Audiometry Health Article

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 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.