Testing the Hearing

There are three stages to testing the hearing and all are important. Audiograms can be wrong. 1 Clinical assessment of the degree of deafness. 2 Tuning fork tests. 3 Audiometry. CLINICAL ASSESSMENT OF THE DEGREE OF DEAFNESS By talking to the patient, the examiner quickly appreciates how well a patient can hear and this assessment continues throughout the interview. A more formal assessment is then made by asking the patient to repeat words spoken by the examiner at different intensities and distances in each ear in turn. The result is recorded as, for example, whispered voice (WV) at 150 cm in a patient with slight deafness, or conversational voice (CV) at 15 cm in a deafer individual. If profound unilateral deafness is suspected, the good ear should be masked with a Barany noise box and the deaf ear tested by shouting into it. The limitations of voice and whisper tests must be borne in mind; they are approximations but with practice can be a good guide to the level of hearing and will confirm the audiometric findings. TUNING FORK TESTS Before considering tuning fork tests it is necessary to have a basic concept of classification of deafness. Almost every form of deafness (and there are many) may be classified under one of these headings: • conductive deafness; • sensorineural deafness; • mixed conductive and sensorineural deafness. Conductive deafness (Fig. 3.1) Conductive deafness results from mechanical attenuation of the sound waves in the outer or middle ear, preventing sound energy from reachingplaced firmly on the mastoid process and the patient is asked to state whether it is heard better by BC or AC. Interpretation of Rinne’s test If AC>BC–called Rinne positive–the middle and outer ears are functioning normally. If BC>AC–called Rinne negative–there is defective function of the outer or middle ear. Rinne’s test tells you little or nothing about cochlear function. It is a test of middle-ear function. WEBER’S TEST This test is useful in determining the type of deafness a patient may have and in deciding which ear has the better-functioning cochlea. The base of a vibrating tuning fork is held on the vertex of the head 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 SPEECH AUDIOMETRY Speech audiometry is employed to measure 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 fall off above a certain intensity level. It usually implies the presence of loudness recruitment—an abnormal growth of loudness perception. Above a critical threshold, sounds are suddenly perceived as having become excessively loud.This is indicative of cochlear disorder. IMPEDANCE TYMPANOMETRY Impedance tympanometry measures not hearing but, indirectly, the compliance of the middle-ear structures. A pure tone signal of known intensity is fed into the external auditory canal and a microphone in the ear 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 thecompliance 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. ELECTRIC RESPONSE AUDIOMETRY Electric response audiometry is a collective term for various investigations whereby action potentials at various points within the long and complex auditory pathway can be recorded. The action potential (AP) is evoked by a sound stimulus applied to the ear either through headphones or free field, 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, while the averaging ability of the computer will average out the more random electrical activity, such as the EEG. By making the computer look at different time windows, responses at various sites in the auditory pathway can be investigated. As the response travels from the cochlea to the auditory cortex, the latency increases from about1-4 to 300 ms.

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