Diseases Of The Nose Throat And Ear

268

THE EAR

threshold masking of the opposite ear must always be used because of the ease
with which bone conduction sound is transmitted through the bones of the
skull.

In speech audiometry the ability to understand speech is measured. A
recording of phonetically balanced word tests is used. These are played to
the patient at increasing intensities and the proportion of words correctly
repeated by the patient is expressed as a percentage; the results are charted
(.Fig. 148). When hearing is normal all the words will be understood if they are

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Speech Intensity-Decibels above normal threshold

Fig. 148. Speech audiogram. 1, Normal hearing; 2, Conduction deafness; 3, Perceptive deafness.

played loudly enough. In conductive deafness all the words will be under-
stood but they will require to be played louder than for the normal subject. In
sensorineural deafness 100 per cent understanding of words will not be
achieved and increasing loudness above the optimum may produce a reduc-
tion in understanding. If increasing the loudness can produce an understand-
ing of more than 40 per cent of the words, it may be predicted that 90 per cent
of sentence intelligibility can be obtained. This gives a useful method of
assessing the degree of disability produced by the deafness and is also helpful
in predicting the usefulness of a hearing aid or the benefit which might be
obtained by an operation.

Tests for Recruitment

Recruitment is the inability to hear quiet sounds, yet loud sounds can be heard
as loud or even louder than normal. Recruitment produces diminished
understanding of speech and intolerance of loud sounds, particularly when
a hearing aid is worn. Recruitment is present when the cause of the deafness
is in the organ of Corti. Demonstration of recruitment may be used to
localize the site of the lesion in sensorineural deafness and, in particular, to
differentiate deafness due to a cochlear lesion from that due to an auditory
nerve lesion. Recruitment in unilateral deafness is best demonstrated by the
Fowler's Alternating Loudness balance test in which a tone is played
alternately into the normal ear and into the deaf ear, the intensity in the deaf
ear being adjusted until the sounds in both ears are heard equally loudly
(Fig. 149).