Audiological Aspects of the Diagnosis of Acoustic Neuromas*

E. W. JOHNSON, PHD., AND J. L. SHEEHY, ]V[.D. University of Southern California School of Medicine, Los Angeles, California

DITORY symptoms are usually the first sensori-neural, indicating that the area in- indication of a developing acoustic volved may be either sensory (that is coch- neuroma. A high index of suspicion lear or inner ear) or truly neural. Tests of and the use of sophisticated auditory tests speech discrimination, auditory fatigue, and will frequently make it possible to diagnose loudness function help to identify the exact this tumor at a time when it is still contained site of the lesion, cochlear or retrocochlear. in the internal auditory canal and presents It is the retrocochlear type of pattern that is only auditory clues. The advent of micro- found in most cases of acoustic neuroma. surgical techniques for the approach to these small tumors has emphasized the diagnostic Pure Tone Audiometry value of precise auditory testing. 4,9,1~ The establishment of the pure tone audio- The purpose of this article is to review gram is the first step in audiologic evaluation. briefly the auditory tests used in the diag- The patient with an acoustic neuroma will nosis of acoustic neuroma. The typical test have a sensori-neural type of hearing im- findings will be discussed and screening tests pairment, usually limited to or worse on the which may be performed in the general affected side. Furthermore, the configura- physician's office will be described. tion or pattern for loss of frequency may be What Are We Evaluating? significant. Seventy per cent of 53 surgically confirmed tumors in a recent series s had Auditory evaluation in the past has been audiograms that showed a loss of high fre- limited to a pure tone audiogram, testing quency tone perception (Fig. 2). air and bone conduction. The resulting au- diogram showed either a conductive or nerve Speech Audiometry type of impairment (Fig. 1). A nerve im- The second step in auditory evaluation is pairment was found in patients with acous- testing the patient's ability to hear and to tic neuroma. The audiogram was of little understand speech. Understanding of speech help in early diagnosis because there were (speech discrimination) is usually grossly many other conditions which could be the impaired in retrocochlear lesions. cause of a similar audiogram. The tests may be administered by "live It became apparent, however, that there voice," record or tape recording through a were many differences in the way a patient speech circuit in the audiometer. We prefer with an acoustic neuroma responded to tape recording 5 because it reduces distortion speech, to loud tones, and to prolonged and is free from the surface noise present stimulation of soft tones. These differences after records have been played a number of were correlated with the clinical picture and times. began to show a typical pattern. Patients The speech reception threshold (SRT) is whose nerve impairment was due to a coch- established by using bisyllabic words (spon- lear disorder such as Meni6re's disease showed dee words) such as baseball, cupcake, wash- one type of pattern, whereas those with a board, etc. The intensity level at which the true nerve or retrocochlear lesions such as an patient repeats 50 per cent of these words acoustic neuroma showed quite a different correctly is the SRT and should approximate response. the threshold for pure tones. We now refer to a nerve impairment as The patient's ability to understand speech is of greater importance. To determine the Received for publication June ~, 1965. * Sponsored by the Los Angeles Foundation of speech discrimination, monosyllabic words Otology. (phonetically balanced or PB words) such as 6~1 6~ E.W. Johnson and J. L. Sheehy

FIG. 1. Audiogram illustrating conductive impairment in the right ear and sensori-neural (nerve) impairment in the left ear. Solid line represents air conduction and dotted line bone conduction. rip, goose, earth, nudge, etc. are presented to tion. More than half of the words will be the patient ~0 to 40 decibels above the SRT. understood in most eases. This increased intensity is to insure that the The patient with an acoustic neuroma will words are heard loud enough to be under- usually show gross discrimination impair- stood if possible. ment. In the series of confirmed tumors, 9 A list of fifty words is presented. The per- two thirds had a PB score of 30 per cent or centage correct is recorded as the PB or dis- less. Half of those in this group had absolutely crimination score. Normal patients, and no capacity for understanding speech. those with a conductive impairment, will have perfect scores (90 to 100 per cent). Auditory Fatigue Tests Those with a sensori-neural hearing impair- ment will show a varying degree of impaired A person with normal hearing or one with discrimination. a conductive impairment is able to hear a Cochlear lesions such as Meni~re's disease continuous tone, presented just above thresh- or hereditary or aging changes in the inner old, for a prolonged period of time without ear will show a moderate loss of discrimina- developing significant auditory fatigue. In sensori-neural impairments a varying amount of adaptation or auditory fatigue will occur, depending upon the location of the lesion. Auditory fatigue is minimal, and usually limited to the higher frequencies in cochlear disorders. In eases of acoustic neuroma there may be severe or total tone decay (au- ditory fatigue). There are two tests designed to measure this auditory fatigue or adaptation : the mod- ified tone decay test (MTDT), 3 and diag- nostic B6k6sy audiometry. 6 Modified Tone Decay Test. The MTDT is the simpler of the ~ tests. It may be carried FIG. ~. Audiogram showing high-frequency loss con- out with any standard audiometer in a very figuration in a verified acoustic neuroma (solid circles). brief period of time. The procedure is as The opposite ear (open circles) is normal. follows :