Acoustic neuroma INTRODUCTION — Acoustic neuromas, also known as acoustic schwannomas, acoustic neurinomas, vestibular schwannomas, and vestibular neurilemomas, are Schwann cell derived tumors commonly arising from the vestibular portion of the eighth cranial nerve. They account for approximately 8 percent of intracranial tumors in adults and 80 to 90 percent of cerebellopontine angle tumors (CPAs). In comparison, they are rare in children without neurofibromatosis. EPIDEMIOLOGY AND RISK FACTORS — The overall incidence of symptomatic acoustic neuroma is one per 100,000 person-years; however, subclinical acoustic neuroma has been found in as many as 1 percent of patients at autopsy . Most patients present between the ages of 30 and 50. The tumors are almost always unilateral, affecting the right and left sides with equal frequency. Bilateral acoustic neuromas are primarily limited to patients with autosomal dominant neurofibromatosis. One risk factor for the development of acoustic neuroma is acoustic trauma. In one case-control study, acoustic neuroma was associated with acoustic trauma (exposure to extremely loud noise) of ten years duration. Noise exposure was based upon a blinded review of job histories. A dose relationship was observed as twenty or more years of exposure was associated with an odds ratio of 13.2 . Experimental studies of tissue injury and repair following acoustic trauma support the biological plausibility of this association. Patients with a history of parathyroid adenoma also have been reported to have a significantly increased risk for developing acoustic neuromas. In contrast to these data, exposure to radiofrequency energy by use of a handheld cellular telephone does not appear to increase the risk of acoustic neuroma. Neurofibromatosis type 2 (NF2) accounts for approximately 10 percent of patients with schwannomas. It is typically characterized by bilateral vestibular schwannomas and/or a family history of NF2. The NF2 gene is a tumor suppressor gene on chromosome 22 which encodes a membrane cytoskeletal protein called merlin or schwannomin that appears to be involved in actin-cytoskeleton organization. Up to 60 percent or more of schwannomas in patients with NF2 carry inactivating mutations. In addition, most sporadic schwannomas are associated with mutations in or inactivation of merlin. HISTOPATHOLOGY — Acoustic neuromas arise from the schwann cell perineural elements of the affected nerve and are similar pathologically to peripheral schwannomas found in other parts of the body. They occur with equal frequency on the superior and inferior branches of the vestibular nerve; only rarely are they derived from the cochlear portion of the VIII nerve. Microscopically, zones of alternately dense and sparse cellularity, called Antoni A and B areas, are characteristic of acoustic neuromas. Malignant degeneration to a fibrosarcoma is extremely rare with only six cases having been reported. Immunohistochemical staining for S100 protein is usually positive in both the benign and the rare malignant forms of this tumor. Acoustic neuromas have a variable natural history as illustrated by serial imaging studies. The average growth rate is 2 mm/year, but rates as high as 25 mm/year have been described in a few patients. However, up to 40 percent of tumors overall and a higher percentage of small tumors show no growth or even shrinkage on serial imaging studies . There is no predictive relation between growth rate and tumor size at presentation. The growth rate tends to be constant over time, but sudden changes in tumor size can occur if there is hemorrhage into a tumor. CLINICAL PRESENTATION — Symptoms associated with acoustic neuroma are due to cranial nerve involvement and to tumor progression. In a series of 1000 acoustic neuromas treated at a single institution, the acoustic nerve was involved in almost all cases, followed by the vestibular, trigeminal and facial nerves. Acoustic nerve — Symptomatic acoustic nerve involvement occurred in 95 percent of patients. The two major symptoms were hearing loss and tinnitus. Hearing loss was present in 95 percent but only two-thirds of these patients were aware of this limitation. The hearing loss was usually chronic, with an average duration of 3.7 years. Although acute sensorineural sudden hearing loss (SHL) is unusual as a presenting feature, acoustic neuroma appears to be a common cause of this problem. In one series of 40 patients with sensorineural SHL, 19 (48 percent) had a CPA tumor. Tinnitus was present in 63 percent with an average duration of 3.1 years. The incidence of tinnitus was higher in hearing than in deaf patients but was still present in 46 percent of preoperatively deaf patients. Vestibular nerve — Involvement of the vestibular nerve occurred in 61 percent of patients. Affected patients frequently acknowledged having unsteadiness while walking, which was typically mild to moderate in nature and frequently fluctuated in severity. True spinning vertigo was uncommon because these slow growing tumors cause gradual rather than acute asymmetries in vestibular function. In this setting, the central vestibular system can often compensate for the gradual loss of input from one side. The most nondescript vertiginous sensations, such as brief tilting or veering, sometimes suggest the presence of an acoustic neuroma. In addition, acoustic neuroma accounts for approximately 1 percent of patients with vertigo. Trigeminal nerve — Trigeminal nerve disturbances occurred in 17 percent of patients. The most common symptoms were facial numbness (paresthesia), hypesthesia, and pain. The average duration of symptoms was 1.3 years; the symptoms usually occurred after hearing loss had been present for more than two years and vestibular symptoms for more than one year. Facial nerve — The facial nerve was involved in 6 percent of patients. The primary symptoms were facial paresis and, less often, taste disturbances. Tumor progression — Other presenting signs are the result of tumor progression, leading to pressure on adjacent posterior fossa structures. Very large tumors can press on the cerebellum or brainstem and result in ataxia. Brainstem compression, cerebellar tonsil herniation, hydrocephalus and death can occur in untreated cases. The functions of the lower cranial nerves (nerves IX, X, and XI), such as speaking and swallowing, can also become impaired, leading to dysarthria, dysphagia, aspiration, and hoarseness. DIAGNOSIS — The diagnosis of acoustic neuroma is made by the demonstration of asymmetric sensorineural hearing loss or other cranial nerve deficits followed by imaging with MRI or CT scan. Acoustic neuroma accounts for 80 to 90 percent of posterior fossa lesions. Meningioma accounts for 4 to 10 percent; the remainder are accounted for by facial nerve schwannomas, gliomas, cholesterol cysts, cholesteatomas, hemangiomas, aneurysms, arachnoid cysts, lipomas, and metastatic tumor. Physical examination — Hearing tests are typically abnormal due to involvement of the acoustic nerve. Physical examination reveals a normal-appearing external and middle ear. The presence of sensorineural loss can be confirmed with the Rinne test, which involves placing the tuning fork on the mastoid bone behind the ear of the affected side. The vibrating fork placed near the ear is much louder than when placed on the mastoid bone (ie, air conduction greater than bone conduction) in patients with sensorineural loss. In contrast, sound should be at least equally loud or louder when the fork is placed on bone as compared to held next to the ear in a conductive hearing loss. The Weber test should also be performed. A tuning fork is placed in the midline of the forehead: sensorineural hearing loss is present if the vibratory sound is louder on the "good" side; conductive hearing loss is present if the vibratory sound is louder on the "bad" side. The tuning fork can be moved short distances around the forehead to establish that the test is reliable. Further neurologic testing may reveal other cranial nerve deficits. An early classic sign, a decreased or absent ipsilateral corneal reflex and facial twitching or hypesthesia, may be present as cranial nerves V and VII become affected. Other cranial nerve deficits are uncommon unless the tumor is large. Romberg, Hall-Pike and other common office balance tests are typically normal. Audiometry — Audiometry is the best initial screening laboratory test for the diagnosis of acoustic neuroma, since only 5 percent will have a normal test. Pure tone and speech audiometry should be performed in an acoustically shielded area. Test results typically show an asymmetric sensorineural hearing loss, usually more prominent in the higher frequencies. Hearing loss does not necessarily correlate with tumor size. The speech discrimination score is usually markedly reduced in the affected ear and out of proportion to the measured hearing loss. Many other auditory tests have been used historically to try to diagnose acoustic neuromas. These include acoustic reflex testing, impedance audiometry, and Bekesy audiometry. They have limited accuracy and diagnostic value and their utility has diminished with the advent of brainstem-evoked response audiometry (AER/ABR). Brainstem-evoked response audiometry can be used as a further screening measure in patients with unexplained asymmetries in standard audiometric testing. Test results show a delay in nerve conduction time on the affected side, reflecting the probable presence of a tumor. Prior to magnetic resonance imaging (MRI), ABR was the most accurate screening modality. In centers experienced with its use, the test shows abnormalities in 90 to 95 percent of patients with tumors. However, the false negative rate can be as high as 30 percent with small acoustic neuromas, and there is a 10 percent false positive rate . Vestibular testing — Vestibular testing now has limited utility as a screening test for the diagnosis of acoustic neuroma because of the accuracy of evoked response audiometry. When testing is performed, a decreased or absent caloric response on the affected side may be seen. Imaging — Magnetic resonance imaging is the imaging procedure of choice because it is more sensitive than CT and is able to detect smaller tumors (as small as 1 to 2 mm in diameter). Thus, when brainstem testing is abnormal or the suspicion for an acoustic neuroma is high for another reason, MRI scanning with gadolinium contrast should be performed. If a patient cannot tolerate MRI, high resolution CT scanning with and without contrast can be substituted. Acoustic neuromas are seen on MRI and CT scans as enhancing lesions in the region of the internal auditory canal (IAC) with variable extension into the cerebellopontine angle. CT scans with bone windows can also be of prognostic significance as the extent of widening of the IAC and the extent of tumor growth anterior and caudal to the IAC are predictive of postoperative hearing loss. It has been suggested that fast spin echo MRI can be useful as a screening test due to its low cost compared to gadolinium MRI, noninvasiveness, and high sensitivity and specificity. In one study of 25 patients and 50 ears, there were 11 true positives and 39 true negatives on gadolinium MRI. There were no false positives or false negatives with fast spin echo MRI. This test is useful when performed specifically for evaluation for an acoustic neuroma, not as a general screen. TREATMENT — Once the diagnosis of an acoustic neuroma has been established, the three major treatment options are surgery, radiation therapy and observation. Chemotherapy has yet to be proven beneficial. Surgery — Surgery usually results in cure of acoustic neuroma, with only rare recurrences following total resection. As a result, the 1991 National Institutes of Health Vestibular Schwannoma (Acoustic Neuroma) Consensus Development Conference established surgery as the recommended primary modality for the treatment of acoustic neuromas. There are three standard operative approaches: Selection of a particular approach is determined by the size of the tumor and whether hearing preservation is being attempted. The suboccipital approach can be used for any size tumor with or without attempted hearing preservation. The translabyrinthine approach has been recommended for acoustic tumors larger than 3 cm and for smaller tumors when hearing preservation is not an issue. The middle fossa approach is suitable for small (<1.5 cm) tumors when hearing preservation is a goal. In many institutions, a team consisting of a neurosurgeon and an otologist perform the procedure and care must be taken in selecting experienced surgeons to maximize good patient outcomes. Several studies have suggested that, for a new surgical team, there is a significant learning curve for the removal of vestibular schwannomas consisting of 20 to 60 patients as judged by postoperative facial nerve function. With technologic advances, operative mortality has been reduced to virtually zero for this benign but potentially fatal tumor. Complete tumor removal is accomplished in almost all patients and there is little if any recurrence. The likelihood of surgical morbidity, which includes hearing loss, facial weakness, and vestibular disturbances, depends upon tumor size. Facial nerve function can be preserved in most patients even with large tumors and serviceable hearing can be preserved in many patients. However, only rarely does hearing improve after acoustic tumor surgery. Intraoperative facial nerve and auditory monitoring have alerted surgeons to potential injury, thereby improving the final outcome . The current outcomes that are achievable with an experienced team can be illustrated by the following observations: As noted in these studies, cerebrospinal fluid leaks are a common complication. Among patients treated with the enlarged translabyrinthine approach, it has been suggested that modifications in the surgical procedure can virtually eliminate this problem. The outcome is less favorable in patients who undergo subtotal removal in an attempt to preserve anatomical continuity of the facial or acoustic nerves. Regrowth and/or recurrence, which are usually asymptomatic, occur in up to 15 percent. Stereotactic radiosurgery — Stereotactic radiosurgery is a technique that utilizes multiple convergent beams to deliver a high single dose of radiation to a radiographically discrete treatment volume, thereby minimizing injury to adjacent structures. This can be accomplished with either a gamma-emitting Co60 unit, the gamma knife, or with a linear accelerator. Radiosurgery is a viable treatment option for selected patients with smaller tumors (<3 cm) or for enlarging tumors in patients who are not candidates for surgery. One of the largest experiences with stereotactic radiosurgery comes from 829 patients with acoustic neuroma treated at the University of Pittsburgh. The overall tumor control rate was 98 percent. No radiation-related second neoplasms were observed in this series, although others have reported such cases. In the original report of 162 patients from the University of Pittsburgh normal facial and trigeminal nerve function was preserved at five years in 79 and 73 percent of patients, respectively, using radiation doses of 18-20 Gy. By comparison, with the decreased radiation doses (13 Gy) used in the more recent extended series, the risk of new facial weakness was less than one percent, the risk of trigeminal sensory loss was three percent, and the percentage of patients with preservation of hearing increased from 50 to 70 percent. Stereotactic radiotherapy — Another approach, fractionated stereotactic radiotherapy, utilizes focused doses of radiation given over a series of treatment sessions. The intent is to reduce radiation injury to critical neural structures while preserving tumor control. In a series of 125 patients, tumors 3 cm or greater in diameter (n = 14) were treated with 30 Gy in 10 daily 3 Gy fractions, while tumors less than 3 cm (n = 111) received 25 Gy in five daily 5 Gy fractions. Tumor growth was controlled in all patients, and no patient developed facial weakness. The tumors responded similarly regardless of size, and the probability of maintaining useful hearing was not different for larger or smaller tumors. Of the 56 patients who underwent pretreatment and posttreatment audiograms, hearing improved in 10, was stable in 26, and was diminished in 20 after treatment. Proton beam therapy — Proton beam therapy may provide maximal local tumor control while minimizing cranial nerve injuries. The physical characteristics of the beam result in the majority of the energy being deposited at the end of a linear track, called a Bragg peak, with the dose falling rapidly to zero beyond the Bragg peak. Thus, the use of proton beam therapy permits the delivery of high doses of RT to the target volume while limiting the "scatter" dose received by surrounding tissues. In one report of 88 patients who were treated with fractionated proton beam therapy, the two- and five-year local control rates were 95 and 94 percent, respectively. Salvage therapy was necessary in five (radiosurgery in one, craniotome in one, and shunting for hydrocephalus in three). Seven of 21 patients with functional hearing retained serviceable hearing ability, while facial and trigeminal nerve function was preserved in 91 and 89 percent of patients, respectively. Observation — Since acoustic neuromas are typically slow growing, observation with follow-up MRI scans every 6 to 12 months may be warranted in patients with significant medical problems. A retrospective review of the literature identified 21 studies involving 1345 patients who were managed conservatively. With an average follow-up of 3.2 years, 43 percent of tumors showed evidence of growth, with an average growth rate of 1.9 mm/year. One problem with observation is increased hearing loss. Thus, if hearing preservation is an issue, earlier treatment is preferred. Observation is contraindicated in patients with large tumors and brainstem compression or hydrocephalus. In addition, patients with good hearing should be encouraged to undergo therapy to maximize the preservation of hearing. It may be preferable to delay surgery in elderly patients who do not meet criteria for immediate intervention. However, if there are signs or symptoms of tumor progression, age should not be considered a contraindication to surgery or an indicator of adverse prognosis. In one series of 61 elderly patients, for example, complete removal was achieved in all but two, there was no mortality, and the facial nerve and hearing were preserved in 95 and 41 percent, respectively. The American Society of Anesthesiology score, the preoperative Karnofsky score, and the size of the tumor exerted a significant influence on the outcome, but not patient age. |
Retromastoid suboccipital (retrosigmoid) route