Diagnostic Procedures

The following procedures are used to test for hearing loss and to diagnose or rule out certain possible underlying causes of hearing loss:

• Audiometry. This is the most commonly used hearing test. The first part measures how well you can hear sounds conducted through the air and indicates the condition of your overall hearing. The test is usually administered in a soundproof room using a machine called an audiometer. Through headphones, you listen to a series of sound tones that range from high to low, one tone at a time. Each tone begins at an easily audible sound level. You indicate with a prearranged signal when you hear the tone. The sound level decreases gradually until you are no longer able to hear the tone; this point is your hearing threshold for that frequency. The second part of the test measures how well you can hear sounds conducted through your head and indicates whether your hearing loss is conductive or sensorineural (see page 397). The procedure is the same as for the first part of the test, but this time you wear special vibrating headphones. Next you are tested for words to establish the lowest threshold at which you can hear two-syllable words (called your speech-reception threshold) and to determine the percentage of one-syllable words you can repeat back correctly (called speech discrimination).Your hearing thresholds for all parts of the test are recorded on a graph called an audiogram.

• Impedance audiometry. This hearing test measures how well your eardrums reflect sound waves. During the test, a probe that is covered with soundproof material is placed into your outer ear canal, sealing off the entrance to both sound and outside air pressure. The probe then transmits a continuous sound as air is pumped into the ear canal through the probe at various pressure levels (from low to high), and a microphone in the probe measures reflected sound waves. These reflections are recorded on a graph called a tym-panogram. This test is used to detect fluid in the middle ear, a perforated eardrum, and disorders of the three tiny sound-conducting bones (the malleus or hammer, the incus or anvil, and the stapes or stirrup) of the middle ear.

• Auditory evoked response testing (also called auditory brain stem response testing). This computerized hearing test is used to measure the electrical activity of the vestibulocochlear nerve by determining how long it takes nerve impulses traveling along the nerve to reach the brain stem. During the test, electrodes are placed on your scalp to analyze your brain's response to sound stimulation produced by an audiometer. This test is sometimes used to rule 405

out an acoustic neuroma (a noncancerous tumor in the ear canal). Ears

• Electrocochleography. This hearing test measures the electrical activity of the sensory hair cells in the inner ear in response to sound waves. During the test, the eardrum is anesthetized and a very fine needle is passed through the eardrum until it is very near the sensory hair cells. Sound tones of varying frequency (low to high) and loudness are then transmitted into the ear through headphones, while the needle detects the electrical activity of the sensory hair cells. The electrical activity is recorded on a graph called an electro-cochleogram. This test is sometimes used to diagnose Meniere's disease (see page 402).


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