HEARING TESTING

Timothy C. Hain, MD

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Figure 1. The Ear. Hearing can be impaired due to lesions in the external ear canal, the middle ear, or the inner ear. The purpose of hearing testing is to evaluate hearing function and, if it is impaired, to attempt to localize the site of lesion.

 

What is Hearing Testing?

Hearing testing is a means of evaluating an individual's overall hearing function. The tests are used to determine if there is something wrong with the hearing (auditory) portion of the inner ear. They are often used as an initial screening to decide if more expensive tests like magnetic resonance imaging (MRI) are needed. They are sometimes used in conjunction with vestibular testing to diagnose specific disorders, such as Meniere's disease. Finally, hearing tests can be used to decide if a hearing aid might be helpful.

The following hearing tests are briefly described below:

Bedside Testing of Hearing

There are many ways of testing hearing in the office. Bedside testing may be used as a screening procedure. While formal audiometry is preferable, for reasons of expense or access, it may not always be possible. In bedside testing, tuning forks (Figure 2) are often used to test at chosen frequencies. A variety of other methods (whisper, rubbed fingers, ticking watch, and so forth) can be used to quantify hearing using readily accessible sources of noise.

Figure 2:

Three different sized tuning forks. The higher pitched forks (such as the 512 hz fork) are more appropriate for hearing testing. (c) Timothy C. Hain, MD. 2002

The single most common office test is a tuning fork test called the Rinne, named after Adolf Rinne of Gottingen, who described this test in 1855. In the Rinne test, a comparison is made between hearing elicited by placing the base of a tuning fork applied to the mastoid area (bone), and then after the sound is no longer appreciated, the vibrating top is placed one inch from the external ear canal (air). A positive Rinne indicate an air-bone gap and, therefore, presumed presence of a conductive hearing loss.   Frequencies from 256 to 1024 Hz have been advocated. When the Air-bone gapis less than 17.5 dB or greater than 30 dB, the Rinne is usually negative (Jacob et al, 1993). Because of this, the Rinne has limited utility.

In the Weber test, a 512 Hz tuning fork is placed on the patient's forehead. If the sound lateralizes (is louder on one side than the other), the patient may have either an ipsilateral conductive hearing loss or a contralateral sensorineural hearing loss.

Other tuning fork tests include the Bing and Schwabach tests. In the Bing test, the fork is struck and placed on the patient's mastoid tip. The examiner alternately occludes the patient's external meatus. If the patient has normal hearing or a sensorineural loss, he or she will notice a change in intensity with occlusion. If the patient has a conductive hearing loss, he or she will notice no change.

The Schwabach test compares the patient's bone conduction to that of the examiner's. If the patient stops hearing before the examiner, this suggests a sensorineural loss. If the patient hears it longer than the examiner, this suggests a conductive loss. Of course, this test is contingent on the examiner having normal hearing.

Other methods can be used. The Hearing Handicap Inventory is a questionaire that has been well studied and thought to provide a good indication as to whether the person taking the test has impaired hearing.

The Welch Allyn Audioscope is a hand-held device that can be used for hearing screening. Tones are presented to each ear between 500 and 4000 hz and the listener is asked to indicated whether he or she can hear the tone.

Audiometry

Figure 3:

Audiogram for a Patient With Normal Hearing

Audiogram for a patient with normal hearing

Tympanometry for a patient with normal hearing

Acoustic reflex thresholds for a patient with normal hearing

The key for the audiogram for a patient with normal hearing Speech audiometry for a patient with normal hearing

COMMENTS: DNT = did not test. Pounding heartbeat right ear - constant. No evidence of heartbeat rhythm in tympanogram.


Audiometry is the term used to describe formal measurement of hearing. The measurement is usually performed using an "audiometer" by an "audiologist". An audiologist is a non-medical healthcare professional specializing in the evaluation and rehabilitation of people with hearing loss. Audiologists have either a master's or doctoral degree in audiology. Coursework includes anatomy and physiology of the ear, psychoacoustics, behavioral and electrophysiologic testing, hearing aids, and the study of lip-reading, auditory training and other rehabilitation techniques. There are several subtests that are in aggregate called the audiogram.

Pure tone audiometry

In pure tone audiometry, hearing is measured at frequencies varying from low pitches (250 Hz) to high pitches (8000 Hz). An example of an audiogram in a person with normal hearing is shown in figure 3. The hearing level (HL) is quantified relative to "normal" hearing in decibels (dB), with higher numbers of dB indicating worse hearing. The dB score is not really percent loss, but neverthless 100 dB hearing loss is nearly equivalent to complete deafness for that particular frequency. A score of 0 is normal. It is possible to have scores less than 0, which indicate better than average hearing. Pure-tone average (PTA) is the average of pure tone hearing thresholds at 500, 1000, and 2000 Hz.

Figure 4:Audiogram for a Patient With an Acoustic Neuroma

Audiogram for a patient with an acoustic neuroma

Tympanometry for a patient with an acoustic neuroma

Acoustic reflex thresholds for a patient with an acoustic neuroma

The key for the audiogram for a patient with an acoustic neuroma Speech audiometry for a  patient with an acoustic neuroma

 

Figure 4 illustrates an audiogram of a person with a tumor called an acoustic neuroma. Hearing is worse for the left ear (squares) than the right ear (circles) although both ears are at least partially outside the normal range.

Hearing loss is often described in words as follows:

When there is a hearing loss, the next step is to try and determine whether the loss is caused by a sensory problem (sensorineural hearing loss) or a mechanical problem (conductive hearing loss). This distinction is made by using a bone vibrator, which bypasses the mechanical parts of the middle ear. If hearing is better using bone than air, this suggests a conductive hearing loss.

Routine audiometry, like routine eye care, is often not covered by health insurance in the United States. When audiometry is performed for specific medical reasons (for example, to follow Meniere's disease), it may be covered.

You may wish to see the more extensive discussion of pure tone audiometry on the emedicine site.

Speech audiometry

There are a number of special subtests that are optionally included in the audiometry procedure. Speech audiometry involves reading a list of words to see if patients can discriminate words. By comparing speech comprehension with anticipated speech comprehension, inferences can be made about central processing and central hearing deficits. One of the most basic measurements as the speech reception threshold. This test determines the lowest intensity level at which the patient can correctly identify 50% of common two-syllable words such as: baseball, airplane, mushroom. The SRT should be in close agreement with pure-tone threshold results. As a rule of thumb, the pure tone average or PTA (see above) should match the SRT, within 5 dB, and the speech detection threshold (SDT), within 6-8 dB. A significant difference between the two thresholds would raise questions about the validity of the pure tone thresholds, such as in cases of exaggerated hearing loss. There are many other methods that can be used to detect exaggerated hearing loss..

Word recognition tests (also known as speech discrimination tests) assess the person's ability to understand speech when presented at a loudness that is well above their threshold. The result is presented as a percentage score. There should be a correlation between the type and degree of hearing loss and the word recognition score (WRS), but this depends on the cause of the hearing loss. For example, a person with a moderate conductive loss might score 88% on a word recognition test, but a person with a similar moderate retrocochlear hearing loss, might only score 28%. The WRS can be helpful in predicting the usefulness of a hearing aid. An increase in the WRS with amplification, suggests that a hearing aid might be useful.

Complex speech tests are mainly used in evaluations of central auditory processing (CAP). Persons with CAP may have normal pure tone thresholds, and perhaps even normal word recognition ability, but are unable to process complex speech signals. One commonly used test presents two different words to each ear simultaneously (a dichotic task). Persons with normal CAP can repeat both words easily, while someone with a temporal lobe problem might be unable to repeat the word presented to the ear contralateral to the lesion. This results resembles the results of simultaneous visual or sensory stimuli in persons with parietal lobe disturbances.

Acoustic reflex testing

Acoustic reflex testing consists of subjecting the ear to a loud sound and determining if it causes the stapedius muscle to tighten the stapes. Acoustic reflexes are mainly useful as a crude but non-subjective method of evaluating hearing, as the stapes should tighten for a given level of perceived loudness. Acoustic reflexes can also be a sign of brainstem dysfunction. Click here for more detail.

Tympanometry

Tympanometry for a patient with an acoustic neuroma

Figure 5:

Tympanometry from case of figure 4 above.

Tympanometry is a measure of the stiffness of the eardrum and thus evaluate middle ear function. This test can be helpful in detecting fluid in the middle ear, negative middle ear pressure, disruption of the ossicles, tympanic membrane perforation, and otosclerosis.

To perform the test, a soft probe is placed into the ear canal and a small amount of pressure is applied. The instrument then measures movement of the tympanic membrane (eardrum) in responses to the pressure changes.

The result of the test is recorded in a visual output, called a tympanogram. If there is fluid in the middle ear, the tympanic membrane will not vibrate properly and the line on the tympanogram will be flat. If there is air in the middle ear (the normal condition) but the air is at a higher or lower pressure than the surrounding atmosphere, the line on the tympanogram will be shifted in position.

The pressure readings produced by tympanometry do not reflect true middle ear pressure and are subject to substantial errors, especially in persons with small mastoid sinus cavities (Cinamon and Sade, 2003).

References:

Cinamon U, Sade J. Tympanometry versus direct middle ear pressure measurement in an artificial model: is Tympanometry an accurate method to measure middle ear pressure ? Otology and Neurotology 24:850-53, 2003

Otoacoustic Emissions

This is a relatively new test used to assess hearing in newborns as well as being a method of determining whether the cochlea is functioning. With this test, a probe that contains both a tiny speaker and a tiny microphone is inserted into the ear canal. Quiet tones are sent from the speaker, which travel through the middle ear and stimulate the hairs in the cochlea The hairs respond by generating their own minute sounds, which are detected by the microphone. If there is a hearing loss, the hairs in the cochlea do not generate these minute sounds.

Detection of Malingering

Unfortunately, because hearing reduction can be the basis for lawsuits and compensation, exaggeration of hearing loss is not uncommon in these situations. Fortunately, there are numerous methods of measuring hearing independently of subjective responses. These include purely electrical tests such as brainstem or cortical auditory evoked responses, as well as strong psychophysical tests such as the Stenger. (Hone et al, 2003)

Acknowledgments

Figures 1,3 and 4 are courtesy of Northwestern University, and were assembled by Pam Fiebig, Senior lecturer in Audiology. The remainder of the figures are copyrighted to Timothy C. Hain, MD

References