associated schwannomas [especially vestibular schwannomas (VSs)] to the NF2 tumor suppressor gene located on chromosome 22. Approximately half of NF2 cases represent new mutations, suggesting a high mutation rate in this gene.

Biallelic inactivation (the classic "two-hit mechanism") of the NF2 gene is also detected in nearly all sporadic vestibular schwannomas and 50-70% of meningiomas.

Inherited Tumor Syndromes. The most common tumor predisposition syndrome that causes multiple schwannomas is NF2. The presence of bilateral VSs is pathognomonic of NF2. One VS and a first-degree relative

with NF2 or a VS in combination with another cranial nerve schwannoma, meningioma, or glioma is also indicative of NF2.

Schwannomatosis is a condition with multiple peripheral, often painful schwannomas without other features of NF2. These patients have no evidence of VS, no first-degree relative with NF2, and no known constitutional NF2 mutation.

Schwannomatosis occurs in 2-4% of patients with schwannomas. Both sporadic and familial forms of schwannomatosis occur and can be associated with both nonvestibular intracranial and spinal schwannomas.

Patients with schwannomatosis tend to be younger than those who present with solitary schwannomas. The average age at onset is 28.5 years.

Plexiform schwannoma, also known as multinodular schwannoma, is a Schwann cell tumor in which multiple (2-50) circumscribed lesions occur along an affected nerve fascicle. Most are dermal-subcutaneous tumors of the extremities, trunk, head, and neck. Brain lesions have not been reported.

Approximately 90% of plexiform schwannomas are sporadic with 5% associated with NF2 and 5% with schwannomatosis. Unlike patients with NF1 and plexiform neurofibromas, there

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(23-44) Olfactory nerve (I), optic chiasm (II), oculomotor nerve (III), trochlear nerve (IV) coursing anteriorly in the ambient cistern, and the trigeminal nerve (V) with its ophthalmic (V ), maxillary (V ), and mandibular (V ) branches are shown. Abducens (VI) and hypoglossal (XII) nerves are not visualized. CPA segments of the facial (VII) and vestibulocochlear (VIII) nerves are shown. Glossopharyngeal (IX), vagus (X), and spinal accessory (XI) nerves course toward the jugular foramen. (Courtesy M. Nielsen, MS.)

is no known predilection for malignant degeneration of plexiform schwannomas.

Carney complex (CNC) or Carney syndrome (MIM) is a rare autosomal-dominant syndrome characterized by pigmented lesions of the skin and mucosa, cardiac, cutaneous, and other myxomas, and multiple endocrine tumors. Melanotic schwannomas occur in up to 10% of patients with CNC.

Pathology

Location. Schwannomas arise at the glial-Schwann cell junction of CNs III-XII. The distance from the brain to the interface where the glial covering terminates and Schwann cell ensheathing begins varies with each cranial nerve. In some—such as the oculomotor nerve (CN III)—the junction is in close proximity to the brain. Here schwannomas arise close to the exit of the parent nerve from the brain. In others—such as the vestibulocochlear nerve (CN VIII)—the junction lies at some distance from the nerve exit or entrance into the brainstem.

Sensory nerves are much more commonly affected by schwannomas compared with pure motor cranial nerves. The vestibulocochlear nerve is by far the most common

(23-45) Axial (L) and sagittal (R) graphics show a schwannoma arising within a unifascicular nerve. The tumor displaces other nerve fibers peripherally .

intracranial site (90%). The second most common site is the trigeminal nerve (CN V) (2-4%).

INTRACRANIAL SCHWANNOMAS

Synonyms

• Neurilemoma, neurinoma

Epidemiology

•Vestibular (CN VIII) most common (95%)

○All other sites combined (1-5%)

•Trigeminal (CN V) second most common

•Jugular foramen (CNs IX, X, XI) third

•Hypoglossal (CN XII) fourth

•All others rare except in neurofibromatosis type 2

•Intraparenchymal schwannomas very rare

Pathology

•Arise at glial-Schwann cell junction

○Distance from brain varies according to cranial nerve

•Benign encapsulated nerve sheath tumor

•Well-differentiated neoplastic Schwann cells

•Biphasic histology with 2 components

○Compact, highly ordered cellularity ("Antoni A")

○Less cellular, myxoid matrix ("Antoni B")

Schwannomas of cranial nerves other than CN VIII and CN V are very rare, accounting for just 1-2%. As a group, jugular foramen schwannomas (i.e., schwannomas arising from the glossopharyngeal, vagal, and spinal accessory nerves) are the third most common, followed by facial (CN VII) and hypoglossal (CN XII) schwannomas. In the absence of NF2, schwannomas of CNs III, IV, and VI are all very rare.

(23-46) Schwannomas are encapsulated by perineurium and epineural collagen and grow eccentrically to the nerve of origin. (From DP: Neuropathology, 2e.)

Intraparenchymal and intraventricular schwannomas occur but are extremely uncommon.

Size and Number. Most intracranial schwannomas are small, especially those that arise from motor nerves. Some, especially trigeminal schwannomas, can attain huge size and involve both intraand extracranial compartments.

Most schwannomas occur singly in otherwise healthy individuals and are termed "sporadic" or "solitary" schwannomas. The presence of multiple schwannomas in the same individual suggests an underlying tumor predisposition syndrome.

Gross Pathology. Schwannomas arise eccentrically from their parent nerves and are smooth or nodular well-encapsulated lesions (23-45) (23-46). Cystic change is common, as is yellow discoloration due to lipidization. Microhemorrhages occur, but gross macroscopic bleeds are rare.

Microscopic Features. A biphasic pattern is typical of conventional schwannoma. The "Antoni A" pattern consists of compact fascicles of elongated spindle cells that demonstrate occasional nuclear palisading (Verocay bodies). A less cellular, loosely textured, more haphazard arrangement with clusters of lipid-laden cells is called the "Antoni B" pattern (23-48). Mitotic figures are rare. Immunohistochemistry is characterized by strong diffuse positivity for S100 protein.

Cellular schwannoma consists mostly of "Antoni A" tissue but lacks Verocay bodies. Such tumors may demonstrate hypercellularity and minor nuclear atypia. Frequent mitotic figures and increased proliferative indices can be seen in young children. Cellular schwannomas do not undergo malignant transformation.

(23-47) Graphic of a large vestibular schwannoma shows the typical "ice cream on cone" morphology. Note the prominent CSF-vascular "cleft" between the middle cerebellar peduncle and the cerebellar hemisphere .

Plexiform schwannoma can be either conventional or cellular type.

Staging, Grading, and Classification. Conventional schwannomas correspond to WHO grade I.

Clinical Issues

Epidemiology. The vast majority of schwannomas occur outside the CNS, most often in the skin and subcutaneous tissues. Intracranial schwannomas are relatively uncommon, constituting about 7% of all primary neoplasms.

Demographics. All ages are affected, but the peak incidence is in the fourth to sixth decades. Schwannomas do occur in children but are uncommon unless associated with NF2. There is no sex predilection.

Presentation. Many schwannomas are asymptomatic, and any symptoms are location specific. As schwannomas favor sensory nerves, motor symptoms are rare. Vestibular schwannoma, the most common intracranial schwannoma (23-47), presents with sensorineural hearing loss (see below).

Natural History. Schwannomas are benign tumors that tend to grow very slowly. The exception is VSs in young patients with NF2. These tumors have higher MIB-1 indices.

Recurrence after complete surgical removal of a conventional schwannoma is uncommon. Roughly 30-40% of cellular schwannomas recur after subtotal resection but do not undergo malignant transformation.

Schwannomas rarely—if ever—undergo malignant degeneration. Most "malignant intracerebral schwannomas"

(23-48) The juxtaposition of the cellular "Antoni A" and loose "Antoni B" patterns is classic for conventional schwannoma. (From DP: Neuropathology, 2e.)

are probably malignant peripheral nerve sheath tumors (see below).

Approximately 10% of melanotic schwannomas are malignant. In approximately half of these cases, the patients have Carney complex, an autosomal-dominant disorder characterized by lentiginous facial pigmentation, cardiac myxoma, and endocrine overactivity (e.g., precocious puberty, pituitary adenoma with acromegaly, and Cushing syndrome with multinodular adrenal hyperplasia).

Treatment Options. Depending on size, location, and symptoms, treatment options range from watchful waiting to surgery and stereotactic radiosurgery.

Imaging

General Features. Neuroimaging findings reflect their slow growth and benign biologic behavior. A well-circumscribed extraaxial mass that originates within or near a cranial nerve and displaces but does not invade adjacent structures is typical.

CT Findings. Most schwannomas exhibit low to intermediate attenuation on NECT scans and show smooth expansion/remodeling of osseous foramina on bone CT. Cystic change is common. Gross intratumoral hemorrhage is uncommon, and calcification is rare. Strong moderately heterogeneous enhancement after contrast administration is typical.

MR Findings. Schwannomas are generally isointense with cortex on T1WI and heterogeneously hyperintense on T2WI and FLAIR (23-50A). Although macroscopic intratumoral hemorrhage is rare, T2* (GRE, SWI) scans often reveal "blooming" foci of microbleeds (23-56).

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Secondary changes of muscle edema or denervation with atrophy and fatty infiltration can occur with motor nerve schwannomas. Vagal nerve schwannomas may cause vocal cord paralysis.

Virtually all schwannomas enhance intensely. Approximately 15% have nonenhancing intratumoral cysts. Nonneoplastic peritumoral cysts occur in 5-10% of cases, especially with larger lesions (23-55).

Differential Diagnosis

The differential diagnosis of a solitary enlarged enhancing cranial nerve includes schwannoma, multiple sclerosis, viral and postviral neuritis, Lyme disease, sarcoid, ischemia, and malignant neoplasms (metastases, lymphoma, and leukemia).

The most common cause of multiple enhancing cranial nerves is metastasis. NF2, neuritis (especially Lyme disease), lymphoma, and leukemia are significantly less common than

(23-49A) Thin-section CISS demonstrates a classic vestibular schwannoma (VS) with "ice cream on cone" appearance and thin CSF-vascular "cleft"where the tumor slightly indents the adjacent pons and middle cerebellar peduncle. (2349B) Strong, relatively uniform enhancement is seen on T1 C+ FS scan. The tumor extends to the IAC fundus .

(23-50A) Axial T2 FS shows heterogeneously hyperintense signal of typical large VS . The tumor indents the pons and cerebellar peduncle, deforming the fourth ventricle. Note CSF "cleft"between the tumor and brainstem. (23-50B) Precontrast T1WI (L) and postcontrast T1 C+ FS (R) in the same case show that the tumor enhances intensely but somewhat heterogeneously. This is a VS with conventional histology.

metastasis. Rare but important causes include multiple sclerosis and chronic inflammatory demyelinating polyneuropathy, a disorder that usually affects spinal nerves but may occasionally involve cranial nerves.

Vestibular Schwannoma

Terminology

Vestibular schwannoma (VS) is the preferred term for a CN VIII schwannoma. VSs are also known as acoustic schwannomas and acoustic neuromas.

Focal intralabyrinthine schwannomas, also known as inner ear schwannomas, form a special subgroup of CN VIII schwannomas (23-51). Intralabyrinthine schwannomas are named according to sublocation. Schwannomas within the cochlea are termed intracochlear. Lesions within the vestibule are called intravestibular schwannomas (23-53). If a schwannoma involves both the vestibule and the cochlea, it is

termed vestibulocochlear. A schwannoma that crosses the modiolus from the cochlea into the internal auditory canal (IAC) fundus is a transmodiolar schwannoma. If a lesion crosses from the vestibule into the IAC fundus, it is termed transmacular. Finally, an extensive schwannoma that crosses the entire inner ear from the IAC fundus to the middle ear is called transotic (23-54).

Etiology

General Concepts. VSs arise from the vestibular portion of CN VIII at the glial-Schwann cell junction, inside the IAC near the porus acusticus. Schwannomas rarely arise from the cochlear portion of CN VIII.

Genetics. The pathogenesis underlying both familial and most sporadic VSs has been linked to mutation in a single gene, the neurofibromin 2 (NF2) gene located on chromosome 22.

Nearly 60% of sporadic VSs have inactivating mutations of

NF2.

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Pathology

Location. VSs may occur at any location along the course of the nerve. Small VSs are often completely intracanalicular. Larger lesions frequently protrude medially through the porus acusticus into the cerebellopontine angle (CPA) cistern.

Size and Number. Small VSs are round or ovoid lesions that generally measure 2-10 mm in length. VSs that extend into the CPA cistern can become very large, up to 5 cm in diameter. Bilateral VSs are pathognomonic of NF2.

Clinical Issues

Epidemiology. VS is by far the most common intracranial schwannoma. VS is also the most common cerebellopontine cistern mass, accounting for 85-90% of lesions in this location.

Demographics. Peak presentation is 40-60 years of age. There is no sex predilection.

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Presentation. The most common presentation is in an adult with slowly progressive unilateral sensorineural hearing loss (SNHL). Small VSs may present initially with tinnitus. Large lesions often present with trigeminal and/or facial neuropathy.

Natural History. The growth rate of VSs varies. On average, they tend to enlarge between 1 or 2 mm per year. Approximately 60% grow very slowly (under 1 mm per year), whereas 10% of patients experience rapid enlargement of their lesions (more than 3 mm per year). Size and position at presentation also affect VS growth rate; intracanalicular VSs grow more slowly than extracanalicular lesions.

The growth rate of NF2-associated VSs is generally considered more aggressive compared with that of sporadic VSs.

Treatment Options. Treatment options vary. Watchful observation of small lesions with interval follow-up imaging is common, especially in older patients. Surgical removal or

(23-55A) Axial FIESTA in a

31y man with headaches, left facial numbness shows large VS with intratumoral and marginal cysts with fluidfluid level suggesting hemorrhage . Peritumoral cysts are present, exhibit different signal from normal CSF in CPA cistern . (23-55B) T1 C+ FS shows nonenhancing intratumoral cysts , rim enhancement along marginal and peritumoral cysts . This is cystic VS with intracystic hemorrhage.

(23-56A) Axial T2WI in a

67y man with left-sided sensorineural hearing loss (SNHL) shows a very heterogeneous-appearing mass with a large CPA component , smaller intracanalicular component (classic "ice cream on cone"). (23-56B) T2* GRE shows multiple blooming foci of intratumoral hemorrhage. The lesion enhanced very heterogeneously on T1 C+ (not shown). This is hemorrhagic VS.

stereotactic radiotherapy are other possibilities. The surgical approach of choice varies with tumor size and location, as well as whether hearing preservation is possible.

Imaging

General Features. The classic imaging appearance of VS is an avidly enhancing mass that looks like "ice cream on a cone" (23-49). Many VSs extend medially from their origin within the IAC. The intracanalicular part of the tumor represents the "cone." If a VS passes through the porus acusticus, it typically expands when it enters the CPA, forming the "ice cream" on the cone.

Precisely defining the size and extent of a VS is one of the most important goals of imaging. Some VSs remain as small slow-growing lesions that are entirely intracanalicular (23-51) (23-53). Many intracanalicular VSs have a distinctive fundal "cap" of CSF interposed between the lesion and the modiolus (23-52). Others grow laterally, extending deep into the IAC

fundus, and may eventually pass through the cochlear aperture into the modiolus (23-54).

CT Findings. CT is generally negative unless lesions are large enough to expand the IAC or protrude into the CPA cistern. VSs are generally noncalcified, appear mildly hyperdense on NECT, and enhance strongly and relatively uniformly on CECT. Bone CT may show IAC enlargement on the symptomatic side.

MR Findings. Full-brain FLAIR with axial and coronal fatsaturated T1 C+ imaging of the CPA and ICA is the standard. A "screening" study for adults with uncomplicated unilateral SNHL is a common option and is generally limited to highresolution T2WI, CISS, or FIESTA sequences. Detailed evaluation of the CPA/ICA can be performed with these sequences, reserving contrast-enhanced studies for patients with equivocal screening studies.

VSs are generally iso/hypointense with brain on T1WI (2350B). An intracanalicular VS appears as a hypointense filling

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defect within the bright CSF on CISS (23-49A). Larger VSs are isoto heterogeneously hyperintense on T2WI. Microhemorrhage on T2* is common although macroscopic hemorrhage is rare (0.4% of all newly diagnosed VSs but 5-6% of anticoagulated patients).

Virtually all VSs enhance strongly following contrast administration (23-49B). A schwannoma-associated "dural tail" sign occurs but is rare compared with CPA meningiomas.

Special imaging sequences may be helpful in the preoperative planning for VS surgery. MR tractography with threedimensional tumor modeling can depict the precise location of the CNs that surround large VSs.

Differential Diagnosis

The major differential diagnosis of VS is CPA meningioma. Most meningiomas "cap" the IAC and do not extend deep to the porus. However, a reactive dural "tail" in the IAC may make

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distinction between VS and meningioma difficult unless other dural "tails" along the petrous ridge are also present.

A facial nerve schwannoma confined to the IAC may be difficult to distinguish from a VS. Facial nerve schwannomas are much less common and usually have a labyrinthine segment "tail." Beware: extension along the labyrinthine segment of the facial nerve means the schwannoma arises from CN VII and is not a VS.

Metastases can coat the facial and vestibulocochlear nerves within the IAC. Metastases are usually bilateral with other lesions present.

Other CPA masses such as epidermoid cysts, arachnoid cysts, and aneurysms can usually be distinguished easily from VS. VSs occasionally have prominent intramural cysts, but a completely cystic schwannoma without an enhancing tumor rim is very rare.

(23-61) Coronal graphic of a vagal schwannoma shows the tumor enlarging and remodeling the bony margins of the jugular foramen . The "beak" of the "eagle" is eroded. (23-62A) NECT scan shows osseous remodeling of the right jugular foramen. The jugular spine is eroded, but the surrounding cortex appears intact (compare with Figure 23- 6).

(23-62B) Axial T1 C+ FS scan shows an enhancing mass in the jugular foramen . (23-62C) Coronal T1 C+ FS shows the intensely enhancing mass . Contrast this with the normally enhancing left jugular bulb and vein . At surgery, this jugular foramen schwannoma proved to be arising from the vagal nerve (CN X).

Trigeminal Schwannoma

Although trigeminal schwannomas are the second most common intracranial schwannoma, they are rare tumors. They may involve any part of the CN V complex, including extracranial peripheral divisions of the nerve. Nearly twothirds of all Meckel cave tumors are schwannomas.

The principal presenting symptoms involve sensory impairment in one or more of the three divisions. Trigeminal neuralgia can occur but is uncommon.

Imaging

Trigeminal schwannomas arise from the junction of the gasserian ganglion and the trigeminal nerve root (23-57). Small lesions may be confined to Meckel cave. They have a very characteristic appearance on coronal T2WI, the "winking Meckel cave" sign. Because at least 90% of each Meckel cave is normally filled with CSF, any lesion that fills the cave with soft

tissue contrasts sharply with the bright signal on the opposite normal side (23-58).

Bicompartmental tumors are common. Schwannomas that originate in Meckel cave can extend into the posterior fossa (through the porus trigeminus). These tumors have a characteristic "dumbbell" configuration (23-59).

Less commonly, bicompartmental tumors extend from the middle fossa anteroinferiorly through the foramen ovale into the masticator space. Tumors that involve all three locations are uncommon and are termed "three-compartment" trigeminal schwannomas (23-60).

Schwannomas that involve the mandibular division (CN V ) may cause denervation atrophy of the muscles of mastication.

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Differential Diagnosis

The appearance of a bior tricompartmental CN V schwannoma is distinctive. The major differential diagnoses of a Meckel cave schwannoma are meningioma and metastasis.

Jugular Foramen Schwannoma

Although schwannomas account for approximately 40% of all jugular foramen (JF) neoplasms, JF schwannomas constitute only 2-4% of all intracranial schwannomas.

Glossopharyngeal schwannomas are the most common JF schwannoma but are still rare, with just 42 cases reported between 1908 and 2008. The vast majority (85%) presented with vestibulocochlear symptoms secondary to compression and displacement, not CN IX symptoms. Glossopharyngeal schwannomas can occur anywhere along the course of CN IX, but the majority of symptomatic cases are intracranial/intraosseous.

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Compared with their extracranial counterpart, intracranial vagal schwannomas are rare. Purely intracisternal ones are even more unusual. Most vagal schwannomas are "dumbbell" lesions that extend from the basal cistern through the jugular foramen into the high deep carotid space (23-61) (23-62). When large, they may compress the ventrolateral medulla and cause refractory neurogenic hypertension.

Spinal accessory nerve schwannomas not associated with neurofibromatosis are very rare. They can be either intrajugular or intracisternal.

The major differential diagnoses of JF schwannoma include meningioma, glomus jugulare tumor, and metastasis. Only a JF schwannoma smoothly enlarges and remodels the jugular fossa.

Facial Nerve Schwannoma

Facial nerve schwannomas (FNSs) are rare lesions that can arise anywhere along the course of the facial nerve, from its origin in the CPA to its extracranial ramifications in the parotid space (23-63) (23-64). Depending on their location along CN VII, FNSs display several imaging patterns. The most common presentation is facial neuropathy.

CPA-IAC FNSs are radiologically indistinguishable from vestibular schwannomas if they do not demonstrate extension into the labyrinthine segment of the facial nerve canal. Lesions that traverse the labyrinthine segment often have a "dumbbell" appearance.

Almost 90% of FNSs involve more than one facial nerve segment (23-65). The geniculate fossa is the most common site, involved in more than 80% of all FNSs. The labyrinthine and tympanic segments are each involved in slightly over half of FNSs.

Geniculate fossa FNSs typically appear as a round or tubular enhancing mass in a smoothly enlarged facial nerve canal.

FNSs that track along the greater superficial petrosal nerve are seen as a round middle cranial fossa extraaxial mass (2366).

Tympanic segment FNSs often pedunculate into the middle ear cavity, losing their tubular configuration.

When the mastoid segment is involved, tumor may break into adjacent mastoid air cells and assume a more aggressive appearance, mimicking a malignant invasive tumor.

Schwannomas of Other Intracranial

Nerves

Motor nerve schwannomas are much less common than those arising from sensory or mixed sensory-motor nerves. Less than 1% of intracranial schwannomas arise from CNs III, IV, VI, and XII.

"Other" cranial schwannomas resemble their more common counterparts on imaging studies. Most motor nerve schwannomas are small, round or ovoid, well-delineated, strongly enhancing lesions.

Olfactory (CN I) "Schwannoma"

The cells that ensheathe the intracranial olfactory nerve are actually modified glial cells, not Schwann cells (see above). Primary tumors of the CN I nerve sheath are very rare. Once termed "olfactory groove schwannoma" or "subfrontal schwannoma," many of these neoplasms are more accurately called "olfactory ensheathing cell (OEC) tumors." True schwannomas of the olfactory groove may arise from meningeal branches of the trigeminal nerve or anterior ethmoidal nerves, not CN I.

Tumors of CN I typically present with anosmia. Many reach large size, causing frontal lobe signs such as emotional lability and complex partial seizures (23-67).

Esthesioneuroblastoma, also known as olfactory neuroblastoma, is a rare malignant tumor that arises in the superior nasal cavity from olfactory mucosa. Esthesioneuroblastoma is discussed with embryonal and neuroblastic tumors (see Chapter 21).

Optic Nerve (CN II) "Schwannoma"

Neoplasms of the optic nerve (a brain tract) are astrocytomas, not schwannomas. Intraorbital schwannomas arise from peripheral branches of CNs IV, V , or VI or from sympathetic or parasympathetic fibers (not the optic nerve).

Oculomotor (CN III) Schwannoma

Oculomotor schwannomas are rare but are the most common of all the pure motor nerve schwannomas (23-68). They can be asymptomatic or present with diplopia.

The most frequent location of a CN III schwannoma is in the interpeduncular cistern near the nerve exit from the midbrain (23-69). The second most common site is the cavernous sinus. Most intracranial oculomotor schwannomas are small, generally measuring less than 0.5 cm in diameter. Combined orbitocavernous sinus schwannomas are somewhat larger, extending from the orbit through the superior orbital fissure into the cavernous sinus.

Intraorbital schwannomas are rare. As a group, they account for just 1% of all orbital tumors. They typically present with slowly progressive proptosis and seldom develop diplopia.

Trochlear (CN IV) Schwannoma

Trochlear nerve schwannomas are uncommon (23-70). They cause diplopia (isolated unilateral superior oblique palsy) and compensatory head tilt that may be misdiagnosed clinically as "wry neck." Most CN IV schwannomas are small and are either simply watched or treated with prism spectacles.

Abducens (CN VI) Schwannoma

Schwannomas of the abducens nerve are extremely rare. Most patients present with a history of diplopia.

Schwannomas can occur anywhere along the entire length of the nerve, including its intracavernous and orbital segments. Most are found in the cerebellopontine angle cistern, adjacent

to the pontomesencephalic junction. They typically displace the facial/vestibulocochlear nerve complex posterosuperiorly and may be difficult to distinguish on imaging studies from schwannomas that arise from the cisternal segments of these nerves.

Hypoglossal (CN XII) Schwannoma

Hypoglossal tumors are the rarest of the "other" schwannomas, accounting for only 5% of all nonvestibular intracranial schwannomas (23-71). Over 90% present with denervation hemiatrophy of the tongue.

Most CN XII schwannomas originate intracranially but can also extend extracranially as a "dumbbell" tumor that expands and remodels the hypoglossal canal (23-72). Most are solid enhancing masses although cystic and even hemorrhagic CN XII schwannomas have been reported (23-73).

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Parenchymal Schwannomas

Because the brain parenchyma does not normally contain Schwann cells, so-called "ectopic" schwannomas not associated with cranial nerves are very rare (<1% of cases). Parenchymal schwannomas may arise from ectopic neural crest stem cells that become displaced during embryogenesis. Other possible etiologies include origin in the sympathetic nerve plexus that surrounds cerebral blood vessels.

Most intraparenchymal schwannomas are solitary and nonsyndromic. Approximately 15% are associated with NF2. The most common reported sites are the frontal and temporal lobes. Parenchymal schwannomas have also been described in the cerebellar hemispheres, vermis, brainstem, and cerebral ventricles (23-75).

Pathologically, two-thirds of cases are either cystic or contain areas of cystic degeneration. The histology of parenchymal

(23-74A) Axial T1 C+ scan shows a well-demarcated, enhancing parenchymal mass without edema. (23-74B) Sagittal T1 C+ scan in the same patient shows that the lesion is clearly intraaxial. Parenchymal schwannoma was diagnosed at histopathology.

(23-75A) Axial T2WI in an

84y man with headaches shows a mass in the fourth ventricle extending anterolaterally through an enlarged lateral recess. (23-75B) T1 C+ FS in the same case shows that the mass enhances strongly but heterogeneously. Preoperative diagnosis was subependymoma. Conventional schwannoma was found at histopathology.

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schwannoma is indistinguishable from that of other schwannomas.

On imaging studies, most intracranial parenchymal schwannomas appear well demarcated. The most common imaging pattern is that of a cyst with a mural nodule and peripheral enhancement. One-third are solid tumors with strong homogeneous or heterogeneous enhancement (2374). Peritumoral edema varies from none to moderate.

Melanotic Schwannoma

Melanotic schwannoma (MS) is a rare tumor composed of melanin-producing cells that have ultrastructural features resembling Schwann cells (23-76). Cytological atypia with hyperchromasia and macronucleoli is common. Two histologic MS subtypes, psammomatous and nonpsammomatous, are recognized.

Paraspinal and extraneural locations (e.g., skin, soft tissues, bone, and viscera) are the most common sites. Craniofacial and intracranial MSs are very rare.

Peak age at diagnosis is about a decade younger compared with conventional schwannoma. MSs can be sporadic or syndromic. Approximately half of all patients with psammomatous MS have Carney complex, an autosomaldominant disorder characterized by facial pigmentation, cardiac myxoma, and endocrine hyperactivity (including Cushing syndrome). Loss-of-function germline mutations of the PRKAR1A gene on chromosome 17q are characteristic. Malignant degeneration occurs in approximately 10% of PMSs associated with Carney complex.

Because melanin causes T1 shortening, most MSs are hyperintense on T1WI (23-77) and hypointense on T2WI (2378). Enhancement varies from minimal to striking.