Medical Neuroscience Module -- AUDIOMETRY PRACTICAL

David McAlpine, Dept. Physiology, UCL,

Aim

To give experience of simple clinical assessment of hearing.

Objectives

At the end of this session you should be able to

  • Describe, with the aid of diagrams, the basic principles of tests used in the assessment of hearing loss
  • Make and interpret an audiogram.

Important Note After the practicals, the web discussion forum (access from ) is available to discuss and clarify any issues and problems arising from this practical. See the notes at the start of the VISION Practical for more details.

Session content

Clinical assessment of hearing loss

Evaluation of the patient’s hearing problems begins with simple observation. Observation of manner, voice and his hearing for conversation at the first consultation may provide a rough indication of the nature and degree of hearing disability. The patient who watches a speaker closely may be proficient at lip reading. The voice is often soft in conductive disability and flat and toneless with sensori-neural loss. When a louder than normal voice is required for communication in a small consulting room, significant hearing handicap is probable.

Clinical History

The clinical history is the first step in investigation. The following points are of particular importance.

  • Time and age of onset of loss of hearing
  • Illness, accident or other unusual circumstances at onset
  • Concurrent symptoms, vestibular disturbances, pain or ‘fullness in ear’, headache, neurological disturbance, infection of ear or respiratory tract, tinnitus
  • Personal history:

acute infectious diseases

aural infections.

respiratory infections

noise exposure – Industrial (e.g.factory machine noise, road drills)

Rock ‘n’ Roll (e.g. musicians etc)

Drugs:

neomycin

gentomycin

streptomycin

  • Trauma to the head
  • Family history

Loss of hearing in patient or siblings

German measles during first trimester of mother’s pregnancy

  • Conditions in which disability is apparent, e.g. talking in groups, noise.

The second step in clinical investigation is to carry out simple tests of hearing as set out below. Work in pairs and take it in turn to act as patient and investigator.

Rinne’s Test and Weber’s Test

These tests determine whether the deafness is conductive or sensori-neural. In the former there is a defect in the transmission of sound from the outside world to the cochlea, in the latter the defect is in the transmission of information from the cochlea to the auditory cortex.

Procedure

Perform both tests on both a normal subject and one with conductive deafness simulated by blocking one ear with cotton wool. Tabulate the results. Treat the tuning forks gently.

Weber’s Test

This test is used to classify unilateral hearing loss.

The stem of a vibrating tuning fork is held at the midline of the skull and the tone is best heard in the affected ear in conductive loss, and in the non-affected ear in sensory-neural loss.

Rinne’s Test

The tone of a tuning fork is alternately presented by air conduction 1 cm form the pinna and by bone conduction with the stem of the fork pressed firmly against the skull behind and above the ear. The opposite ear should be masked. 256, 512 and 1024 forks are most useful. Forks of lower frequency may be felt as vibration, while those with higher frequency are heard by air conduction when bone conduction is tested.

In normal (POSITIVE RINNE) response, air conduction is heard approximately twice as long as is bone conduction.

In an abnormal (NEGATIVE RINNE), bone conduction is heard longer that air conduction. Responses may be of any degree, as decreased positive or equal. Abnormal response indicates decreased sound conduction in the external or middle ear.

Pure tone audiometry

Pure tone audiometry is employed principally to:

  • Assist in making the diagnosis or auditory pathology (including malingering and psychogenic aberrations).
  • Ascertain the type and severity of the impairment in relation to suitability for rehabilitative middle ear surgery.
  • Periodically check the progress of auditory sensitivity following surgery or drugs
  • Serve as a guide in the selection of rehabilitative procedures for patients with handicapping irreversible hearing impairments.

Use of AMPLIVOX Diagnostic Audiometer

Switch on. The black toggle power switch is on the rear panel immediately above the entry of the mains cable. The machine will go through a series of internal checks before setting itself up to be ready for a pure tone audiometry test, starting with the left ear.

Air conduction threshold measurement

Fit the air conduction headphones with RED marked earphones on the Right ear and the BLUE on the Left ear, adjusting the headband as necessary. Ensure that the opening in the ear cushion is directly over the ear canal and that moderate pressure is applied by the headband, for better results.

Set the Frequency control to 1000 Hz and the hearing level control to 60 dB. Depress the interrupter button. The subject should indicate that the tone is heard in the ear selected for test. If there is severe impairment, the tone will not be heard and the hearing level control should be increased in 10 dB steps until the tone is heard.

The hearing level control should be lowered in 5dB steps until the tone is lost and then increased by 5dB for the threshold to be checked and rechecked.

The threshold of hearing is the quietist level at which the subject can identify the tone when it is re-introduced by means of the interrupter.

It is important to limit the duration of the test tone to one or two seconds, and always to interrupt when re-setting the Hearing Level Control. On no account should the Hearing Level Control be changed until the tone interrupter push button had been released.

The dB reading is plotted on the audiogram chart, a small circle being used as a symbol to indicate Right ear measurements, and a small cross for Left ear measurements. Use a pencil in case of mistakes. It is also desirable to mark the audiogram ISO zero.

Having established the threshold for 1000Hz, the threshold for higher pure tone frequencies should be taken in sequence up to 8000Hz. Then go back to 1000Hz to re-check the threshold initially established, and proceed to check the remaining frequencies below 1000Hz, i.e. 750, 250, 125 Hz.

The same procedure can be repeated for the left ear.

Understanding the Audiogram

The results of the audiogram are plotted in such a way that compares the hearing of the patient with that of the average person. The audiometer is calibrated to ISO (International Organization for Standardization) threshold levels for air conduction. According to this standard the absolute intensity of sound in watts/m2 produces by the audiometer at each frequency is set in such a way that when an intensity of 0dB is indicated the average person will be at the threshold for hearing at that frequency. Thus points plotted at 0dB are ‘normal’. Points lying at + 5dB or greater indicate a hearing deficit. Points lying at –5dB or -10 dB indicate better than normal average hearing.

Understanding the measurement of sound intensity in decibels (dB)

Sound intensity is frequently expressed in terms of decibels (dB). The decibel (1/10 of a Bel) was named in honour of Alexander Graham Bell. It expresses the logarithm to the base 10 of the ratio between two sound intensities or (sound pressures)2.

Relative intensity in dB = 10 log10Sound intensity (watts/m2)

Reference intensity (watts/m2)

OrdB = 10 log10(Sound pressure)2

(reference pressure)2

= 20 log10sound pressure

reference pressure

The internationally recognised reference standard intensity for sound is 10-12 watts per m2 which corresponds to a sound pressure of 2.10-5 N/m2 (or Pascal). For a pure tone at 1000 Hz this is close to the threshold of hearing for young healthy adults.

Some examples of sound intensity levels:

dB / Sound pressure (N/m2; Pa) / Relative sound Pressure
Jet aeroplane, 80 ft from tai; hair cell damage / 120 dB / 20 / 10-6
Busy traffic, shouting / 80 dB / 2.10 -1 / 104
Conversational speech / 60 dB / 2.10 -2 / 103
Residential area at night / 40 dB / 2.10 -3 / 102
Whisper at 5ft / 20 dB / 2.10 -4 / 101
Threshold for hearing 1000 Hz young adult / 0 dB / 2.10 -5 / 1.0

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