Core Curriculum Syllabus: Audiology

Accurate assessment of hearing (audiometry) is vital to the diagnostic evaluation of patients with suspected otologic disorders for the determination of the underlying process, as well as in the planning of rehabilitation of hearing loss. Originally, audiometry was limited to the psychophysical measurement of the sensation of hearing; thus, patient cooperation was essential. However, other tests have been developed over the years which permit more objective assessment of hearing even in infants, small children, malingerers, and hysterics.

I. Human Hearing

Most humans hear sounds in the range of 20 to 20,000 Hz. Sensitivity varies as a function of frequency, with sounds in the middle frequencies being heard best. The ability to hear higher frequencies declines with age.

For a discussion of the causes of hearing loss, see the section on diseases of the middle ear, or diseases of the inner ear.

II. Basic Audiometry

Adequate testing requires an audiometer (device for presenting sounds to the patient at precisely controlled intensity), a sound-proof environment, a competent audiologist, and a cooperative patient. The standard testing battery includes Pure Tone Audiometry, Speech Audiometry, and Immittance Audiometry.

A. Pure Tone Audiogram: This is a graphic plot of the patient's thresholds of auditory sensitivity for pure tone (sine wave) stimuli. Threshold hearing levels are indicated for each frequency tested. By convention, normal hearing levels are shown at top of the graph; a decrease in hearing sensitivity is indicated by larger values of hearing level. Hearing level is plotted on a logarithmic decibel scale. Sounds are tested with presentation by air conduction (earphones) as well as bone conduction (skull vibrator). An air bone gap indicates a conductive component of hearing loss. A decrease in threshold sensitivity by bone conduction reflects a sensory or neural loss.

The format for recording audiometric findings is shown in the legend below:

Air conduction thresholds are represented by circles, and bone conduction by triangles. A solid symbol indicates that masking noise was presented to the opposite ear to minimize the chance of responses due to crossover of sound.

This sample audiogram indicates normal hearing in the right ear, and a conductive loss on the left.

RIGHT EAR	LEFT EAR
Normal Hearing	Conductive Hearing Loss

The second audiogram demonstrates a sensorineural loss in the right, and a mixed loss on the left.

RIGHT EAR	LEFT EAR
Sensorineural Hearing Loss	Mixed Hearing Loss

B. Speech Audiometry: These tests utilize spoken words and sentences rather than pure tones. Tests are designed to assess sensitivity (threshold) or understanding (intelligibility).

• Threshold - the level at which the patient can correctly repeat 50% of test materials--phoneme-balanced words (PB), synthetic sentences, etc.
• Intelligibility - by convention, the percentage of words or sentences a patient can correctly repeat when presented at supra-threshold levels.
  • Provides information about hearing handicap. Problem maybe worse than indicated by pure tone average (PTA) for the speech frequencies.
  • Useful to determine candidacy for hearing aid.
  • Very poor results, out of proportion to PTA, suggests probable retrocochlear cause of hearing loss.

C. Immittance Audiometry: These hearing tests utilize the electroacoustic immittance bridge, as shown in this schematic diagram below. This device is designed to quantify the impedance (resistance to movement) of the conductive mechanism of the ear by bouncing a probe tone off the tympanic membrane and measuring the proportion of reflected sound.

Maximal reflection of sound occurs when the mechanism is very stiff, while a compliant system transmits more sound and reflects less. There are two principal applications of this device.

• Tympanometry: A tympanogram is a graphic representation of the relationship of external auditory canal air pressure to impedance; the latter is usually reported in terms of tone of its derivatives, compliance in arbitrary units. Pressure in the external auditory canal is varied from -200 daPa* through +200daPa while monitoring impedance. Impedance is lowest (maximal compliance) when pressure in the canal equals pressure in the middle ear. Ears can be classified into three basic groups on the basis of the configuration of the tympanogram.
  • Type A. The peak compliance occurs at or near atmospheric pressure indicating normal pressure in the middle ear. There are three subgroups.
    • A - normal shape reflects a normal mechanism
    • AD - A deep curve with a tall peak indicates an abnormally compliant middle ear, as seen in ossicular dislocation or erosion, or loss of elastic fibers in the tympanic membrane.
    • AS - A shallow curve indicates a stiff system, as in otosclerosis.
  • Type B - No sharp peak, with little or no variation in impedance over a wide range, usually secondary to non-compressible fluid in the middle ear (otitis media), tympanic membrane perforation or obstructing cerumen.
  • Type C - Peak compliance is significantly below zero, indicating negative pressure (sub-atmospheric) in the middle ear space. This finding is often indicative eustachian tube dysfunction.

  *daPa = decaPascal = mm H20 One system of tympanogram classification. A = normal AS = stiffened tympano-ossicular system; AD = disarticulation B = Middle ear effusion, tympanic membrane perforation or impacted cerumen C = negative middle ear pressure.

• Acoustic Reflex measures (AR): Contraction of the stapedius muscle occurs with loud sounds, producing a measurable change in compliance. Abnormalities of hearing may be suspected by the following results:
  • Elevated threshold - indicates cochlear sensitivity loss or VIII nerve disorder
  • Absent reflex
    • Abnormal middle ear system
    • Severe sensitivity loss
    • VIII nerve lesion
    • Ipsilateral VII nerve lesion
    • Some otherwise "normal" ears
  • Threshold low in proportion to sensitivity level ("recruitment")- seen in cochlear loss
  • Abnormal "shape" of reflex.

III. DIAGNOSTIC AUDIOMETRY

A battery of tests intended to determine the site of lesion inpatients with otologic or neurotologic disorders. The constellation of tests varies according to the available test battery and provisional diagnosis.

A. Immittance audiometry (see above)

B. PI-PB functions - Speech discrimination is plotted as a function of sound intensity. Normally, discrimination improves with intensity up to a maximal level, then plateaus. In VIII nerve disorders, discrimination often declines dramatically as intensity increases above the level yielding maximum performances.

PI-PB Function
RIGHT EAR	LEFT EAR
NORMAL P1 - PB	ROLLOVER

C. Bekesy Audiometry: This test has significant historical interest in development of assessment of hearing. However, today it is used predominately only in industrial and military hearing screening situations. Patient traces his own auditory threshold by means of a self-recording audiometer. Tracings are obtained for pulsed as well as continuous tones. The relationship between the two categories can be categorized into diagnostic patterns.

• Type I: interwoven - normal or cochlear
• Type II: slight separation, especially in high frequencies- cochlear
• Type III: initially interwoven, but continuous sensation falls off dramatically at higher frequencies - VIII nerve lesion
• Type IV: continuous tracing at markedly lower level than that for pulsed signal - usually associated with VIII nerve lesion
• Type V: sensitivity lower for intermittent tone - malingering

D. Tone decay tests: Abnormal adaptation to a continuous tone is seen in retro-cochlear lesions.

E. Stenger Test: Performed to detect malingering of unilateral loss. If sound is presented to both ears, patient will deny hearing in the ear with the feigned loss. If sound is presented to the good ear at a suprathreshold level, simultaneous to a louder sound in the questionable ear, a malingerer will localize the sound to his "bad" ear, and therefore deny hearing anything at all.

F. ABR - evoked auditory brainstem responses: Scalp electrodes measure electrical activity in response to sound clicks. The response is quite small in relation to other ongoing brain activity, but by presenting a large number of clicks and averaging the responses by computer, unrelated events can be canceled out.

• This is useful for documenting hearing in uncooperative or unresponsive patients. The disadvantage is that it tests mainly the 1,000-4,000 Hertz frequency range of hearing and is a poor indicator of the overall auditory function.
• Abnormal ABR is seen in VIII n or brainstem lesions.

Normal Auditory Brainstem Response

G. ECOG (electrocochleography): Electrical activity is measured from the promontory, and responses to a large number of clicks are averaged. These will be abnormal in eighth cranial nerve lesions and certain cochlear disorders.

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Bobby R. Alford Department of Otolaryngology-Head and Neck Surgery
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Last modified: May 21, 2006