Chapter 62. Nonacoustic Lesions of the Cerebellopontine Angle

Vestibular schwannomas (acoustic neuromas) account for 80% of all lesions of the cerebellopontine angle (CPA). This chapter discusses some of the other common neoplasms (meningiomas and epidermoid cysts) as well as uncommon tumors of the CPA that commonly present with injury to the cochlear–vestibular system. Each of these tumors has a similar clinical presentation; they are primarily differentiated by their imaging characteristics.

The CPA consists of a potential space filled with cerebrospinal fluid (CSF) in the posterior cranial fossa bounded by the temporal bone, the cerebellum, and the brainstem. The CPA is traversed by cranial nerves V–XI and most prominently the facial (CN VII) and vestibulocochlear (CN VIII) nerves. CPA tumors account for 10% of all intracranial tumors (Table 62–1). Nearly 90% of all CPA tumors include vestibular schwannomas (acoustic neuromas) and meningiomas. Other CPA lesions include congenital rest lesions (eg, epidermoid cysts, arachnoid cysts, and lipomas), schwannomas of other cranial nerves, intra-axial tumors, metastases, vascular lesions (eg, paragangliomas and hemangiomas), and lesions extending from the skull base (cholesterol granulomas and chordomas). CPA lesions become clinically symptomatic by causing compression of the neurovascular structures in and around the CPA. The classic description of these symptoms initially includes unilateral hearing loss, vertigo, altered facial sensation, facial pain that later progresses to nystagmus, facial palsy, vocal cord palsy, dysphagia, diplopia, respiratory compromise, and death (Table 62–2).

Essentials of Diagnosis

• Asymmetric (unilateral) sensorineural hearing loss, tinnitus, or both
• More likely than vestibular schwannomas to have facial or trigeminal nerve findings, or both
• Dural tail and calcification are distinctive on imaging.

General Considerations

Meningiomas are the second most common CPA tumors and account for 3–10% of neoplasms at this location. Compared with schwannomas, meningiomas are a more heterogeneous group of tumors with regard to pathology, anatomic location, and treatment outcome. Most of these tumors are benign and slow growing; 1% will become symptomatic. Meningiomas differ in pathogenesis, anatomic location, and imaging characteristics from vestibular schwannomas but are nearly indistinguishable in terms of clinical presentation and audiovestibular testing. Meningiomas are primarily managed by surgical excision.

Pathogenesis

Meningiomas arise from arachnoid villi cap cells and are located along dura, venous sinuses, and neurovascular foramina. Meningiomas are most commonly sporadic but may occur in familial syndromes such as NF2, Werner syndrome, and Gorlin syndrome. More than one-third of patients with NF2 have meningiomas. Molecular studies have shown deletions in chromosome 22 in nearly 75% of meningiomas. Specifically, mutations in the NF2 gene encoding the merlin protein have been shown in 30–35% of meningiomas. Although most meningiomas are benign, 5% are malignant. Chromosomal abnormalities in 1p, 6q, 9p, 10q, and 14q are seen in more aggressive or malignant meningiomas.

Clinical Findings

Symptoms and Signs

From the onset of symptoms, meningiomas take an average of 5 y to be diagnosed. The typical patient is a woman in her fourth or fifth decade of life. Unlike vestibular schwannomas, there is a 2:1 female bias. Meningiomas presenting in younger patients or multiple meningiomas in the same patient should prompt an evaluation for NF2. The most common complaints are the same as vestibular schwannoma and include unilateral hearing loss (80%), vertigo or imbalance (75%), and tinnitus (60%). The symptoms and signs more common to meningiomas relative to vestibular schwannomas include trigeminal neuralgia (7–22%), facial paresis (11–36%), lower cranial nerve deficits (5–10%), and visual disturbances (8%).

Imaging Studies

Imaging provides the diagnosis of meningioma and allows the differentiation between meningioma and vestibular schwannoma (Table 62–3).

Computed Tomography Scanning

Computed tomography (CT) scanning without contrast shows an iso- or hyperdense mass with areas of calcification in 10–26% of cases and provides information regarding hyperostosis or bony invasion. Meningiomas enhance homogeneously with CT contrast, and 90% of these neoplasms can be detected by contrast-enhanced CT.

Magnetic Resonance Imaging

MRI is the study of choice for the diagnosis of meningiomas. Meningiomas are hypo- to isointense on MRI T1-weighted images and have a variable intensity on T2-weighted images. Areas of calcification appear dark on both T1- and T2-weighted images (Figure 62–1A and B). Unlike vestibular schwannomas, meningiomas are broad based (sessile) and usually not centered over the porous acoustics. The broad-base attachment to the petrous wall leads to an obtuse bone-tumor angle. There is no widening of the internal auditory canal (IAC). Also unlike vestibular schwannomas, meningiomas more commonly herniate into the middle fossa. T1-weighted enhanced images can show an enhancing dural tail (meningeal sign) adjacent to the bulk of the tumor in 50–70% of meningiomas.

Special Tests

The hearing loss on an audiogram has no characteristic pattern. The speech discrimination scores suggest a retrocochlear pathology in 50% of cases. The auditory brainstem response may be normal in 25% of cases.

Treatment

The management options include surgery, stereotactic radiotherapy, and observation. The latter two are indicated in patients with limited life expectancy or in whom the expected morbidity of surgical excision is not justified.

Surgical Measures

Surgical treatment ideally consists of total meningioma removal, excision of a cuff of surrounding dura, and drilling of the underlying bone. The surgical approach is based on the tumor location and the patient's hearing status.

In contrast to vestibular schwannomas, the anatomic location of posterior fossa meningiomas is varied. The site of the meningioma is a major determinant of types of morbidity from the tumor and the success of treatment. A simple classification differentiates whether the tumor is medial or lateral to the IAC. Meningiomas medial to the IAC are more common. These meningiomas commonly arise along the inferior petrosal sinus and may involve the petrous apex, the lateral clivus, and Meckel cave. Meningiomas lateral to the IAC involve the sigmoid sinus, the jugular bulb, and the superior petrosal sinus. In an uncommon pattern, meningiomas may be centered on the IAC and most closely mimic a vestibular schwannoma. IAC meningiomas may invade the inner or middle ear. Meningiomas may also be superior to the IAC and considered a midpetrosal meningioma.

Meningiomas lateral to the IAC are approached via a retrosigmoidal approach. The facial nerve in lateral meningiomas is most often displaced anteriorly and so does not lie between the surgeon and the tumor. Therefore, the facial nerve is less traumatized during tumor removal. The retrosigmoidal approach also allows for hearing preservation. Limited intracanalicular meningiomas may be managed by the middle cranial fossa approach, especially if hearing preservation is possible. Meningiomas involving the IAC in patients with poor hearing are approached via the translabyrinthine approach. If the tumor invades the cochlea and has an anteromedial extension to the clivus or Meckel cave, then a transcochlear approach should be considered. The transcochlear approach sacrifices hearing and requires rerouting of the facial nerve. Sixty percent of CPA meningiomas involve the middle fossa and may require a craniotomy of the combined middle and posterior fossae. The type of posterior fossa craniotomy in the combined approach depends on the need for hearing preservation and the extent of the surgical exposure required.

Adjunctive Therapies

Adjunctive therapies include external-beam radiation and stereotactic radiation therapy. Radiation therapy should be considered in cases of inoperable tumors, subtotal resection, recurrent tumors, and malignant tumors.

Prognosis

Total tumor removal is accomplished in 70–85% of meningioma cases. Incomplete tumor removal is often associated with either adherence of the meningioma to the brainstem or cavernous sinus involvement. The long-term recurrence after total tumor removal is between 10% and 30%, whereas that of subtotal removal is more than 50%. In contrast to vestibular schwannoma, hearing preservation is more likely and approaches 70%. The facial nerve function has a 17% rate of deterioration from preoperative levels. Less frequently, gait disturbance and CSF leak may occur (8%). The mortality rate is between 1% and 9%.

Essentials of Diagnosis

• Asymmetric (unilateral) sensorineural hearing loss, tinnitus, or both
• More likely than vestibular schwannomas to have facial or trigeminal nerve findings, or both
• A distinguishing characteristic relative to vestibular schwannomas and meningiomas is that epidermoid cysts show no enhancement with intravenous contrast.

General Considerations

Epidermoid cysts are much less common than vestibular schwannomas or meningiomas. They account for approximately 5% of CPA lesions. Epidermoid lesions are slow growing and often grow to a significant size before causing CPA symptoms because they initially grow around structures via pathways of least resistance rather than cause compression. Epidermoid cysts are treated by surgical excision, but total removal is more difficult than vestibular schwannomas because they become adherent to normal structures.

Pathogenesis

Epidermoid cysts likely develop from ectodermal inclusions that become trapped during embryogenesis. These ectodermal inclusions lead to a keratinizing squamous epithelium in the CPA. The squamous epithelium produces a cyst filled with sloughed keratinaceous debris. The gross appearance is of a nodular cyst. The cyst is lined with squamous epithelium and filled with lamella of desquamated keratinaceous debris. Most epidermoid cysts are benign, with rare reports of squamous cell carcinoma arising in epidermoid lesions.

Clinical Findings

Symptoms and Signs

The presentation of epidermoid cysts is similar to other CPA lesions, with hearing loss being the most common symptom. Epidermoid cysts have a higher rate of preoperative facial and trigeminal nerve involvement–40% and 50%, respectively–compared with vestibular schwannomas. Patients may present with hemifacial spasm, facial hypesthesia, neuralgia, or wasting of the muscles of mastication.

Imaging Studies

On CT scanning, epidermoid cysts are hypodense compared with the brain. A distinguishing characteristic relative to vestibular schwannomas and meningiomas is that epidermoid lesions show no enhancement with intravenous contrast. Epidermoid cysts have irregular borders, are not centered on the IAC, and do not usually widen the IAC (Figure 62–2). Epidermoid cysts have imaging characteristics similar to CSF on MRI (hypointense on T1-weighted imaging and hyperintense on T2-weighted imaging) and do not enhance with gadolinium contrast.

Special Tests

Audiovestibular testing does not show any patterns distinguishing for epidermoid cysts.

Treatment

The primary treatment of epidermoid cysts is surgical. The approaches include a retrosigmoidal approach for hearing preservation and a translabyrinthine approach in patients with significant hearing loss. Any extension into the middle fossa can usually be removed via a posterior fossa craniotomy. The ability to completely remove the tumor is limited by the propensity of epidermoid cysts to adhere to neurovascular structures. Attempts at complete tumor removal may increase the rate of postoperative transient or permanent cranial nerve palsies.

Prognosis

Total tumor removal is accomplished in less than 50% of cases, and the recurrence rate may be as high as 50%. Regardless, most patients have excellent or good postoperative function.

Nonvestibular schwannomas represent more than 95% of all CPA schwannomas. In addition to CN VIII (the vestibulocochlear nerve, schwannomas of cranial nerves V (the trigeminal nerve), VII (the facial nerve), IX (the glossopharyngeal nerve), X (the vagus nerve), XI (the accessory nerve), and XII (the hypoglossal nerve) can involve the CPA. CPA schwannomas share clinical, pathological, and imaging characteristics. The primary treatment, similar to that for vestibular schwannoma, is surgical resection. The surgical approach is based on the location of the schwannoma and the patient's hearing status. Resection of cranial nerve schwannomas may lead to significant cranial nerve dysfunction; therefore, preoperative cranial nerve function and postoperative rehabilitation are important issues to consider.

Facial Nerve Schwannomas

Facial nerve schwannomas most commonly occur at the geniculate ganglion, but can involve any portion of the facial nerve. Similar to a vestibular schwannoma, a facial nerve schwannoma presents with hearing loss, tinnitus, and imbalance (CPA symptoms). Facial nerve symptoms such as facial spasm or weakness usually present with larger tumors. Audiovestibular testing shows an abnormality in acoustic reflex testing because of impairment of the CN VII motor supply to the stapedius muscle. Facial nerve schwannomas are associated with the reduction of electroneuronography (ENOG) potentials on the ipsilateral side, even when there is no clinically evident palsy. Imaging often does not allow for the differentiation between a vestibular and a facial nerve schwannoma.

Distinguishing features on imaging of facial nerve schwannomas include expansion of the fallopian canal, extension from the geniculate ganglion into the middle fossa, and location of the schwannoma in the anterior superior portion of the IAC (a position eccentric to the axis of the IAC). These schwannomas may be observed until facial nerve function has deteriorated or a neurological complication becomes imminent since resection usually requires division and grafting of the facial nerve. Mimetic function following interposition grafting is poor and is limited to House–Brackmann Grade 3 functioning at best.

Trigeminal Nerve Schwannomas

Trigeminal nerve schwannomas initially present with ipsilateral facial hypesthesia, paresthesias, neuralgia, and difficulties with chewing. Trigeminal schwannomas arise from the gasserian ganglion in the middle fossa and grow posteriorly to involve the CPA or arise from the root of the nerve and directly involve the anterior CPA and Meckel cave. These tumors frequently involve both the middle and the posterior fossa, and a combined approach may be necessary for resection.

Lower Cranial Nerve Schwannomas

Schwannomas of CN IX, X, and XI cause smooth enlargement of the jugular foramen and may grow superiorly into the CPA or inferiorly into the parapharyngeal space. Schwannomas of these cranial nerves produce symptoms based on their cranial nerve functions, thereby causing hypesthesia and weakness of the palate, vocal cords, and shoulders, respectively. Patients present with dysphagia, hoarseness, and shoulder weakness. CPA involvement also leads to CPA symptoms. Schwannomas of cranial nerve XII cause hemiatrophy of the tongue and expansion of the hypoglossal canal. The treatment is surgical removal and rehabilitation of the patient's functional deficit.

Congenital rest lesions involving the CPA include epidermoid cysts, arachnoid cysts, and lipomas. These lesions occur because of errors in embryogenesis that allow vestigial structures to remain and grow during adult life. These lesions are not aggressive but rather slow growing and have a tendency to envelop the neurovascular structures in the CPA. The presenting symptoms are very similar to, if not indistinguishable from, those of vestibular schwannoma, and only imaging allows their differentiation. The imaging characteristics on CT scan are very similar and include a well-encapsulated, hypodense mass that does not enhance with contrast. MRI allows differentiation based on the signal characteristics of desquamated epithelium, CSF, or fat. The treatment is surgical, but total removal is more difficult than in vestibular schwannoma and is not always necessary.

Arachnoid Cysts

Arachnoid cysts are CSF-filled cysts surrounded by an epithelial lining. This epithelial lining originates from a duplication of the arachnoid membrane and has secretory capabilities. The rate of growth is unpredictable and patients may present with arachnoid cysts in the CPA or IAC at any age. The key imaging point is that these cysts match the signal intensity of CSF on every MRI sequence and do not enhance with gadolinium. The treatment involves diuretics, shunting procedures, and marsupialization of the cyst into the subarachnoid space.

Lipomas

Lipomas are rare lesions of the CPA and IAC. They are due to congenital malformations that lead to proliferation of adipocytes in subarachnoid cisterns or ventricles. MRI parallels the intensities of fat, and so lipomas are hyperintense on T1-weighted imaging, show no enhancement with gadolinium, are hypointense on T2-weighted imaging, and become hypointense on T1-weighted imaging with fat suppression. The neurovascular structures of the CPA travel through the lipoma. Therefore, the surgical treatment of these lesions, if they become symptomatic, is conservative debulking.

A variety of vascular lesions may directly or via extension involve the CPA. These include paragangliomas (glomus jugulare neoplasms), hemangiomas, and aneurysms.

Paragangliomas (Glomus Jugulare Neoplasms)

Glomus jugulare tumors arise from paraganglionic tissues (chief cells) and have intracranial extension in 15% of cases. These neoplasms are slow growing and present initially with pulsatile tinnitus and conductive hearing loss. Further growth at the jugular foramen causes lower cranial nerve neuropathies, and then intracranial extension into the posterior fossa may lead to sensorineural hearing loss and dizziness. Paragangliomas have a variable appearance on CT scans, but bone algorithms show the extent of temporal bone involvement (Figure 62–3). Paragangliomas cause irregular expansion of the jugular foramen, whereas lower cranial nerve schwannomas cause smooth enlargement of the jugular foramen. Paragangliomas have a “salt and pepper” appearance on T2-weighted MRI scans, and MRI shows the extent of intracranial involvement. There is presence of flow voids and pronounced enhancement with gadolinium. Magnetic resonance angiography (MRA) and angiography provide information on the involvement of the great vessels and allow for preoperative embolization of larger neoplasms. The treatment requires surgical excision. Jugular bulb involvement may be addressed by a transmastoid-neck approach. Extension to the carotid artery or intracranial extension requires an infratemporal fossa approach.

Hemangiomas

Hemangiomas of the temporal bone often involve the geniculate ganglion and the internal auditory meatus. Hemangiomas are benign, slow-growing vascular hamartomas. The hemangiomas involving the geniculate ganglion cause progressive facial paresis. Patients also complain of facial twitches, tinnitus, and facial pain. Hearing loss, if present, is conductive because of middle ear involvement. The facial paresis occurs sooner with hemangiomas than with facial nerve schwannomas.

CT imaging shows a small, soft tissue mass at the geniculate ganglion, with surrounding smooth or irregular bony enlargement of the fallopian canal. The small size of the soft tissue mass, irregular bony erosion, and presence of calcium in the tumor are all suggestive of a geniculate ganglion hemangioma versus a facial nerve schwannoma. MRI shows isointense T1-weighted images, intense enhancement, and hyperintense T2-weighted images.

Hemangiomas in the IAC present similarly to vestibular schwannomas, and a preoperative differentiation may be very difficult. The treatment involves surgical removal when there is significant facial nerve dysfunction. Because hearing is often intact, the middle fossa approach provides good surgical exposure and allows for hearing preservation. The chance of an intact facial nerve after removal of a hemangioma is higher than with a facial nerve schwannoma. Regardless, facial nerve anastomosis or grafting is often still required.

Aneurysms

Aneurysms and vascular anomalies of the posterior circulation (anterior and posterior cerebellar artery, carotid artery, vertebral artery, and basilar artery) are rare but produce CPA symptoms by causing compression on neurovascular structures. Aneurysms are seen as enhancing lesions on CT scanning. In addition to MRI, MRA and angiography allow the characterization of these vascular lesions.

Intra-axial tumors of the brainstem (gliomas), cerebellum (medulloblastomas, astrocytomas, and hemangioblastomas), and fourth ventricle (ependymomas and choroid plexus papillomas) may extend into the CPA and present with CPA symptoms. The CPA extension occurs as a result of exophytic growth, growth into the CPA via the foramen of Luschka, and, rarely, extra-axial origin directly in the CPA from an embryonic rest.

To appropriately counsel and treat patients, intra-axial tumors involving the CPA must be differentiated from extra-axial CPA masses. The differentiation is based primarily on imaging characteristics. Imaging characteristics suggestive of an extra-axial neoplasm include bony changes, widening of the subarachnoid cistern, displacement of brain and blood vessels away from the skull or dura, and sharp definition of the tumor margin. Characteristics suggestive of intra-axial tumors include irregular and poorly defined brain tumor margins, widening of the foramen of Luschka, brain edema out of proportion to the CPA component of the tumor, and hydrocephalus. The management of intra-axial lesions includes angiography, conservative surgical resection, and adjunctive therapy.

Lesions involving the skull base–specifically, the petrous apex (cholesterol granulomas), the clivus (chordomas), and the petrooccipital fissure (chondrosarcomas)–may grow posteriorly and laterally to involve the CPA. Each of these lesions has characteristic presentations and imaging criteria but at times may present solely with CPA symptoms. The treatment is primarily surgical.

Cholesterol Granulomas

The petrous apex of the temporal bone lies anterior and medial to the inner ear and posterior and lateral to the clivus and contains pneumatized air cells in one-third of temporal bones. The obstruction of these air cells leads to inflammation and hemorrhage into the air cells. The phagocytosis of red cells leads to deposition of cholesterol crystals and a foreign body reaction in the petrous apex. This process leads to a cholesterol granuloma that may extend beyond the petrous apex to involve the CPA. Patients may have CPA symptoms in addition to symptoms of headache and sixth cranial nerve (the abducens nerve) dysfunction.

The key differential diagnosis in the petrous apex and CPA of a cholesterol granuloma is an epidermoid cyst. The distinguishing factor is that cholesterol granulomas are hyperintense on T1- and T2-weighted MRI scans, whereas epidermoid cysts are hypointense on T1-weighted images. When symptomatic, the treatment is surgical drainage rather than complete excision. The petrous apex may be accessed via a transmastoid or transcanal approach.

Chordomas

Chordomas arise from remnants of the notochord, and skull base chordomas occur at the clivus (sphenooccipital synchondrosis). Patients usually present with headache and diplopia, but posterolateral extension to the CPA may lead to CPA symptoms. CT scans show an isodense mass with bone destruction, intratumor calcifications, and marked enhancement with contrast. MRI shows hypointense T1-weighted images, marked gadolinium enhancement, and hyperintense T2-weighted images. The midline location and bony destruction without sclerosis are characteristics of chordomas. The treatment is complete surgical excision and radiation for subtotal removal or recurrence.

Chondrosarcomas

The main differential diagnosis of chordomas is chondrosarcomas. These tumors arise along the sphenooccipital synchondrosis and are laterally located relative to chordomas. Chondrosarcomas may arise from embryonal cartilage rests located at the skull base synchondrosis. Tumor growth laterally causes involvement of the IAC and CPA. Hearing loss may very well be the presenting complaint. The differential diagnosis from chordoma may require immunohistochemistry stains since chondrosarcomas, unlike chordomas, do not stain positively with epithelial tissue markers.

Metastatic disease from the lungs, breasts, skin, prostate, nasopharynx, and kidney are the most common extra-axial malignant neoplasms of the CPA. In contrast to benign lesions of the CPA, malignant lesions of the CPA cause rapid progression of CPA symptoms. On imaging, the lesions are small, isointense to brain on T1- and T2-weighted images, and enhanced with gadolinium. There is high likelihood of parenchymal brain metastases and bilateral CPA lesions. The extent of treatment is based on the extent of the metastatic and primary disease and includes multimodality treatment with surgical biopsy or resection, radiation, and chemotherapy. The other aspect of treatment includes relieving symptoms of hydrocephalus or brainstem compression. The primary extra-axial malignant neoplasms of the CPA are exceedingly rare and include lymphomas, squamous cell carcinomas, malignant acoustic neuromas, and malignant meningiomas.