Acoustic neuroma

Acoustic neuroma is a rare but important cause of sensorineural hearing loss. Patients with asymmetric hearing loss or unilateral tinnitus should be evaluated expeditiously to prevent further neurologic damage. Gadolinium-enhanced magnetic resonance imaging is the best tool for making the diagnosis of acoustic neuroma. Surgical removal using a translabyrinthine approach, is the favored treatment, although radiation therapy and expectant management may be chosen for patients at high surgical risk.

Acoustic neuroma, also referred to as vestibular schwannoma, arises from the Schwann cells covering the vestibular branch of the eighth cranial nerve as it passes through the internal auditory canal (Figures 1 and 2). The tumor compresses the cochlear (acoustic) branch, causing hearing loss, tinnitus and disequilibrium. Continued growth results in expansion of the tumor into the cerebellopontine angle, where the facial and trigeminal nerves may be damaged. Larger tumors may encroach further into the brainstem, resulting in signs of increased intracranial pressure.[1]

The annual incidence of acoustic neuroma is one case in 100,000 persons. The female-to-male ratio is 1.5:1. Less than 5 percent of acoustic neuromas are bilateral, occurring in association with type II neurofibromatosis, a rare autosomal dominant disorder.[2] Acoustic neuromas are histologically benign. Both the sporadic neuromas and the more rare familial bilateral tumors are thought to result from a functional loss of a tumor-suppressor gene on chromosome 22.[3]

Illustrative Cases

CASE 1

A 35-year old woman presented with a "hum" in her right ear. Otoscopic examination and Weber's and Rinne testing revealed normal results. An otolaryngologist noted a mild symmetric low-tone hearing loss and a family history of Meniere's disease. The patient's symptoms persisted, and repeat audiograms one year later showed a marked loss of hearing in the right ear (Figure 3). Magnetic resonance imaging (MRI) disclosed a 0.9-cm tumor in the right internal auditory canal. An acoustic neuroma was surgically removed using a translabyrinthine approach.

COMMENT

While asymmetric sensorineural hearing loss is the hallmark of acoustic neuroma, up to one-third of patients eventually diagnosed as having acoustic neuroma initially present with symptoms such as unilateral tinnitus, imbalance, headache, fullness in the ear, otalgia or facial nerve weakness.[4] Up to 15 percent of patients with acoustic neuroma have normal results on an audiogram.[1] Patients with asymmetric symptoms should be followed closely, even if results on the audiogram are normal.

CASE 2

A 51-year-old man, being treated for hypertension, mentioned difficulty hearing soft voices. The audiogram showed symmetric pure tone thresholds of 30 to 45 dB. The patient returned for a physical examination four years later and reported increased hearing loss on the right side. Audiogram showed a high frequency loss pattern with thresholds up to 80 dB, worse in the right ear (Figure 4). MRI revealed a 2.5-cm acoustic neuroma (Figure 5), which was surgically removed using a translabyrinthine approach.

COMMENT

This patient presents a slow onset of sensorineural hearing loss lateralizing gradually to the affected side. A series of 136 patients diagnosed in the late 1980s showed that the average time from onset of symptoms to discovery of acoustic neuroma is about four years.[1] Another review showed that 38 (6 percent) of 614 patients with acoustic neuroma had normal hearing or a symmetric hearing loss preoperatively.[5] In patients whose hearing seems to be deteriorating without an evident cause, evaluation for acoustic neuroma may be warranted.

Diagnostic Evaluation

After physical examination, the pure tone audiogram is the first step in evaluating hearing loss. Sensorineural hearing loss can usually be distinguished from conductive hearing loss by tuning fork tests (Weber and Rinne) and confirmed by bone conduction audiometry Acoustic neuroma will cause a high-tone loss pattern in 66 percent of patients. Low-tone, flat or trough-shaped loss patterns each account for about 10 percent of patients with neuromas.[6] Speech discrimination testing may increase sensitivity of the audiogram but does not improve specificity.[1]

A variety of tests, such as auditory brain-stem response, acoustic reflex decay and electronystagmography, have been used to evaluate patients with acoustic neuroma. Of these, only auditory brainstem response testing has been shown to have useful sensitivity (93 to 98 percent) and specificity (90 percent).[1] Unfortunately, the auditory brainstem response requires a pure tone threshold of less than 70 dB to provide meaningful results.[4] It may be best used in patients with low-probability symptoms and well-preserved hearing.[7]

Imaging of acoustic neuroma has improved markedly with the advent of gadolinium-enhanced MRI scans. From 1975 to 1989, mean tumor size at the time of detection decreased from 27.9 mm to 16.5 mm, probably as a result of the increased use of MRI.[7] High-resolution computed tomographic (CT) scanning has a sensitivity of only 48 percent for acoustic neuromas less than 15 rnm.[8] A CT scan using gas-contrast enhancement of the posterior fossa is a more invasive alternative if MRI is not available.[4]

Differential Diagnosis

The diagnostic challenge presented by hearing loss was discussed in a recent review.[9] In general, most other causes of sensorineural loss result in a deficit that remains symmetric and is less rapidly progressive (Table 1). The tinnitus and vertigo of Meniere's disease may present a dilemma. Usually the tinnitus of Meniere's disease is intermittent and distracting, compared with the continuous and less disturbing tinnitus caused by an acoustic neuroma. Patients with an acoustic neuroma more often present with imbalance or disequilibrium than with true vertigo.[1]

The increased use of MRI has led to the incidental discovery of acoustic neuroma in asymptomatic patients. Depending on underlying medical status and whether the patient has serviceable hearing in the other ear, the potential of improved hearing preservation that is associated with early surgery should be weighed against the benefits of simple observation. A study of 50 patients followed by serial imaging showed a mean growth rate of 1.1 mm per year. Tumor growth varied considerably, and some tumors were actually diminished. No factors, including initial tumor size, were predictive of subsequent tumor growth.

REFERENCES

[1.] Selesnick SH, Jackler RK. Clinical manifestations and audiologic diagnosis of acoustic neuromas. Otolaryngol Clin North Am 1992;25:521-51. [2.] Chandler CL, Ramsden RT. Acoustic schwannoma. Br J Hosp Med 1993;49:335-43. [3.] Consensus Development Panel, National Institutes of Health. Consensus Development Conference Statement on Acoustic Neuroma. Arch Neurol 1994;51:201-7. [4.] Harvey SA, Haberkamp TJ. Pitfalls in the diagnosis of CPA tumors. Ear Nose Throat J 1991;70:290 8,303-4. [5.] Roland PS, Glasscock ME 3d, Bojrab DI, Josey AF Normal hearing in patients with acoustic neuroma. South Med J 1987;80:166-9. [6.] Johnson EW. Auditory test results in 500 cases of acoustic neuroma. Arch Otolaryngol 1977,103:152-8. [7.] Welling DB, Glasscock ME 3d, Woods CI, Jackson CG. Acoustic neuroma: a cost-effective approach. Otolaryngol Head Neck Surg 1990;103:364-70. [8.] Mikhael MA, Wolff AP, Ciric IS. Current concepts in neuroradiological diagnosis of acoustic neuromas. Laryngoscope 1987;97:471-6. [9.] Nadol JB Jr. Hearing loss. N Engl J Med 1993;329: 1092-102. [10.] Lalwani AK. Meningiomas, epidermoids, and other nonacoustic tumors of the cerebellopontine angle. Otolaryngol Clin North Am 1992;25:707-28. [11.] Jackler RK, Pitts LH. Selection of surgical approach to acoustic neuroma. Otolaryngol Clin North Am 1992;25:361-87. [12.] Wiet RJ, Teixido M, Liang JG. Complications in acoustic neuroma surgery. Otolaryngol Clin North Am 1992,25:389-413. [13.] Haines SJ, Levine SC. Intracanalicular acoustic neuroma: early surgery for preservation of hearing. J Neurosurg 1993;79:515-20. [14.] Shelton C. Hearing preservation in acoustic tumor surgery. Otolaryngol Clin North Am 1992;25:609-21. [15.] Lunsford LD, Linskey ME. Stereotactic radiosurgery in the treatment of patients with acoustic tumors. Otolaryngol Clin North Am 1992;25:471-91. [16.] Nedzelski JM, Schessel DA, Pfleiderer A, Kassel EE, Rowed DW. Conservative management of acoustic neuromas. Otolaryngol Clin North Am 1992;25:691-705.

The Author

GERARD D. SPOELHOF, M.D. is clinical assistant professor at the University of Minnesota-Duluth School of Medicine. Dr. Spoelhof earned a medical degree from the University of Michigan Medical School, Ann Arbor, and completed a residency in family medicine at Duluth (Minn.) Graduate Medical Education Council.

Address correspondence to Gerard D. Spoelhof, M.D., Skyline Family Medical Center, 2900 Piedmont Ave., Duluth, MN 55811

Adapted from Nadol JB. Hearing loss. N Engl J Med 1993;329:1092-102. Used with permission.

COPYRIGHT 1995 American Academy of Family Physicians
COPYRIGHT 2004 Gale Group