Skull Base Tumor and Other CPA Tumors

Early in the 20th century, CPA lesions were difficult to diagnose and rarely completely excised. Indeed, the mortality rate for operative intervention in the posterior fossa approached 50%. However, revolutionary advances in neuroradiologic imaging and surgical techniques and approaches have made these lesions almost uniformly treatable with acceptable morbidity and very low rates of mortality. Surgical intervention remains the most definitive means for complete tumor removal for most of these lesions. Radiation therapy has gained popularity to arrest tumor growth with specific tumor types and is also an option for patients who are unwilling or medically unable to undergo surgery.

The density of vital neurologic structures in the posterior fossa requires careful assessment and treatment planning to avoid unnecessary morbidity and mortality.

Frequency

See the list below:

• CPA tumors other than vestibular schwannoma (percentage of total CPA occurrence)

  • Meningiomas (3-13%)

  • Epidermoids (2-6%)

  • Facial and lower cranial nerve schwannomas (1-2%)

  • Arachnoid cysts (1%)

• Rare tumors

  • Lipomas

  • Dermoid tumors

  • Neuroepithelial cysts

  • Trigeminal schwannoma

  • Endolymphatic sac carcinomas

  • Ependymoma

  • Brainstem glioma

  • Astrocytoma

  • Medulloblastoma

  • Choroid plexus papilloma

  • Large or giant aneurysms of anterior inferior cerebellar artery (AICA), superior cerebellar artery (SCA), or the midbasilar artery

  • Brainstem arteriovenous malformation (AVM)

  • Brainstem cavernous malformation originating in the brain stem (may reach a pial surface along the CPA)

  • Petroclival chondrosarcoma

  • Clivus chordoma (usually arise in midline but may extend laterally into the CPA)

  • Osteosarcoma of the lateral skull base

  • Metastatic tumors (can reach CPA by hematogenous spread or direct extension)

  • Granulomatous inflammatory masses

See the list below:

• Meningioma

  • Meningiomas arise from cap cells gathered in clusters around the tips of arachnoid villi.

  • They are benign but locally aggressive.

  • The 4 histologic variants of meningioma are (1) meningothelial (syncytial), (2) fibroblastic, (3) transitional, and (4) angioblastic. Although the angioblastic variant has been associated with more aggressive behavior, no significant correlation has been found between the other histologic subtypes and clinical behavior.

  • In the posterior fossa, they usually arise on the posterior surface of the petrous bone, near the porous acusticus, or along the lateral venous sinus.

  • They displace but do not invade adjacent neural tissue.

  • Meningiomas usually have a thin investing capsule.

  • They can invade bone without destruction by extension along haversian canals.

  • Sixty percent extend into the middle fossa.

  • Psammoma bodies are observed on microscopic examination.

• Epidermoid

  • Epidermoids originate from epithelial rests within the temporal bone or CPA.

  • They consist of stratified squamous epithelial linings surrounding desquamated keratin.

  • They usually grow very slowly.

  • Patients often do not present until the second to fourth decade of life.

  • Epidermoids tend to envelop rather than displace adjacent neural and vascular structures.

  • Shapes are variable, with irregular patterns.

  • Epidermoids frequently develop multiple fingerlike projections between cranial nerves and brainstem fissures and sulci.

  • They may extend through the tentorium into the middle fossa, forming a dumbbell shape.

• Facial and lower cranial nerve schwannoma: These arise from Schwann cells anywhere along the course of the nerve.

• Arachnoid cysts: These are thin-walled sacs containing yellow cerebrospinal fluid (CSF) that most likely originate as congenital developmental anomalies.

A study by Berkowitz et al found a tendency for patients diagnosed with vestibular schwannoma to be white, aged 50-60 years, and in a professional occupation, but there seemed to be no specific sex preference in the disease. A strong association appeared to exist between hay fever and vestibular schwannoma, while tobacco use seemed to have an inverse relationship to the likelihood of developing the condition.[2]

Presenting symptoms of CPA tumors (including acoustic neuromas) include the following:

• Hearing loss - 95%
• Tinnitus - 80%
• Vertigo/unsteadiness - 50-75%
• Headache - 25%
• Facial hypesthesia - 35-50%
• Diplopia - 10%

Presenting symptoms can vary according to the size and location of the lesion.

Meningioma

See the list below:

• Trigeminal or facial nerve symptoms are likely to occur earlier than hearing loss.

• Patients with larger tumors can present with obstructive hydrocephalus and/or symptoms of brainstem compression.

Epidermoid

See the list below:

• These can become quite large without symptoms.

• Facial twitching (hemifacial spasm) and progressive facial paralysis is more prominent than with other tumors in the CPA.

• Patients may present with cranial nerve or cerebellar dysfunction that develops over a number of years.

Facial nerve schwannoma

See the list below:

• Clinical findings depend on the portion of the nerve affected by the neoplasm.

• Patients can present with conductive hearing loss from middle ear involvement, parotid mass from extratemporal involvement, or sensorineural loss from internal auditory canal (IAC) or CPA involvement.

• CPA lesions do not cause facial weakness until tumor is very large (hemangiomas of the facial nerve usually cause symptoms at an earlier stage).

• Hemifacial spasm is relatively common.

Lower cranial nerve schwannoma

See the list below:

• Patients may have weakness and hypesthesia of the palate, vocal cord, and shoulder (cranial nerves [CN] IX, X, XI) or hemiatrophy of the tongue (CN XII).

• Large tumors may cause deficits of all the lower cranial nerves.

Malignant lesions

In a study of nine patients with malignant lesions of the CPA and/or internal acoustic meatus (IAM), Eliezer et al found that eight individuals (89%) presented with severe facial palsy in association with severe cochleovestibular symptoms, seven patients (78%) were observed to have rapid tumor growth (mean time interval = 4.6 months), five patients (56%) showed atypical magnetic resonance imaging (MRI) aspect with bilateral tumors, and 4 patients (44%) had a history of malignancy. Thus, the investigators state, “any association between cochleovestibular symptoms and facial weakness, which can be clinically severe with rapid onset,” should raise suspicions for a malignant lesion.[3]

Because of the location of these tumors within the cerebellopontine angle (CPA) and the significant number of closely associated vital neurologic structures, the risk of complications from cranial nerve damage and brainstem compression increases with delays in diagnosis and treatment. Patient age and medical condition, specific tumor growth rate, and pathologic behavior are taken into account when recommending a mode of therapy. Surgical resection is considered for any patient in good medical condition with a benign or malignant lesion in the CPA if the boundaries of the tumor are resectable with minimal risk to the local neurologic structures. Surgical resection renders the greatest chance for complete tumor eradication of the vast majority of CPA lesions. Occasionally, planned partial resection is considered in patients with extensive tumors associated with increased intracranial pressure.

A study by Goebel and Mehdorn indicated that untreated CPA tumors can lead to neurocognitive impairment. The investigators found that out of 45 patients with an untreated CPA tumor, including 27 with a vestibular schwannoma and 18 with a meningioma, 69% demonstrated neurocognitive problems, with attention and visuomotor speed impairment revealed in 62% of cases. Visuoconstruction was affected in 44% of patients. Clinically relevant depression and/or anxiety was reported by approximately one third of individuals in the study, while 48% stated that they suffered from high fatigue levels.[4]

See the Pathophysiology section.

Although patient age and medical condition, specific tumor growth rate, and pathologic behavior are taken into account when recommending a mode of therapy, surgical resection is considered for any patient in good medical condition with a benign or malignant lesion in the cerebellopontine angle (CPA) if the boundaries of the tumor are resectable with minimal risk to the local neurologic structures. Occasionally, complete resection is impossible because of the intimate involvement of surrounding structures, which may impose unwarranted morbidity if complete excision is attempted, and planned partial resection is considered.

See the list below:

• Meningioma

  • Computed tomography (CT) scanning findings

    • Hyperintense compared with the cerebellum on noncontrast images

    • May demonstrate calcifications within or at the periphery of the tumor

    • May be a broad attachment to the petrous ridge

    • Less likely to show enlargement of the internal auditory canal (IAC) than vestibular schwannomas

  • MRI findings

    • Variable intensity on T2-weighted images and either isointense or slightly hypointense to brain on T1-weighted images

    • Can be heterogeneous as a result of calcifications and cystic foci within the tumor

    • Dural tail sign, characterized by enhancement of the dura adjacent to an enhanced primary lesion, possible on contrast-enhanced MRI

• Epidermoid

  • CT scanning findings

    • Hypointense to brain

    • Do not enhance (enhancement suggests an associated malignancy or infection)

    • Irregular margins with variable involvement of the IAC

  • MRI findings

    • Heterogeneous and hypointense to brain on T1-weighted images

    • Homogeneous and isointense or hyperintense to brain on T2-weighted images

    • Nonenhancing (allows them to be differentiated from schwannomas, meningiomas, and chondromas, which have similar intensity characteristics)

• Facial nerve schwannoma

  • CT scanning findings

    • Identical to vestibular schwannoma within the IAC except for possible anterosuperior IAC erosion or erosion of facial nerve canal in labyrinthine segment

    • May have enlargement of geniculate ganglion and fallopian canal (more common)

  • MRI findings

    • Isointense or mildly hypointense to brain on T1-weighted images

    • Mildly hyperintense to brain on T2-weighted images

    • Enhances with gadolinium contrast

• Lower cranial nerve schwannoma

  • Enlargement of jugular foramen with cranial nerve IX, X, and XI tumors

  • Enlargement of hypoglossal canal with cranial nerve XII tumors

• Arachnoid cyst[5]

  • CT scanning findings

    • Typically smooth surface lesion isointense with CSF

    • Nonenhancing

  • MRI findings

    • Isointense or hypointense with brain on T1-weighted images

    • Hyperintense to brain on T2-weighted images

• Cerebral angiography

  • Cerebral angiography is used when involvement of a large vessel is suspected or preoperative embolization is required.

  • It is used to assess patency in vessels encased by tumor.

  • Balloon test occlusion can aid in determining likelihood of catastrophic stroke if internal carotid artery sacrifice is contemplated.

  • Preoperative embolization can be performed in vascular tumors to effectively decrease amount of blood loss.

Diuretic therapy may provide symptomatic relief in a minority of patients with arachnoid cysts. Otherwise, options for management of lesions in the cerebellopontine angle (CPA) include observation with serial imaging, radiation therapy, or surgery. Chemotherapy is an option only for occasional malignant lesions. Therapeutic embolization for vascular tumors has had limited success.

A study by Tan et al indicated that patients with small vestibular schwannomas can be managed with a watch, wait, and rescan approach, with 67.3% of these individuals not exhibiting growth over a 5-year period. The report found that if serial scanning reveals that small tumors have grown by over 2 mm, radiotherapy can provide good tumor control, with such management controlling 95% of these cases in the study. The investigators also determined that microsurgical excision by the translabyrinthine route resulted in a radiologically clear resection in 93% of individuals with larger vestibular schwannomas (over 20 mm in size).[6]

Surgical approaches vary depending on the pathologic entity, as well as the size and involvement of adjacent structures. Although complete excision is planned for most cases, the intimate involvement of surrounding structures may impose unwarranted morbidity if complete excision is attempted. Meningiomas are excised completely more readily than epidermoids, whereas adequate therapy for arachnoid cysts is drainage.[1]

Standard approaches to the CPA include the translabyrinthine, suboccipital (retrosigmoid), or middle fossa craniotomies (see Skull Base, Acoustic Neuroma (Vestibular Schwannoma)). The choice of approach is based on specific location and hearing status. Occasionally, these craniotomies can be combined or performed in addition to an infratemporal fossa dissection for larger tumors.[7]

See the list below:

• Neurotologic evaluation

  • Obtain a patient history as it relates to presenting symptoms.

  • Perform a complete physical examination including cranial nerve and cerebellar testing.

• Audiologic testing

  • Testing most commonly reveals high-frequency sensorineural loss.

  • Flat mid- and low-frequency sensorineural losses have been described.

  • Speech discrimination is usually diminished out of proportion to degree of loss on affected side.

  • Little relationship exists between severity of hearing loss and size of lesion.

  • Conductive loss occurs with middle ear involvement of facial nerve schwannomas or paragangliomas.

• Auditory-evoked brainstem response (eg, ABR, BAER)

  • Abnormal I-V interpeak latencies, a significant interaural latency difference for wave V, and poor waveform morphologic characteristics are suggestive of retrocochlear pathology on ABR.

  • Preoperative assessment is required when hearing preservation surgery is to be attempted.

• Vestibular testing

  • Testing may be helpful in patients presenting with vestibular symptoms.

  • Abnormal electronystagmography (ENG) findings may result from direct compression of the vestibular nerve, brain stem, or cerebellum.

  • Localizing specific vestibular nerve involvement based on laboratory vestibular testing alone is unlikely.

• Radiographic imaging: Plain radiography has virtually no role in the diagnosis of these lesions.

• CT scanning: Bone algorithms can help delineate bony involvement and destruction. CT scanning helps determine the location of jugular bulb.

• Contrast MRI of the head and internal auditory canals

  • This is the criterion standard for diagnosis.

  • Perform magnetic resonance angiography (MRA) or magnetic resonance venography (MRV) on patients with suspected vascular lesions as a screening technique.

  • Angiography prior to embolization is the criterion standard in assessing the vascular tributaries to various tumor types.

• Ventricular shunting or drainage: This is considered for large tumors.

See the list below:

• Use neuroanesthetic techniques with continuous blood pressure monitoring.

• Positioning of patient is based on approach to be used.

• Administer perioperative prophylactic antibiotics for 24 hours.

• Use applicable cranial nerve monitoring techniques (eg, intraoperative ABR, facial and lower cranial nerve monitoring).

• Obtain proximal control of the internal carotid artery and internal jugular vein in the neck in patients with lesions with involvement of these vessels.

• Consider the administration of mannitol and/or diuretics before craniotomy.

• Stereotactic operative techniques can be used for selected lesions.

• Meticulously wax temporal bone air cells.

• Seal off the eustachian tube if the middle ear is opened.

• Attempt watertight closure of the dura and/or skin.

See the list below:

• Continuous intensive care unit monitoring with frequent neurologic checks is imperative in the immediate postoperative period.

• Maintain meticulous eye care when facial nerve function is compromised.

• Appropriately manage comorbid medical conditions.

• Institute speech and physical therapy as soon as possible.

See the list below:

• Guidelines for follow-up care depend on the type of lesion and morbidity.

• Typically, an enhanced MRI is obtained 6 months to 1 year after surgery and at variable periods thereafter.

• Malignant lesions require more frequent follow-up care.

Contemporary surgical and anesthetic techniques have minimized morbidity for these lesions. Careful preoperative assessment and control of comorbid conditions can further diminish the risks of complication. Complications from these procedures can include the following:

• Permanent hearing impairment or complete hearing loss on the operative side

• Temporary or permanent facial paralysis[8, 9]

• Exposure keratitis or corneal ulcer

• Ipsilateral facial hypesthesia

• Lower cranial nerve deficits (eg, dysphagia, hoarseness, aspiration)

• CSF leak (eg, otorrhea, rhinorrhea)

• Infection (eg, wound, meningitis, intracranial abscess

• Intraoperative hemorrhage

• Hematoma

• Seizures

• Stroke

A retrospective study by Shew et al suggested that the chance of CSF fistula following CPA tumor resection is greater in patients with isolated petrous apex (PA) pneumatization. The investigators found the rates of CSF leak and CSF rhinorrhea to be 25.0% and 15.0%, respectively, in patients with PA pneumatization who underwent resection, compared with 13.7% and 5.9%, respectively, in resection patients without pneumatization.[10]

The outcome and prognosis for cure from surgical resection depends on the histology, size, and location of the tumor and the approach chosen. Cessation of tumor growth from primary radiation also depends on the specific factors listed above. Expected facial palsy rates are high for primary facial schwannomas. The effect on quality of life from facial nerve palsy is quite significant. Results also depend on the experience and skills of the surgeon.

In a study of 34 patients who underwent surgery for CPA meningiomas, Agarwal et al found that the rate of permanent cranial nerve deficits was significantly greater in patients with tumors of more than 3 cm in size than in those with smaller meningiomas (45.5% vs 5.9%, respectively). It was also found that deficits of the lower cranial nerves occurred only in patients whose tumors extended into the jugular foramen. No association was found between tumor extension into the internal acoustic canal and either postoperative complications or cranial nerve deficits. Among all patients, 5.9% suffered postoperative facial nerve palsy.[11]

Further advances in surgical techniques and radiotherapy are being made. Endoscopic surgical techniques in the cerebellopontine angle (CPA) have improved visualization of the tumor and adjacent structures. Stereotactic methods have improved accuracy of the delivery of microsurgical dissection and radiation therapy, thus minimizing damage to adjacent tissue. New radiation delivery protocols, although incompletely followed up, show some promise as primary or adjuvant therapy of some of these lesions.