Clinical Practice Guideline
Developed for the
Aerospace Medical Association
by their constituent organization
Overview: Acoustic neuroma (AN), also known as acoustic schwannoma, acoustic neurinoma, vestibular schwannoma, and vestibular neurilemoma, is a benign Schwann cell derived tumor commonly arising from the inferior vestibular branch of the eighth cranial nerve. These tumors account for approximately 8 percent of intracranial tumors in adults and 80 to 90 percent of all cerebellopontine angle (CPA) tumors. The overall incidence of symptomatic AN is about 1:100,000 , and appears to be increasing, possibly due to the incidental diagnosis of asymptomatic lesions with the widespread use of magnetic resonance imaging (MRI) and computed tomography. Symptoms associated with AN are typically associated with cranial nerve involvement, cerebellar compression, and tumor progression. One large study revealed that the acoustic nerve was involved in almost all cases, with the vestibular, trigeminal and facial nerves involved less frequently. The median age at diagnosis is approximately 50 years. The tumors are unilateral in more than 90 percent of cases4, affecting the right and left sides with equal frequency. Bilateral AN is primarily limited to patients with autosomal dominant neurofibromatosis type 2 (NF-2). Any patient over 18 years of age who has a unilateral AN and another neurologic tumor in the brain or spine should be screened for NF-2.
The aviator with asymmetric hearing loss should be evaluated for AN. The individual is often unaware of any hearing deficit and many of the cases seen by the Air Force at the Aeromedical Consultation Service (ACS) were discovered by observing changes in the annual audiogram.
AN has 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. However, up to 40 percent of tumors overall and a higher percentage of small tumors show no growth or even shrink on serial imaging studies. There is no predictive relation between growth rate and tumor size at presentation. Symptoms associated with AN are due to cranial nerve involvement, cerebellar compression, and tumor progression. In a series of 1,000 acoustic neuroma cases treated at a single institution, the acoustic nerve was involved in almost all cases (95%) presenting with hearing loss and tinnitus. This was followed by the vestibular nerve (61%) associated with unsteadiness while walking and the trigeminal nerve (17%) presenting with facial numbness (paresthesia), hypesthesia, and pain. Finally, least involved was the facial nerve (6%) which may present with facial paresis and, less often, taste disturbances8. Direct extension of the tumor to surrounding anatomic structures may induce ataxia (brainstem), or involve the functions of the lower cranial nerves (IX, X, and XI), leading to dysarthria, dysphagia, aspiration, and hoarseness.
Once the diagnosis of an acoustic neuroma has been established with a thorough history/physical examination, audiometry, vestibular testing and imaging, three major treatment options are currently available: 1) observation, 2) surgery, and 3) radiation therapy. Pharmacotherapy has yet to be proven beneficial; however, research with Cox-2 inhibitors (OSU-03012 and OSU-HDAC-42) at Ohio State University show potential for tumor control and regression.
Tumor growth rate typically falls within the range of slow (0.02cm/yr) or medium (0.2cm/yr). Patients may elect observation, especially if they have minimum symptoms. Obviously, tumors that are removed when they are small offer better outcomes.
With technologic advances, operative mortality has been reduced to less than 1% at high volume centers for this benign but potentially fatal tumor. Complete tumor removal can be accomplished in most patients (depending on tumor size) and there is rare chance for 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.
Finally, most widely touted are the varieties of radiation therapy options offered to AN patients. Over 10,000 AN patients have been treated worldwide by radiation. These may be delivered by gamma knife (Cobalt60) or via linear accelerators. Furthermore, the treatment may be further modified by fractionation and by reducing tumor radiation dose from 16 Gy to 12-13 Gy. Fractionation appears to decrease risk of injury to other cranial nerves. Though all tumor sizes may be treated, smaller, non-cystic tumors tend to do better. While it is presumed that the tumor observed is an AN, other lesions such as neurosarcoidosis have been known to mimic these tumors. Radiated tumors tend to swell/expand at 6 to 24 months. Hearing loss post-radiation generally is less than with that of surgery. Imbalance may be seen in 5-10% of patients and facial palsy in less than 1%. However, the biggest drawback of radiation therapy is the small but not insignificant risk of malignancy, which is currently estimated at 1:100,000. Also, some tumors fail to respond to radiation and continue to grow. Those requiring surgery tend to have poorer cranial nerve outcomes due to operating in an irradiated field. Finally, these patients will probably require lifelong follow-up.
Aeromedical Concerns: Cochlear and vestibular symptoms are of obvious importance in the aviator; hearing loss and tinnitus impact communications, while vertigo and disequilibrium may affect control of the aircraft. Because of the wide range of progressive and sometimes abrupt symptomatology, conservative observation therapy is not consistent with safe performance of cockpit duties. All post-operative or post-radiation vestibular compensation symptoms need to be resolved prior to waiver consideration and any hearing loss needs to be stabilized and well documented by competent audiology services. In the military services an in-flight hearing evaluation will most likely be required prior to clearing an aviator for flying duties.
Medical Work-up: Evaluation needs to include a complete history to include all symptoms, hearing exams prior to any treatment, treatment course, post-surgical vertigo symptoms (if any) and how resolved. The exam needs to mention all audiograms and an eye exam with an emphasis on eye tracking. All surgical reports and pathology reports need to be included as well as an ENT consultation report. All imaging tests need to be reviewed. If the aviator is military there may be a need for a tumor board and all military will need to accomplish a medical evaluation board.
Air Force: Acoustic neuroma is not specifically addressed in AFI 48-123, but can be covered under several other headings. A18.104.22.168. discusses a “history of surgery involving the middle ear, excluding cholesteatoma”; A22.214.171.124. covers “any conditions that interfere with the auditory or vestibular functions”; A4.5.1. states “hearing loss greater than H-1 profile, or asymmetric hearing loss, requires work-up by an audiologist (audiology evaluation for initial waiver and waiver renewals must have been accomplished within 12 months of submission to waiver authority); and A126.96.36.199. addresses “history of tumor involving the brain or its coverings”. Waivers are required for H-3 hearing loss or greater. Waiver requests may be submitted six months after successful treatment of the AN provided any post-treatment sequelae are within acceptable respective flying-class limits. The tumor must have been unilateral, and there must be complete resolution of vertigo post-treatment. Residual cranial nerve deficits should allow full ocular movements without tracking deficits or strabismus, and allow for acceptable protective mask sealing. ENT and neurology consultations are required for waiver consideration (also audiology if hearing deficit occurs). Confirmation of tumor pathology is requested with invasive surgical cases, and MRI with follow-up is needed in cases treated non-invasively, since acoustic neuromas have characteristic findings on MRI.
Army: Acoustic Neuroma is discussed an Aeromedical Policy Letter by the same name. The aeromedical concerns of the US Air Force are the same as those of the US Army. The long term sequelae of this tumor and condition of the aviator after treatment carry the most impact. Up to 50% of patients will have no useful hearing in the involved ear after surgery. Facial paralysis may make wearing of an oxygen mask difficult, may result in speech problems, and can cause eye symptoms due to inability to close the eyelids. However waiver is possible for Army aircrew and depends on the capability of the aviator 6 months after successful removal of the tumor. Specifically, the tumor must have been 2.5 cm diameter or less; unilateral, postoperative vertigo must have completely resolved; and any damage to cranial nerves should allow full eye movement without strabismus or tracking deficit and acceptable mask sealing. Psychomotor performance should be within normal limits for aircrew members. If a waiver is granted, annual otolaryngology evaluation is a waiver requirement.
Navy: One year following successful excision of a unilateral tumor, a waiver may be considered if there are no serious sequelae. Vertigo, ataxia, and facial paralysis are examples of unacceptable complications. Unilateral hearing loss, even total loss, may well be waiverable provided adequate hearing remains in the other ear and the hearing loss is compatible with the member's mission.
Civilian: The civilian Aviation Medical Examiner should also be concerned over the disease particulars mentioned above. Before a medical certificate can be issued to a civilian pilot, they will require a complete ENT evaluation after treatment and full recovery from the condition. The evaluation should be submitted in writing to the FAA and must also include audiologic tests. If any neurologic sequelae are present, a neurologic evaluation should also be performed and a report submitted. The airman must be able to successfully pass one of the three hearing tests (Conversational Voice at six feet, Audiogram, or Speech Discrimination) required by the FAA. It is unlikely that medical certification will be granted if a conservative wait and watch approach is taken. No follow up is required unless there is some complication. The FAA has granted medical certification to airmen who have not had their tumor completely removed. These airmen are placed on an Authorization and followed with serial status reports and MRI of the tumor.
Air Force: Review of AIMWTS cases showed 17 cases of AN; 0 FC I/IA, 14 FC II, 3 FC III. All but one case was granted a waiver. One FC III aviator was disqualified due to residual extremity weakness and facial nerve weakness. Five of the FC II cases (all pilots) were granted a FC IIC waiver which stated they were not to be assigned to any aircraft requiring stereoacusis. Of the 17 cases, 2 were treated with stereotactic radiation therapy and no surgery, 12 were treated with surgery alone, one was treated initially with surgery and later with radiation therapy and two had small lesions and had had no surgery at the time of the most recent aeromedical summary. One pilot had the diagnosis of an acoustic hamartoma which is similar to an acoustic neuroma in location and treatment.
Army: Since 1990 there have been 123,259 aviators of all types, including applicants, enrolled in the Aeromedical Epidemiological Data Repository. Among them there have been 36 cases of acoustic neuroma; 28 were in rated aviators and only 7 of these were disqualified. There was one case in an applicant with was waivered.
Navy: Not available at this time.
Civilian: As of January 2010 there were 77 first-class, 143 second-class and 360 third-class airmen currently issued with this condition.
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Prepared by Drs. Richard Serkowski and Dan Van Syoc