Download Audiometry practical

Medical Neuroscience Module -- AUDIOMETRY PRACTICAL
David McAlpine, Dept. Physiology, UCL, [email protected]
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
www.ucl.ac.uk/lapt/med ) 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.
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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.
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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:
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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.
2
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 reintroduced 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.
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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 log10
Or
Sound intensity (watts/m2)
Reference intensity (watts/m2)
(Sound pressure)2
(reference pressure)2
dB = 10 log10
= 20 log10
sound 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
Jet aeroplane, 80 ft
from tai; hair cell
damage
Busy traffic, shouting
Conversational
speech
Residential area at
night
Whisper at 5ft
Threshold for hearing
1000 Hz young adult
120 dB
Sound pressure
(N/m2; Pa)
20
Relative sound
Pressure
10-6
80 dB
60 dB
2.10 -1
2.10 -2
104
103
40 dB
2.10 -3
102
20 dB
0 dB
2.10 -4
2.10 -5
101
1.0
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