Impedance Audiometry

After a review of the development of acoustic impedance measurements, the principles of tympanometry, static compliance and measurements of the middle ear reflexes are described. The interpretation of tympanometry is done by analyzing three essential features: pressure, amplitude and shape. The influence of high probe tones on the shape of the tympanogram and the possibility of analyzing the stiffness-mass relationship are pointed out. Impedance screening has supplemented tone screening on 5,886 seven-year-olds in five separate investigations from 1972 to 1978. In the last and most conclusive investigation on 1,027 children, the otologist found 6% pathological ears. Stapedius reflex threshold testing (ipsilateral stimulation), criterion level 110 dB SPL rated 7.8% of the ears as abnormal. Tympanometry resulted in 6.5% pathological ears and tone screening at 0.5 kHz 1.9%. Based on these results the following screening procedure is recommended: tone screening at 0.5 and 4.0 kHz supplemented by tympanometry. A middle ear pressure of ≤ — 150 mm H₂O, or a flat tympanogram and/or tone screening levels ∓20 dB HTL at 0.5 kHz and/or 4.0 kHz are considered at indicative of ear pathology. Children failing the screening test should have a repeat test after four to six weeks at school and only those who do not pass at this time should be referred to an otologist.