|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.|
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:
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.
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, 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). The Rinne is abnormal when one can hear the tuning fork against the mastoid, but not next to the pinna. Because air conduction is generally better than bone, tested in this fashion, the inability to hear the air as well as the bone suggests that there is an air-bone gap and, therefore, presumed presence of a conductive hearing loss. Frequencies from 256 to 1024 Hz have been advocated. When the audiometric air-bone gap is less than 17.5 dB or greater than 30 dB, the Rinne is usually normal (Jacob et al, 1993). Because of this, the Rinne has limited utility. It is best to use the words "normal" and "abnormal" to describe the Rinne, because the words "positive and negative" can be misinterpreted.
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.
There recently has been suggestions in articles authored in part by audiologists, that medical doctors should not rely upon bedside hearing tests (Boatman et al, 2007; Kauffman et al, 2007). Kauffman suggested that because the bedside tests are not as good as formal audiometry (see below), that "the Rinne and Weber tests results should not prevent neurologists from indicating an auditory test ..." Boatman et al suggested that "when there is suspicion of hearing loss, we agree with Kauffman et al that audiometry should be performed even if bedside tests are normal" The methodology of these papers was to compare to subjective tests - -Rinne/Weber vs. audiometry in a sound booth, and to assume that all mistmatches were due to errors in the beside testing.
Going back to the bigger picture, we see these suggestions, which can be summarized as suggesting that subjective beside testing doesn't always match subjective sound-booth testing, as a little misleading. In our opinion, formal audiometry is appropriate when either the patient fails the bedside testing to a significant degree, or indicates in the history that there is a bothersome hearing disturbance. Audiometry is not infallable either.
We think that the reliabilty of the entire bedside hearing evaluation is rather good, when combined with an alert examiner who is observant and uses his/her voice as a testing instrument during the evaluation. We also think that what matters the most is how well the patient understands speech in the real world. In this regard, newer methods of testing hearing such as the Quick SIN (Speech-In-Noise) test have been developed, and where available, may provide results closer to the "real world". .
Audiogram 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-physician healthcare professional specializing in the evaluation and rehabilitation of people with hearing loss. Audiologists have either a master's or doctoral degree in audiology. The "doctoral degree" of audiologists is generally not a Ph.D type academic doctorate, and also is not the "medical doctor" degree of physicians. Rather, it is similar to the optometry doctoral degree.
Most AuD programs - -that matriculate audiologists, are 3-4 years added onto a bachelors degree in a life science. In other words, graduates of AuD programs generally have about the same number of years of higher education as someone who has just finished medical school (e.g. 8 years). Of course, no freshly graduated medical student would be able to work as a "doctor" without another 4 years of residency and then perhaps 2 years of fellowship.
The audiology "AuD" degree training process is similar to the "DPT" -- doctor of physical therapy training process. Both are 3-4 years added onto a science bachelors degree. Thus the number years of education required to practice for a person with an audiology or physical therapy doctoral degree is substantially less than that required for a physician. There are similar degrees for psychologists, pharmacists, and many other allied health providers. The proliferation of "doctors" can be very confusing to patients. For example, a patient might wonder -- can the "doctor of audiology" take care of my diabetes and prescribe medication?
Coursework in audiology 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.
In pure tone audiometry, hearing is measured at frequencies varying from low pitches (250 Hz) to high pitches (8000 Hz). This is just a part of the entire human auditory range, which extends between 20 and 20,000 hz. Neverthless, most audiometers are designed so that they cannot go as low or high as most good stero systems.
You can get an idea of what an audiogram sounds like by clicking on these sound files:
(While these sounds were designed to be of all the same intensity, computers vary and this should not be relied upon).
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.
Hearing loss is often described in words as follows:
- Normal hearing
- < 25 db HL (adults)
- < 15 db HL (children)
- Mild hearing loss = 25-40 db HL
- Moderate hearing loss = 41-65 dB Hl
- Severe hearing loss = 66-90 db HL
- Profound hearing loss = 90+db HL
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.
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.
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.
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).
More about tympanometry can be found here.
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.
OAE testing, like ASSR (see below), is very suitable to assessment of malingering, as it requires no cooperation.
This is a method of measuring hearing function that does not require cooperation. It is not generally available, and is not a routine test. It is most useful in testing persons who cannot or will not cooperate with testing - -young children, and persons suspected of malingering (see below).
Unfortunately, because hearing reduction can be the basis for lawsuits and compensation, exaggeration of hearing loss is not uncommon in these situations. Routine hearing testing -- raise your hand when you hear the beeps -- is of course very susceptible to persons who are pretending to have hearing loss.
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, otoacoustic emissions, ASSR, as well as strong psychophysical tests such as the Stenger. See this page for more.
Figures are copyrighted to Timothy C. Hain, MD