18 Tumors of the Cerebellopontine Angle
Abstract
This chapter summarizes the most common neoplastic lesions located in the region of the cerebellopontine angle, as well as the key facets of the pathophysiology, epidemiology, and clinical presentation of these lesions. Key aspects of perioperative evaluation and treatment options are also covered. We conclude by reviewing patient outcomes and provide highlights of the best evidence-based recommendations.
Introduction
Sir Charles Ballance is often credited with the first report of surgical resection of a cerebellopontine (CP) angle tumor, 1 which was suspected to be a meningioma given the patient’s lack of hearing loss and Ballance’s description of the wide dural attachment of the lesion. 2 The surgical approach at the time focused on speed, using a blunt finger dissection method because, as Harvey Cushing phrased it, “the cerebellopontine angle, like the fence corner of the Gettysburg battlefield, might well be called the ‘bloody angle’.” 2
Over the past century, surgery of the posterior fossa has advanced considerably, with surgical mortality dropping from the historical 70 to 80% range 3 to 1%. 4 Surgical intervention in this location continues to fascinate and challenge neurosurgeons. Modern surgery in the CP angle is safe and major morbidity is rare as the focus has shifted from preserving life to improving and preserving function, particularly with regard to the vestibulocochlear apparatus and the facial nerve. Although the most common mass lesion in the CP angle is vestibular schwannoma (VS), a variety of neoplastic and other mass lesions may occur in this location. 5 Careful study of the clinical presentation and neuroimaging features often leads to the correct diagnosis and can help in designing a treatment strategy.
Vestibular Schwannomas
Vestibular schwannomas, also referred to as acoustic neuromas or acoustic neurinomas, account for 80% of all tumors occurring in the CP angle, making it the most common lesion in that location. 5 Overall, VSs also account for 6 to 8% of all brain tumors ( Fig. 18.1, Fig. 18.2 ). 6
Pathophysiology, Incidence, Epidemiology, and Natural History
Vestibular schwannomas are considered grade I tumors by the World Health Organization (WHO) classification. 7 The lesions are thought to arise at or near the Obersteiner-Redlich zone, 8 which is the transition point between glial and Schwann myelination areas, although this hypothesis has been widely disputed. 9 In more than 90% of VSs, the lesion arises from the inferior division of the vestibular nerve. 10 A characteristic quality of schwannomas is the splaying and displacement of adjacent nerve fibers, in contrast to nerve fascicle involvement seen with neurofibromas, which are nonencapsulated spindle-cell tumors with proliferation of all elements of peripheral nerves.
In Denmark, every patient with VS has been enrolled in a national database since the 1970s, and these data have contributed significantly to our understanding of the epidemiology and natural history of this disease. 11 In Denmark, the annual incidence of VS is 2.3 per 100,000 population, while epidemiological studies in the United States reveal a similar annual rate of 1.6 per 100,000 persons. 12 Mean patient age at diagnosis is 58 years, and both sexes are affected equally. Histopathological studies of temporal bones demonstrate a significantly higher incidence of 0.57 to 2.5%, suggesting that the true incidence is higher, but the majority of these tumors never reach a symptomatic threshold. 13
The natural history of tumor growth is variable; some lesions demonstrate continued growth while others will stagnate. 11 The mean size at presentation is 11 mm and the mean annual growth rate is 3±1 mm. 14 In a population study, 17% of intrameatal tumors grew into the cisternal space, and 30% of extrameatal tumors continued to grow while being observed. 11 Among conservatively managed VSs, 1 to 8% demonstrate shrinkage at follow-up. 15 Interestingly, there is a suggestion that tumors that did not show growth in the first 5 years of observation were unlikely to grow beyond that period, 16 while the presence of tinnitus at presentation increased the odds of tumor growth three-fold. 17
With regard to hearing preservation, 50% of conservatively managed patients lost functional hearing over a 5-year followup period. 18 The presence of even small discrimination loss at presentation may predict an increased risk of hearing loss over time, with 38% of patients maintaining good hearing after 4.7 years of follow-up. However, 59% of patients who had 100% speech discrimination at presentation maintained good hearing at follow-up. 19 Interestingly, hearing was lost during observation in cases of both tumor growth and stagnation 15 ; this may be related to the known secretion of ototoxic molecules (including tumor necrosis factor alpha) by VSs. 20
Clinical Presentation
The presentation of VS can be related to the tumor growth pattern. Initially, mass effect on the vestibular and cochlear divisions results in subjective hearing loss, with or without tinnitus, as well as dizziness. Although VS causes a similar mass effect on the facial nerve, it is exceedingly uncommon for patients to present with facial weakness; in fact, an alternative diagnosis should be pursued for patients with suspected VS who present with facial weakness.
As the tumor grows, mass effect on the trigeminal nerve develops, which can lead to facial numbness, loss of the corneal reflex, and occasionally facial pain. As tumors grow larger, impingement on the lower cranial nerves is possible, leading to lower cranial neuropathies, including hoarseness and dysphagia. Further growth results in brainstem compression, which often manifests with gait difficulty, and later obstruction of the fourth ventricle, which leads to symptoms of hydrocephalus. Interestingly, communicating hydrocephalus is also seen in patients with VS and has been reported in the preoperative, as well as in the short-term and long-term postoperative periods; it is suspected to be due to secreted tumor factors resulting in elevated cerebrospinal fluid protein. 21 The most common signs and symptoms of VS are summarized in Table 18.1 and 18.2 . 84
Symptom | Number of patients | Percentage |
Hearing loss | 33 | 71.7% |
Headache | 4 | 8.7% |
Altered balance | 3 | 6.5% |
Gait ataxia | 3 | 6.5% |
Facial pain | 1 | 2.2% |
Tinnitus | 1 | 2.2% |
Facial weakness | 1 | 2.2% |
Data from Ojemann et al 1972. 84 | ||
Sign | Number of patients | Percentage |
Auditory/vestibular | 45 | 98% |
Facial weakness | 26 | 57% |
Trigeminal sensory | 26 | 57% |
Altered taste | 26 | 57% |
Gait ataxia | 19 | 41% |
Limb ataxia | 9 | 20% |
Data from Ojemann et al 1972. 84 | ||
A variant of VS, which is referred to as a “medial acoustic” tumor, does not extend into the internal auditory canal (IAC) and has a tendency to grow to a large size before manifesting clinically. These lesions tend to be hypervascular and may be associated with a higher rate of patient presentation with hydrocephalus. The origin and pattern of growth leads to arachnoidal rearrangement and resultant adherence to the brainstem interface at the time of surgery. 22
Perioperative Evaluation
All patients harboring suspected VS should undergo a complete radiographic assessment. At our institution, this typically includes an IAC-protocol magnetic resonance imaging (MRI) with thin cuts through the posterior fossa on T1-weighted sequences both before and after administration of intravenous contrast, as well as a thin-cut heavy T2-weighted sequence such as FIESTA (fast imaging employing steady-state acquisition) or CISS (constructive interference in steady-state). The MRI is complemented by a “dynamic” computed tomography (CT) angiogram, which is well-suited for the study of the three-dimensional bony anatomy, as well as the relationship of the tumor to arterial and venous structures in the region. 23 This information is helpful when designing a surgical approach and for complication avoidance. Particular attention should be paid to anatomical variations, such as the dominance of transverse and sigmoid sinuses as well as the location and shape of the jugular bulb. The recognition of a high-riding jugular bulb is important, especially if the patient is placed in the sitting position.
Some radiographic signs may be relevant to the prognosis of a patient with VS. Primarily cystic lesions may have the potential for sudden and dramatic growth 24 , 25 and may be more adherent to the nerves; treatment for these lesions may result in worse facial 18 , 26 , 27 and hearing 28 outcomes. Medial VSs are associated with marked tumor adherence to the brainstem and are frequently hypervascular. 22
In addition to radiographic evaluation, all patients should undergo a formal audiogram to assess the quality and severity of hearing loss and to serve as a baseline for reference with postoperative results. For patients in whom involvement of the lower cranial nerves is suspected, formal swallow evaluation may also be considered.
Surgical and Nonsurgical Approaches
The options available to patients with VSs consist of watchful observation, microsurgical resection, radiosurgery/radiotherapy, or a multimodal approach. A survey of the current practice pattern in the United States spanning a 10-year period suggests that microsurgery remains the most common treatment, having been performed on 53.4% of tumors. Radiosurgery/radiotherapy and watchful waiting have gained popularity, however, and are being applied to 24.2% and 22.4% of cases, respectively. 29 The same pattern can be seen in Europe, where the enthusiasm for radiotherapy/radiosurgery is even greater. 30 The Congress of Neurological Surgeons recently published a consensus statement for the treatment of patients with VS. 85 The treatment plan must be tailored to each particular patient; general concepts are reviewed in this section.
Patients who present with large tumors with resultant brain-stem compression, hydrocephalus, or neurologic deficits require microsurgical resection. Additionally, documented enlargement of a tumor occupying the CP angle is also an indication for treatment. The goal in both circumstances is the treatment of the neoplasm that induced, or would induce, neurologic deficit. In intracanalicular tumors, however, the only issue at patient presentation is hearing loss; watchful observation has thus become prevalent in the past few years. Observation is based on the rationale that 70% of the lesions remained unchanged. 16 The management, therefore, is more complex if hearing preservation is the goal.
Radiotherapy, and in particular stereotactic radiosurgery, is an increasingly popular treatment option for appropriately sized VSs and is applied in about 24% of cases in the United States. These cases are typically treated with a mean dose of 13 Gy, 31 which is typically administered as a single fraction in an outpatient setting. The rate of control is 97% at 10 years, 31 although the concept of “control” is elusive given the natural history of VSs, in which growth arrest may occur spontaneously. 32 Furthermore, the CP angle houses several radiosensitive structures, including the brainstem and the auditory apparatus; the risk of delayed complications after radiosurgery should be carefully considered, especially when treating younger patients. 33 In an attempt to reduce the risk of neurologic deficit while maximizing the rate of control, some centers have been administering stereotactic radiation over multiple sessions (often referred to as hyperfractionation), although the long-term results are yet to be elucidated. 34
Microsurgical resection of VSs can be accomplished via a variety of approaches. 35 Hearing-sparing approaches include the retrosigmoid and transmastoid approaches, 36 or the middle cranial fossa approach. The translabyrinthine approach involves sacrifice of hearing and should primarily be considered for patients with large tumors who have already lost hearing. The choice of hearing-sparing approach depends, in part, on the surgeon’s experience and comfort level, as excellent outcomes have been well described with both approaches. 4
Patient Outcomes
Facial Function
Preservation of facial nerve function is of cardinal importance in the treatment of VS; facial nerve injury is associated with facial deformity and attendant social implications, as well as functional deficits related to the ability to articulate speech, chewing, and swallowing. In an effort to preserve facial nerve function, Cushing advocated subtotal tumor resection for VSs. 2 The introduction of the operative microscope to VS surgery by William House and the subsequent pioneering work establishing the translabyrinthine and middle fossa corridors were both aimed at maintaining facial nerve integrity while allowing more radical tumor resection. 37 , 38
Contemporary surgical VS series estimate the likelihood of anatomical facial nerve preservation in the 93% range 39 with a correlation between tumor size and risk of injury to the nerve. 40 The routine use of intraoperative facial nerve monitoring allows for early detection of iatrogenic injury to the nerve, and affords the opportunity for its prevention. 41
Contemporary radiosurgical series also report high rates of facial nerve function preservation, in the 99% range, 31 suggesting the relative radioresistance of the facial nerve compared to surrounding structures.
Hearing Preservation
Postoperative quality-of-life surveys of patients with VSs reveal that the primary patient-reported disability is hearing loss. 42 Unlike facial nerve outcomes, the cochlear nerve results are less favorable in the literature. Distortion of cranial nerves from the presence of a mass lesion renders them more vulnerable to surgical manipulation, which is especially true for the cochlear nerve. The rate of hearing preservation in surgical series ranges from 47 to 92% and is optimized in male patients with small- to medium-sized tumors and good baseline hearing. 43 , 44 While any change in the hearing status can be ascertained relatively soon after surgical resection, an understanding of the hearing preservation results related to radiosurgical series requires long-term follow-up as radiation effects will present in a delayed fashion. Recent hearing-sparing microsurgical series report permanent hearing preservation in as many as 80% of patients, while 25 to 30% of patients will lose hearing during observation despite the fact that some tumors will not grow during the follow-up period. 15
Radiosurgery/radiotherapy results in a high rate of hearing preservation initially, but in patients with useful hearing before radiosurgery, hearing is preserved in only 37% of patients at 10-year follow-up. 45 Similar results are reported in other series with long-term follow-up. 46 , 47 Furthermore, even if surgery fails to preserve functional hearing, the call made nowadays for anatomical preservation of the cochlear nerve carries the hope for hearing restoration with cochlear implants in these patients. 48
Related posts:
15 Tumors of the Thalamus
14 Pediatric Brainstem Tumors
13 Adult Brainstem Gliomas
21 Radiotherapy for Pineal, Thalamic, and Brainstem Tumors
20 Stereotactic Radiosurgery for Tumors of the Brainstem, Thalamus, and Pineal Region
22 Neuro-oncologic Considerations for Pineal, Thalamic, and Brainstem Tumors
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