Testing the hearing
There are three stages to testing the hearing:
✓ Clinical assessment of the degree of deafness
✓ Tuning fork tests
✓ Audiometry
Clinical assessment of the degree of deafness
When you are talking to the patient, you should be quickly assessing how well they can hear and this assessment continues throughout the interview. Voice and whisper tests are approximations but with practice can be a good guide to the level of hearing.
Make a more formal assessment by asking the patient to repeat words spoken by the examiner at different intensities in each ear in turn. Sit beside the patient and use one hand to occlude the ear canal gently in the non-test ear (masking). The best way to do this is to press gently on the tragus and occlude the ear canal. This will mean that the examiner’s voice comes from approximately 1 metre from the test ear (Fig. 3.1). Record the result as, for example, whispered voice (WV) at 1 metre in a patient with slight deafness, or conversational voice (CV) at 1 metre in a deafer individual.
If you suspect profound unilateral deafness, the good ear can be more thoroughly masked with a specially designed noise box (Barany noise box) and the deaf ear tested by speaking loudly into it.
Make a more formal assessment by asking the patient to repeat words spoken by the examiner at different intensities in each ear in turn. Sit beside the patient and use one hand to occlude the ear canal gently in the non-test ear (masking). The best way to do this is to press gently on the tragus and occlude the ear canal. This will mean that the examiner’s voice comes from approximately 1 metre from the test ear (Fig. 3.1). Record the result as, for example, whispered voice (WV) at 1 metre in a patient with slight deafness, or conversational voice (CV) at 1 metre in a deafer individual.
If you suspect profound unilateral deafness, the good ear can be more thoroughly masked with a specially designed noise box (Barany noise box) and the deaf ear tested by speaking loudly into it.
Tuning fork tests
Tuning fork tests rely on the basic concept of classification of hearing loss. Deafness may be classified under one of these headings:
• Conductive deafness.
• Sensorineural deafness.
• Mixed conductive and sensorineural deafness.
• Conductive deafness.
• Sensorineural deafness.
• Mixed conductive and sensorineural deafness.
Conductive deafness (Fig. 3.2)
Conductive deafness results from failure of transmission of sound waves across the outer or middle ear, preventing sound energy from reaching the cochlear fluids. Some forms may be improved by surgery and so it is important to recognize conduc- tive deafness as it is more readily corrected by surgery than sensorineural deafness.
Sensorineural deafness (Fig. 3.2)
Sensorineural deafness results from defective function of the cochlea or of the audi- tory nerve. This either prevents neural impulses being generated in the cochlea or getting from the inner ear to the auditory cortex of the brain.Mixed deafness
Mixed deafness is the term used to describe a combination of conductive and senso- rineural deafness in the same ear.
Rinne’s test
This test compares hearing in one ear by air conduction (AC), and bone conduction (BC). It is usually performed as follows: a tuning fork of 512Hz (cycles per second) isstruck and held close to the patient’s ear (AC); the base is then placed firmly on the mastoid process behind the ear (BC) and the patient is asked to state whether it is heard better by BC or AC (Fig. 3.3).
Interpretation of Rinne’s test
If AC > BC (called Rinne positive) the middle and outer ears are fnctioning normally. If BC > AC (called Rinne negative) there is defective function of the outer or mid-
dle ear (conductive deafness).
Try this on yourself. Then gently occlude your outer ear by pressing the tragus, giving yourself a mild temporary conductive deafness. Now repeat the test and you should find that Rinne becomes negative, demonstrating the conductive loss.
Rinne’s test tells you little or nothing about the cochlea. It is a test of middle-ear function.
In conductive deafness the sound is heard in the deafer ear.
In sensorineural deafness the sound is heard in the better-hearing ear (Figs 3.3–3.5).
dle ear (conductive deafness).
Try this on yourself. Then gently occlude your outer ear by pressing the tragus, giving yourself a mild temporary conductive deafness. Now repeat the test and you should find that Rinne becomes negative, demonstrating the conductive loss.
Rinne’s test tells you little or nothing about the cochlea. It is a test of middle-ear function.
Weber’s test
This test is useful in determining the type of deafness and in deciding which ear has the better-functioning cochlea. The base of a vibrating tuning fork is held on the middle of the skull and the patient is asked whether the sound is heard centrally or is referred to one or other ear.In conductive deafness the sound is heard in the deafer ear.
In sensorineural deafness the sound is heard in the better-hearing ear (Figs 3.3–3.5).
Audiometry
Pure tone audiometry
Pure tone audiometry provides a measurement of hearing levels by AC and BC and depends on the co-operation of the subject. The test should be carried out in a soundproofed room. The audiometer is an instrument that generates pure tone signals ranging from 125 to 12 000 Hz (12 kHz) at variable intensities. The signal is presented to the patient through earphones (for AC) or a small vibrator applied to the mastoid process (for BC). Signals of increasing intensity at each frequency are presented to the patient, who indicates when the test tone can be heard. The threshold of hearing at each frequency is charted in the form of an audiogram (Figs 3.6–3.8), with hearing loss expressed in decibels (dB). Decibels are logarithmic units of relative intensity of sound energy. When testing hearing by BC, it is essen- tial to mask the opposite ear with narrow-band noise to avoid cross-transmission of the signal to that ear.Speech audiometrySpeech audiometry measures the ability of each ear to discriminate the spoken word at different intensities. A recorded word list is supplied to the patient through the
audiometer at increasing loudness levels, and the score is plotted on a graph. In some disorders, the intelligibility of speech may fail above a certain intensity level. Above a critical threshold, sounds are suddenly perceived as having become exces- sively loud – loudness recruitment. This suggests a cochlear disorder and is common in elderly patients with presbycusis.
Impedance audiometry (tympanometry)
Impedance audiometry measures not hearing but the compliance or mobility of the middle-ear structures. A pure tone signal of known intensity is fed into the external auditory canal via an ear probe and a microphone in the probe measures reflected sound levels. Thus, the sound admitted to the ear can be measured. Most sound is absorbed when the compliance is maximal, and by altering the pressure in the external canal a measure can be made of the compliance at different pressures. Impedance testing is widely used as a screening method for otitis media with effusion (OME) in children. If there is fluid in the middle ear, the compliance curve is flattened.
Evoked response audiometry
Evoked response audiometry is a collective term for investigations whereby nerve activity in the form of action potentials (APs) at various points within the long and complex auditory pathway can be recorded. The AP is evoked by a sound stimulus applied to the ear and the resulting AP is collected in a computer store. Although each AP is tiny, it occurs at the same time interval after the stimulus (usually a click of very short duration) and so a train of stimuli will produce an easily detectable response. By making the computer look at different time windows, responses at various sites in the auditory pathway can be investigate.
Evoked response audiometry has the unique advantage of being an objective measure of hearing requiring no co-operation from the subject. It is of value in as- sessing hearing thresholds in babies and small children and in cases of dispute such as litigation for industrial deafness.
Evoked response audiometry has the unique advantage of being an objective measure of hearing requiring no co-operation from the subject. It is of value in as- sessing hearing thresholds in babies and small children and in cases of dispute such as litigation for industrial deafness.
Otoacoustic emissions
When the ear is subjected to a sound wave it is stimulated to produce itself an emission of sound generated within the cochlea. This can be detected and recorded and has been used as a screening test of hearing in newborn babies. It is now in routine clinical use.
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