Hearing impairment or hearing loss is a reduction in the ability to perceive sound. The loss may range from slight to complete deafness.
Audiometric studies are diagnostic tests that evaluate sensorineural and conductive hearing losses. Conductive hearing loss is the result of disorders of the external or middle ear; sensory hearing loss is secondary to disturbance of the cochlea; neural hearing loss results from disease of the auditory (eighth) nerve or central auditory channel connections. Sensory and neural hearing losses are frequently included under the term sensorineural hearing loss. Mixed or combined hearing loss involves disturbances of both conductive and sensorineural mechanisms. A variety of tests have been designed to evaluate central auditory processing, which is a higher order cortical function that processes auditory information. Therefore, central auditory processing tests do not strictly evaluate hearing impairment, and thus are not formally considered in this policy. (See Rationale section for further general discussion of central auditory processing.)
The various audiometric tests can be subdivided into standard batteries that are typically used as part of the initial work-up of patients presenting with hearing impairment, as well as specialized tests that are typically used in specific clinical situations. The standard batteries vary according to whether the patient is an adult, child, or infant. The tests are briefly defined as follows.
Standard Battery of Tests for Adults and Children
1. Pure-tone audiometry, air and bone conduction
This test is a standard audiometric study that uses tones of various frequencies and intensities as auditory stimuli to measure hearing. As air conduction is the usual method of sound transmission, air audiometry uses the external and middle ear in the transmission of sound to the cochlea and beyond. Bone-conduction audiometry involves the vibration of the skull by direct contact with an oscillating device that is thought to set the cochlear fluids into motion, bypassing the external and middle ear. When bone-conduction thresholds are better than air-conduction thresholds, the hearing loss is conductive. When bone-conduction thresholds are the same as air-conduction thresholds, the hearing loss is sensorineural. When bone-conduction thresholds are reduced but are still better than air conductions, the loss is mixed or combined.
2. Speech audiometry
This test is a standard audiometric study that measures overall performance in hearing, understanding, and responding to speech for a general assessment of hearing and an estimate of degree of practical handicap. It may include a speech recognition test, in which the patient repeats words back, and a speech reception threshold, which determines when the patient can first hear speech. It may also be used to reaffirm the findings of the pure-tone audiometry and to diagnose pseudohypacusis (a non-existent or false hearing loss).
3. Word recognition tests
This category includes filtered speech tests and synthetic sentence identification. These tests specifically assess the patient’s ability to discriminate spoken words.
4. Acoustic reflex test and acoustic reflex decay
These tests measure the changes in the ear’s ability to conduct sound to the cochlea. Reflexes, called acoustic reflexes, exist and involve middle ear function. Absence of the acoustic reflex may be indicative, among other things, of lesions of the middle ear, acoustic tumor, otosclerosis, facial nerve involvement of the probe ear side, and surgical removal or congenital absence of the stapes. These tests may be used in assessing the hearing of neonates and other children too young to cooperate in the audiometric testing of functional hearing loss.
5. Tympanometry (impedance testing)
This is a standard series of tests to measure the ability of the middle ear to conduct sound. It is particularly useful in the identification of fluid in the middle ear and in the anatomic localization of facial nerve paralysis.
Additional Standard Battery of Tests for Children Only
1. Select picture audiometry
This test is used to evaluate hearing-impaired children. It involves the use of pictures on cards and the child’s ability to correctly identify objects based on audiologic direction.
2. Conditioning play audiometry
This is usually performed to test hearing impairment in children 2 to 4 years of age. The child is taught to put an object in a specific place, e.g., a marble in a box or a cow in the barnyard when a specific sound is heard.
Standard Battery of Tests for Infants Only
1. Auditory evoked potential (also called auditory brainstem response [ABR])
This is an electrophysiologic measure of auditory function that uses responses produced by the auditory nerve and the brainstem and helps differentiate sensory from neural hearing loss. The response is the waveform averaged over many auditory clicks. It may be helpful in the diagnosis of cerebellopontine angle tumors and acoustic neuromas, is used as a monitor in posterior fossa surgery, and may help to establish a hearing threshold for infants and difficult-to-test patients.
2. Visual reinforcement audiometry (VRA)
The VRA is part of a battery of tests used in the determination of infant hearing loss. The premise of the test is that the rate of patient response is increased by the use of reinforcement.
3. Evoked otoacoustic emissions (OAE)
Otoacoustic emissions are sounds measured in the external ear canal that are a reflection of the working of the cochlea. Probe and click stimuli are used in the performance of this test. OAE is used in the screening as well as the diagnosis of hearing impairment in neonates and young children.
4. Acoustic reflex test
See Standard Battery of Tests for Adults and Children Only, No. 4
Specialized Tests for Adults and Children
1. Auditory Evoked Potential
See Standard Battery of Tests for Infants Only, No. 1
2. Electrocochleography (ECochG)
This is a measure of the electrical potentials generated in the inner ear as a result of sound stimulation. This test may be used in the evaluation of endolymphatic hydrops or Meniere’s disease.
3. Tone decay test
This test involves the presentation of a continuous tone to determine whether the threshold for the tone has changed (becomes poorer) over time. The test helps differentiate sensory from neural hearing loss, and is used in the diagnosis of cochlear versus retrocochlear lesions and eighth nerve tumors. While this test is still in use, the auditory evoked potential test is largely used in place of the tone decay test.
4. Stenger test, pure tone or speech
This test uses a tone presented simultaneously to both ears and is based on the principle that the tone is perceived only in the ear that receives the greater intensity. It is useful in the diagnosis of pseudohypacusis (hypacusis is a hearing impairment of a conductive or a neurosensory nature, and pseudohypacusis is a non-existent [false] hearing loss.
5. Sensorineural acuity level (SAL) test
This test measures the extent of sensorineural hearing loss and is used in the detection of pseudohypacusis among children. It is not a commonly performed test. There are several different tests in use, including one in which a bone-conduction vibrator is placed at the center of the forehead and the threshold shift for a normal ear versus an ear with sensorineural hearing loss is analyzed.
6. Evoked otoacoustic emissions
While this test is considered part of the standard battery of tests in infants, it is considered a specialized test in adults and children. For a description, see Standard Battery of Tests for Infants Only, No. 3.
Automated audiology is different from manual audiometric testing (testing conducted by a clinician [audiologist or physician]). Audiologist may use many computer-assisted devices for procedures such as otoacoustic emissions and evoked potentials requiring the constant presence and vigilance of the audiologist to ensure data gathering for test interpretation. In contrast, automated devices refer to equipment that follows a specified algorithm to determine what action to take next and does not include ongoing monitoring and vigilance by the clinician.
One type of automated audiometry device is the Otogram™ which was approved by the FDA in 2007. The Otogram is a computer-controlled, audiometric instrument combining the functions of an Audiometer, Distortion Product Otoacoustic Emissions Analyzer and Auditory Acoustic Impedance/Admittance Tester (i.e. Tympanometer) The FDA notes that the Otogram is indicated for use by trained healthcare professionals on both adult and pediatric subjects for measurement of audiometric parameters to identify and supply data to help diagnose hearing loss and ear disorders.