X. HEARING LOSS AND MEASUREMENT OF HEARING ABILITY

Hearing loss may be categorized by degree. This table below does not take into account some important variables, including age of the individual which, as we will see later impacts critically on language development.

25-40 dB

Misses hearing many consonants, difficulty in auditory learning, mild speech - language problems

40-65 dB

Speech - language retardation, learning disability, hears little or no speech at normal conversational levels

65-90 dB

Voice pathology, aural language seriously compromised, severe learning problems

>90 dB

Profound hearing loss (deaf), voice-speech characteristic of deaf, severe learning disabilities

Objective 2: Hearing Assessment

Accurate otologic diagnosis often depends on reliable and accurate tests of hearing. The results of such tests help identify the sites of lesions and and point to strategies for intervention and treatment. Patients may be screened in a relatively course way, or may be evaluated using more sophisticated testing procedures.

Screening

Some simple procedures can be carried out to give clues as to the nature of the hearing loss:

In conversation, how does the person respond when you speak to him/her? Does the person have to see your face to understand what you are saying? Does the person tend to favor one or the other ear? Using a tuning fork may help evaluate the nature of a hearing loss short of an audiologic examination.

Audiometric Testing of Adults

The basic battery of audiometric tests normally includes pure-tone testing of threshold by both air and bone conduction, speech reception threshold (SRT), and speech discrimination (SDS). These tests normally require the cooperation of the subject whose response to a sound is indicated by some gesture (e.g. raised hand). Testing is typically carried out in a sound-attenuated room with the subject listening to carefully calibrated sounds.

Threshold sensitivity testing using airborne pure tones.

Testing is done using earphones thereby allowing each ear to be examined independently. Tones are reduced in intensity until they are no longer heard, at which point the examiner alternately raises and lowers the intensity of the sound until a just-detectable threshold is determined. This is repeated at several frequencies within the audible range and the results plotted as an audiogram. The shape of the curve is a measure of the frequency sensitivity of both the middle ear and the inner ear. To differentiate between middle ear (conductive) and inner ear (sensorineural) components to a hearing loss it is necessary to conduct additional tests.

Threshold sensitivity testing using bone-conducted pure tones

Here testing is done with a vibrator placed somewhere on the skull (usually the mastoid). The testing and plotting procedures are the same as with air conduction testing. In this case, however, sound is transmitted directly to the cochlea via bone conduction, thereby by-passing the transmission mechanism of the middle ear. Thus, audiograms obtained using both bone and air conducted sounds provide information about the integrity of both the middle and inner ears. Difficulty in hearing only air conducted sounds results in a separation of the bone and air conduction audiograms - the so-called "air-bone gap".

Objective 3:

Threshold sensitivity testing using speech

The threshold for hearing easily-recognized two-syllable words (e.g. baseball, iceberg, eardrum) is called the speech reception threshold(SRT). Since speech is made up of pure tones, this threshold should be very close to pure tone thresholds up to about 4 kHz, which is the range offrequencies occupied by most normal speech sounds.

Speech discrimination testing

Speech discrimination, as opposed to speech sensitivity, is the person's ability to not only hear words but to identify them. The procedure includes presentation of 50 selected monosyllabic words at an easily detectable intensity level. The speech discrimination score (SDS) is the percentage of words correctly identified. Pathology of the inner ear, auditory nerve, and/or central auditory pathways can affect this score. The ability of an individual to discriminate speech is not well predicted by the pure-tone audiogram. An individual may hear a sound well enough, but the neural signals may be altered to the extent that the sound is unintelligible.

Individuals suffering only a conductive hearing loss will be able to identify words if the sound is loud enough. For persons with sensorineural hearing loss, there is a marked drop in the score without a proportionate loss of pure-tone or speech sensitivity.

Auditory assessment of children

Infants indulge in vocal play within the first year of life. If they fail to respond to their auditory world in this time, a parent will often ask questions about the child's ability to hear. Perhaps another year passes before the question is raised of why the child fails to speak. This early period in the child's life is critical for the development of normal communication skills which, if not achieved then, will often result in lifelong communication handicaps. Thus, newborns at risk for hearing loss and those whose hearing is suspect can, and should, be accurately evaluated at the earliest time. Accurate hearing tests can be conducted at any time in a person's life. Obviously, testing procedures used successfully on adults may have to be modified for use on infants and small children

Infants respond to sounds in the free field or through earphones with facial responses (grimaces, smiles, brow raising) or with head turning. A 3-month old child sitting in its mother's lap may exhibit a startle response to sound. A 6-month old child may initiate or cease activity upon hearing a stimulus. These responses are difficult to quantify, but in the hands of an experienced person, can give clues as to the ability of that infant or child to hear sounds. When testing young children it is necessary to remember to apply standards and stimuli appropriate for the child's developmental level.

By 24 months of age other test strategies can be used based on the child's natural curiosity about objects in the environment. Play audiometry is a technique used that makes a game of the test and may be used up to the age of about 6 depending on the abilities of the child. This technique allows for tests of pure-tone detection and speech sensitivity and intelligibility.

Traditionally, persons with a hearing loss have been fitted with a hearing aid, a device that simply amplifies the sound. Hearing aid may be helpful at any time in life and, because they are amplifiers only (and therefore make sounds louder but not clearer), they tend to work best in cases of conductive hearing loss.