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.