FUNCTIONAL EXAMINATION OF THE EAR 269 In bilateral deafness it is customary to use a battery of tests, e.g. (a) Bekesy self-recording audiornetry, (b) SISI (short increment sensitivity index) loud- ness discomfort level, (c) tone decay level, and (d) acoustic impedance measurements. a. Bekesy Audiometry. In this type of audiometry the frequency of the tests signal is continuously altered from low to high frequency. The patient controls the intensity of the signal so that it fluctuates about the threshold and the level is plotted by a pen. This not only gives a record of the threshold Recruitment Fig. 149. Diagram shoving loudness balance test. (.After Dix, Hallpike and Hood.) but it has been shown that in cochlear lesions the excursions of the pen around threshold become much smaller than when the hearing is normal. In lesions of the auditory nerve the plotted threshold may be lower than that demonstrated by normal pure tone audiometry due to abnormal adaptation. b. SISL In this test the patient's ability to distinguish small fluctuations in intensity of a tone is measured. In cochlear deafness smaller increments than normal may be distinguished. c. Tone Decay Level. In cochlear deafness loud sounds may cause discomfort at a lower level above threshold than in normally hearing subjects and a quiet continuous tone may cease to be heard after an abnormally short duration of exposure. d. Acoustic Impedance Measurements. A tone played in to the ear will be partly absorbed by the sound-conducting apparatus and partly reflected outwards from the surface of the drum. The maximum absorption will occur when the contour of the drum is normal with equal pressure on either side. This absorption or impedance of the drum can be measured. By altering the pressure in the external meatus until maximum absorption by the drum is demonstrated, the pressure in the middle ear can be measured, e.g. Eustachian obstruction where the middle ear pressure is reduced. Characteristic patterns can be shown in secretory otitis media, adhesions in the middle ear, ossicular disruption and otosclerosis. Alterations of the contour of the drum can also be demonstrated when the stapedius and tensor tympani muscles contract. The stapedius muscle can be made to contract by a loud sound and the intensity of sound required to induce contraction can be used as a measure of recruitment and it also provides a rough objective measurement of the threshold of hearing. Demonstration of the stapedius reflex can be used to show the level of a facial paralysis. Contraction of the tensor tympani muscle is produced by stimulation of the sensory part of the trigeminal nerve and the usual stimulus is cold air applied to the cornea.