The natural history of VS includes a slow rate of growth in the IAC and then into the cistern of the CPA. Studies show that periods of growth are intermixed with periods of quiescence. The average growth rate is 1.8 mm/y. This slow growth causes progressive and often insidious symptoms and signs since there is displacement, distortion, and compression of the structures first in the IAC and then in the CPA. This slow growth via cellular proliferation provides a predictable progression of symptoms and signs. Occasionally, the tumor may undergo rapid expansion due to cystic degeneration or hemorrhage into the tumor. A rapid expansion causes rapid movement along the subsequent phases of VS symptoms and may cause rapid neurological deterioration.
The initial intracanalicular growth affects the vestibulocochlear nerve in the rigid IAC and causes unilateral hearing loss, tinnitus, and vertigo or disequilibrium. These three symptoms are the typical presenting complaint of not only patients with VS but also patients with other lesions of the CPA. It is interesting that the motor component of the facial nerve is resistant to injury during this phase of growth and patients have normal facial function. The tumor then grows into the CPA cistern and grows freely without causing significant new symptoms because structures in the CPA are initially displaced without injury (see Figure 61–2A). As the tumor approaches 3 cm, it abuts on the boundaries of the CPA and results in a new set of symptoms and signs. Compression of the CN V causes corneal and midface numbness or pain. Further distortion of CN VIII and now CN VII causes further hearing loss and disequilibrium and also facial weakness or spasms. Brainstem distortion leads to narrowing of the fourth ventricle (see Figure 61–2B).
Further growth leads to the final clinical spectrum of CPA syndrome. The patient develops cerebellar signs due to compression of the flocculus and cerebellar peduncle. The patient also develops obstructive hydrocephalus due to closure of the fourth ventricle. The increasing intracranial pressure manifests in ocular changes, headache, mental status changes, nausea, and vomiting. If the VS continues to grow without intervention, death occurs from respiratory compromise.
The treatment of patients with CPA tumors includes surgical removal, observation, and irradiation. Surgical removal has historically been the primary management modality for VS. More recently, observation with serial MRI and radiation therapy have increased in popularity. The size of the tumor, age of the patient, residual auditory function, vestibular dysfunction, and cranial neuropathies are some of the important factors in determining optimal intervention. Observation is reasonable in older patient or in whom the tumor is not growing. Gamma knife is preferentially used in older patient, those who cannot tolerate a surgical procedure or who have a limited life expectancy.
The surgical approaches to the CPA include translabyrinthine, retrosigmoidal, and middle fossa craniotomies (Figure 61–3A). The appropriate approach for a particular patient is based on the hearing status, the size of the tumor, the extent of IAC involvement, and the experience of the surgeon (Table 61–2). The approaches are either hearing preserving or hearing ablating. The retrosigmoidal and middle fossa approaches are hearing preserving. However, they have limitations of exposure to all aspects of the CPA and IAC. The middle fossa approach is well suited for patients with good hearing and a tumor that is <1.5 cm in the CPA. The retrosigmoidal approach is well suited for patients with good hearing and a tumor <4 cm and not involving the lateral IAC. In the retrosigmoid approach, the lateral IAC is usually only directly accessible following the removal of the posterior semicircular canal; the violation of the posterior semicircular canal leads to hearing loss. The translabyrinthine approach causes total hearing loss and so is well suited for patients with poor hearing (pure-tone average >50) or patients with good hearing and tumors not accessible by the hearing-preserving approaches. Generally, hearing preservation is poor with tumors >2 cm and those that involve the lateral IAC.
Surgical approaches to the cerebellopontine angle include (A) translabyrinthine (A and B), retrosigmoidal, and (C) middle fossa craniotomies. The retrosigmoidal and middle fossa approaches can preserve hearing, whereas the translabyrinthine approach necessarily deafens the patient because it involves drilling the balance portion of the inner ear.
Table 61–2. Surgical Approaches to the Cerebellopontine Angle (CPA) and Indications. |Favorite Table|Download (.pdf)
Table 61–2. Surgical Approaches to the Cerebellopontine Angle (CPA) and Indications.
- Retrosigmoidal approach: Patient has good hearing and tumor not involving the lateral internal auditory canal (IAC)
- Middle fossa approach: Patient has good hearing and tumor ≤1.5 cm in the CPA or IAC
- Translabyrinthine approach: Patient has poor hearing or larger tumors not accessible by other approaches
Three critical issues inherent in all three techniques are the extent of exposure of the IAC and CPA, the identification and preservation of the facial nerve, and the extent of brain retraction. These operations use electrophysiological monitoring of CN VII and an ABR in hearing preservation approaches.
The primary approach for removal of VS is the translabyrinthine approach. The boundaries of the approach include the mastoid facial nerve and cochlear aqueduct anteriorly, middle fossa dura superiorly, posterior fossa dura posteriorly, and jugular foramen inferiorly (see Figure 61–3A). These boundaries are approached via the familiar postauricular incision. A complete canal mastoidectomy is accomplished with identification of the incus, tegmen, sigmoid sinus, and facial nerve. A compete labyrinthectomy is then performed with medial skeletonization of the middle and posterior fossa dura and decompression of the sigmoid sinus to the jugular foramen. After bony skeletonization of the IAC, the dura of the IAC is opened, and the facial nerve is identified medial to the transverse crest (Bill's bar). Once the facial nerve is identified in the fundus or lateral aspect of the IAC, tumor removal occurs from a lateral to medial direction along the IAC. In large tumors, the tumor is debulked internally and then the tumor capsule is removed from the surrounding structures, including the facial nerve. After tumor removal, abdominal fat is placed into the defect.
The three advantages of the translabyrinthine approach are the ability to remove tumors of all sizes, minimal retraction of the brain, and the ability to directly visualize and preserve the facial nerve. The rate of facial nerve preservation is 97%. The rate of CSF leakage presenting under the incision or draining through the nose via the eustachian tube is 5–8%. The majority of these CSF leaks resolve with conservative management that includes mastoid dressing and fluid restriction. A minimal risk of meningitis is associated with a CSF leak.
The retrosigmoidal approach is a modification of the traditional suboccipital approach used by neurosurgeons to address most posterior fossa lesions. The retrosigmoidal approach is a versatile approach with a panoramic view of the CPA from the foramen magnum inferiorly to the tentorium superiorly (Figure 61–3B). The medial two-thirds of the IAC are also accessible without violating the inner ear, therefore preserve hearing.
The surgical technique starts with a curvilinear skin incision 6 cm behind the ear over the retromastoid region. The soft tissue and posterior nuchal musculature are elevated to expose the mastoid and retromastoid bone. A 5 × 5 cm craniotomy is performed with the sigmoid as the anterior boundary and the transverse sinus as the superior boundary. An elevation of a bone plate is technically difficult, so the bone may be removed by drilling. The bone fragments are collected and are replaced during closure. The bone fragments will reform a bone plate and prevent adherence of the musculature to the dura. If decompression of the sigmoid sinus is needed for exposure, a mastoidectomy may also be performed. The dura is then opened along the sigmoid sinus and the cerebellum is seen. The CSF from the cisterna magnum needs to be released prior to retracting the cerebellum. Medial retraction of the cerebellum allows visualization of the CPA. To address the IAC component of the tumor, the posterior IAC bone needs to be removed. The bone dust created is carefully confined and removed to prevent meningeal irritation. The extent of IAC skeletonization is limited by the proximity to the inner ear. The endolymphatic duct and sac serve as landmarks to the proximity of the posterior semicircular canal and allow preservation of the inner ear and hearing. The facial nerve is normally anterior to the tumor or its position is ascertained with facial nerve monitoring. The tumor removal is as previously described. After tumor removal and hemostasis, air cells along the IAC and mastoid are closed with bone wax or bone cement to eliminate paths for CSF leak. A fat or muscle graft may also be placed into the petrosal defect to prevent CSF leak. The dura is closed and the bone plate or bone pate is replaced. The musculature and soft tissue are meticulously closed.
The primary advantage of the retrosigmoidal approach relative to the translabyrinthine approach is the ability for hearing preservation in properly selected tumors. If hearing preservation is not an issue, the retrosigmoidal approach allows a versatile approach to the CPA and IAC. The relative disadvantages compared with the translabyrinthine approach include persistent postoperative headache, increased difficulty in resolving CSF leaks, the need for cerebellar retraction, and the inability to have direct access to the facial nerve. The combination of intradural drilling leading to meningeal irritation by bone dust and dissection of suboccipital musculature causes nearly 10% of patients to have a persistent, severe, postoperative headache. In the case of extensive pneumatization of the IAC and mastoid, the air cells may be difficult to completely seal, and the inability to address the aditus-ad-antrum or the eustachian tube causes CSF leaks to be persistent, despite conservative treatment. The extent of cerebellar retraction is minimal in small tumors, but the amount of retraction increases with larger tumors. The surgical control of the facial nerve is adequate in the retrosigmoidal approach, but the exposure of the facial nerve is superior in the translabyrinthine approach.
The middle fossa approach provides a hearing-preserving approach to intracanalicular tumors with a <1.5 cm cisternal component. The surgical technique involves an inverted U-shaped incision centered over the ear. The temporal muscle is reflected inferiorly to expose the squamous portion of the temporal bone. A 5 × 5 cm temporal craniotomy is performed and is centered over the zygomatic root. Extradural elevation of the temporal lobe is accomplished to reveal the floor of the temporal bone. The greater superficial petrosal nerve leading to the geniculate ganglion reveals the anterior, lateral boundary of the IAC, and the arcuate eminence reveals the posterior boundary of the IAC (Figure 61–3C). These landmarks may be difficult to identify, and the IAC dura may have to be identified medially by drilling toward the porus acousticus. Once the IAC is identified and well skeletonized medially, the bone removal continues laterally. However, the extent of IAC skeletonization laterally is limited by the basal turn of the cochlea anteriorly and the superior semicircular canal posteriorly. The IAC dura is opened posteriorly to avoid injury to the facial nerve. The tumor is dissected free of the facial nerve and removed in a medial to lateral direction. Any air cells are sealed and the dural defect is covered with a fat or muscle plug. The craniotomy bone flap is replaced and the incision is closed.
The middle fossa approach is unique compared with the posterior fossa craniotomies because the entire IAC is accessible without violating the inner ear; direct visualization of the medial IAC is difficult even with the middle fossa approach. This exposure allows for the removal of intracanalicular tumors while maintaining hearing preservation. The limitations of the middle fossa approach include tumors with a >1.5 cm cisternal component. In situations of hearing preservation, an extended middle fossa approach with further removal of bone around the IAC as well as the elevation or division of the superior petrosal sinus and tentorium allow improved exposure into the CPA. The relative merits of the procedure with increased temporal lobe retraction and limited access to the posterior fossa in the event of bleeding relative to a retrosigmoidal approach continues to be defined. The disadvantages of the middle fossa approach include temporal lobe retraction and poor surgical position of the facial nerve relative to the tumor. Temporal lobe retraction may cause transient speech and memory disturbances and auditory hallucinations. The facial nerve, especially if the tumor originates from the inferior vestibular nerve, will be between the surgeon and the tumor. The increased manipulation of the facial nerve during tumor removal increases the risk of transient facial paresis.
The predictable correlation between VS size and significant neurological symptoms and the relative slow growth of VS allow observation to be a management option for VS. Patients may be observed if their life expectancy is shorter than the growth time required for the VS to cause significant neurological symptoms. The growth pattern of the VS should be assessed in these patients with a second radiological evaluation in 6 months and then in yearly radiological evaluations. Studies have shown that 15–24% of patients undergoing conservative management require surgery or stereotactic radiation. If the growth rate in the first year exceeds 2–3 mm, then the patient is likely to need the treatment for the VS. The patient should understand that the initial conservative management, rather than immediate surgical intervention, may necessitate a resection of a larger tumor that is less amenable to hearing preservation or stereotactic radiation (or both) if intervention becomes necessary in the future.
The goal of stereotactic radiation is to prevent further growth of VS while preserving hearing and facial nerve function. This goal directly differs from the goal of complete tumor removal in microsurgical therapy. The mechanism of stereotactic radiation relies on delivering radiation to a specific intracranial target by using several precisely collimated beams of ionizing radiation. The beams take various pathways to the target tissue, therefore creating a sharp dose gradient between the target tissue and the surrounding tissue. The ionizing radiation causes necrosis and vascular fibrosis, and the time course of the effect is >1–2 y. There is an expected transient swelling of the tumor in the short term with modest shrinkage over time. The ionizing radiation is most commonly delivered using a 201-source cobalt-60 gamma knife system. The standard linear accelerator can also be adapted to deliver stereotactic radiation. The practical aspects include the patients wearing a stereotactic head frame, computer-assisted radiation planning using an MRI scan, and a single treatment for delivery of the radiation Figure 61–4.
MRI with gadolinium enhancement demonstrating typical appearance of pre- (A) and 1-y post- (B) gamma knife-treated appearance of VS. The tumor is slightly smaller and there is less intense enhancement following gamma knife therapy. The low signal intensity within the posttreated tumor may represent areas of tumor necrosis.
The success of stereotactic radiation in arresting tumor growth depends on the dose of radiation delivered. However, the rate of cranial nerve neuropathies, including hearing loss, is decreased by lowering the radiation dose. The current trend has been to lower the marginal radiation dose, and the long-term tumor control with these current dosing plans is under investigation. Since VS have a slow growth rate, these studies require 5- to 10-y follow-ups to provide reliable data about tumor control. Studies have shown control rates from 85% to more than 95%. The hearing preservation rate decreases each year after radiation and stabilizes after 3 y in 50% of patients. The rate of facial nerve dysfunction varies from 5% to 20% based on the radiation dose at the margin of the tumor and the length of the facial nerve in the radiation field. Approximately 25% of patients have trigeminal nerve neuropathy. The persistence and extent of these neuropathies with the lower dosing protocols continue to be studied. Hydrocephalus is also a complication of radiation.
As the long-term effectiveness and sequelae of stereotactic radiation are further defined, the indications for radiation therapy will become further refined. Radiation therapy is useful in patients in whom the arrest of tumor growth is acceptable. These patients have either short-life expectancies or a high surgical risk. Compared with microsurgery, stereotactic radiation may allow improved hearing preservation in patients with 2–3 cm VS. Radiation therapy in large tumors (>3 cm) or tumors causing brain compression will exacerbate symptoms because of initial tumor swelling.