Audiometry



Definition

Audiometry encompasses those procedures used to measure hearing thresholds.

Purpose

The purpose of audiometry is to establish an individual's range of hearing. It is most often performed when hearing loss is suspected. Audiometry can establish the extent as well as the type of a hearing loss. Audiometric techniques are also used when an individual has vertigo or dizziness , since many hearing and vestibular or balance problems are related. Since those with facial paralysis may also have hearing loss, audiologic testing may be performed on these individuals as well.

Description

The primary purpose of audiometry is to determine the frequency and intensity at which sounds can be heard. Humans can hear sounds in the frequency or pitch range of 20 to 20,000 Hertz (Hz), but most conversations occur between 300 and 3000 Hz. Audiometric testing is done between 125 and 8000 Hz. The intensity levels or degree of loudness at which sounds can be heard for most adults is between 0 and 20 decibels (dB).

Both air conduction and bone conduction of sounds are evaluated by audiometry. Air conduction establishes the extent of sound transmission through the bones of the middle ear. The results of a bone conduction test determine how soft a sound an individual can hear over several frequencies or pitches. Bone conduction audiometry determines the extent to which there is neurosensory hearing loss. An individual with a neurosensory loss may be able to hear sounds but not understand them. Since those with hearing losses often cannot hear sounds at normal decibel levels, intensities as high as 115 dB are used to assess the extent of air conduction loss and as high as 70 dB for bone conduction loss. The difference between bone conduction loss and neurosensory hearing loss is called the air-bone gap.

The most common method of assessing hearing ability is with the audiometer. Audiometric testing with the audiometer is performed while the patient sits in a soundproof booth and the examiner outside the booth communicates to the patient with a microphone. The patient wears headphones when air conduction is tested and a vibrating earpiece behind the ear next to the mastoid bone or along the forehead when bone conduction is tested. One ear is tested at a time, and a technique called masking, in which noise is presented to the ear not being tested, assures the examiner that only one ear is tested at a time. Through the headphones or earpiece pure sounds in both frequency and intensity are transmitted to the patient and the threshold at which the patient can hear for each frequency is established. The patient signals an ability to hear a sound by raising a hand or finger.

When the child is capable of understanding and responding to words, speech discrimination is also assessed as part of audiometry. Speech discrimination establishes one's ability to understand consonant sounds. In speech discrimination testing, two syllable words are read to and then repeated by the patient. This is an important part of audiometry, since much of a child's learning depends on the ability to discriminate speech. Older children of ten to 12 years of age have speech recognition comparable to adults and do well with speech discrimination testing. To insure that speech discrimination only is being assessed, this part of the hearing test is done at decibel levels of 30 to 40 decibels, higher than that of everyday conversation. By age five most children can do some type of speech discrimination testing.

Speech discrimination in the child of three to six years of age may be tested by having the child look at pictures of common objects as a monosyllabic word is read to him or her. The child indicates comprehension of the word by pointing to the corresponding object.

When evaluating infants, rather than testing of threshold levels, the examiner establishes the minimum response level at which the child responds to auditory stimuli. The minimum intensity level at which a neonate responds to sound is 25 dBs. This minimum level gradually decreases through infancy and at 36 months most children respond to sound intensities of less than 10 dBs.

For the young infant under four months of age, audiologists employ behavioral observation audiometry (BOA). The audiologist observes startle responses and motor reflex changes in the child as various noisemakers are employed to elicit these responses. The difficulty with this test is that the noises used are not standardized in frequency or intensity.

Visual reinforcement audiology (VRA) testing evaluates the hearing of infants from six months to two years. Sounds of varying intensity are presented to one of two speakers as the child sits on a parent's lap. If a sound is heard by the child, then he or she turns toward the appropriate speaker and is rewarded by a visual stimulus, such as an animated toy or a flashing light, although video images have been used for older children.

As the child gets older, condition play audiometry (CPA) is useful. The child is instructed to listen for a sound and to respond when a sound is heard by doing varying tasks, such as placing a ball in a cup or placing a peg in a pegboard, when the auditory stimulus is heard. Headphones may be worn by the child for this type of testing.

Because a reliable subjective response is difficult or impossible in a young patient electrophysiological testing is often performed. Electrophysiological testing is a reliable and nonbehavioral method to assess hearing loss in infants and young children and can be done while the child is either sleeping or under sedation. Some electrophysiological

Technician testing a young girls hearing with an audiometer. (Photograph by Jon Meyer. Custom Medical Stock Photo, Inc.)
Technician testing a young girl's hearing with an audiometer.
(Photograph by Jon Meyer. Custom Medical Stock Photo, Inc.)
tests are the auditory brainstem response (ABR) test, auditory steady-state response (ASSR) testing, electroencephalic audiometry (EEG) test, and otoacoustic emission testing (OAE).

To perform the auditory brainstem response (ABR) test, headphones are placed on the infant or child and electrophysiological responses from the scalp and ears are recorded in response to tones sent through the headphones. A computer compiles the findings into a waveform that gives the examiner information about the location of a hearing problem anywhere along this pathway from the ear canal to the brainstem. This test is also called the brainstem auditory evoked response.

Auditory steady-state response (ASSR) testing also involves monitoring recorded responses from the scalp of tones at varying frequencies. This test is a more sensitive test than the ABR and can also measure residual hearing better. The EEG or electroencephalic audiometry test measures tone loss but cannot locate the site of a hearing loss. Otoacoustic emission testing (OAE) records spontaneous emissions from the ear and can detect middle ear problems. It is simpler than ABR, and it can be used to screen infants for severe hearing losses, since if hearing loss of greater than 40 dBs exist, no emission will be recorded.

An adjunct test of audiometry is acoustic immitance testing which assesses the facility with which sound can travel from the external ear to the cochlea inside the ear. The most familiar of this type of testing is the tympanogram, which determines if fluid has built up behind the eardrum.

Precautions

Audiometry is a safe procedure to which there are rarely contraindications.

Preparation

For most audiometric testing no special preparation is required, although the first time that hearing testing is done on a child the procedure should be explained as clearly as possible. If ABR or ASSR testing is done under sedation, then the child may not eat for several hours prior to administration of the drugs.

Aftercare

Audiometric testing, except when sedation is involved, requires no special aftercare.

Risks

If the ABR is used under sedation then the side effects of sedatives must be considered. Otherwise there are no risks associated with audiometry.

KEY TERMS

Audiologist —A person with a degree and/or certification in the areas of identification and measurement of hearing impairments and rehabilitation of those with hearing problems.

Cochlear implantation —A surgical procedure in which a small electronic device is placed under the skin behind the ear and is attached to a wire that stimulates the inner ear, allowing people who have hearing loss to hear useful sounds.

Decibel —A unit of the intensity of sound or a measure of loudness. Normal speech is typically spoken in the range of about 20–50 decibels.

Frequency —Sound, whether traveling through air or the human body, produces vibrations—molecules bouncing into each other—as the sound wave travels along. The frequency of a sound is the number of vibrations per second. Within the audible range, frequency means pitch—the higher the frequency, the higher a sound's pitch.

Parental concerns

Audiometry should be performed on all infants and children since unidentified hearing loss can delay speech and language skills. The earlier that a child with a hearing problem can be identified, the sooner the child's communication skills will develop. The audiometry available as of 2004 can determine the type and extent of a hearing loss as well as identify the location of the hearing problem. The results of audiometric testing can help determine if a hearing aid or cochlear implant may help a child. Audiometric testing can also be an adjunct to diagnosis of more serious problems related to hearing loss such a related syndrome or a tumor.

Parents of a child diagnosed with a hearing loss must be prepared to bring the child back for follow-up evaluations to monitor the hearing loss every three months for the first year after diagnosis and at least annually through the remainder of childhood. As the child gets older, more extensive audiometry testing can be performed.

Resources

BOOKS

Beasley, Donald J., and Ronald G. Amedee. "Hearing Loss." In Expert Guide to Otolaryngology , edited by Karen H. Calhoun. Philadelphia: American College of Physicians, 2001.

Miller, Andre J., and Gernard J. Gianoli. "Dizziness." In Expert Guide to Otolaryngology , edited by Karen H. Calhoun. Philadelphia: American College of Physicians, 2001.

Turkington, Carol, and Allen E. Sussman. Deafness and Hearing Disorders , 2nd ed. New York: Facts On File, 2004.

PERIODICALS

Firszt, Jill B., et al. "Auditory Sensitivity in Children Using the Auditory Steady-State Response." Archives of Otolaryngology—Head & Neck Surgery 130 (May 2004): 536–40.

Schmida, Milton J., et al. "Visual Reinforcement Audiometry Using Digital Video Disc and Conventional Reinforcers." American Journal of Audiology 12, no. 1 (June 2003): 35–40

Martha Reilly, OD



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