What is Test for middle ear function test - and explain in detail?
Ans. is 'c' i.e., Impedence audiometry Impedance audiometryo This most valuable test is now widely used and provides a quick objective measurement of the state of the middle ear. Impedance audiometry' consists of - 1) Tympanometry, 2) Acoustic reflex (Stapedial reflex) measurementTympanometryo It is the most commonly used type of impedance audiometry. The principle of tympanometry is that a tone played into the ear will be partly absorbed by sound-conducting apparatus and partly reflected outwards from the surface of tympanic membrane. A stiffer tympanic membrane reflects more of sound energy than a compliant one and the maximum absorption of sound occurs when the air pressure in the external auditory canal is the same as their pressure in the middle ear. The amount of sound reflected by the drum is measured by a microphone and by varying the pressure in the external meatus by means of a pump attached to a manometer, a tracing of sound absorption against pressure is obtained. Because the passage of sound can be said to be impeded by the drum and ossicles, the technique is known as impedance audiometer.o The equipment consists of a probe which snugly fits into the external auditory canal and has three channels : -To deliver a tone of 220 Hz, it) To pick up the reflected sound through a microphone, and hi) to bring aboutchanges in air pressure in the ear canal from positive to normal and then negative.Types of tympanogramso By charting the compliance of tympano-ossicular system against various pressure changes, different types of graphs called tympanograms are obtained which are diagnostic of certain middle ear pathologies.Type A curveo It is normal bell shaped curve with a peak at zero pressure, i.e., Maximum compliance at zero (ambient) pressure.o There are two variant of * A' curve : -AS curve : - Low compliance tympanogram with low peak, i.e., low compliance at zero (ambient) pressure.AD curve : - High compliance tympanogram with high peak, i.e., high compliance at zero (ambient) pressure.Type B curveo Flat/Dome shaped tympanogram which is seen in cases of middle ear fluid or thick tympanic membrane.Type C curveo It is a bell shaped curve with a peak shift to the negative pressure, i.e., maximum compliance at negative pressure. Acoustic (Stapedial) reflexo When a sound of 70-100 dB above the threshold of hearing is presented to the ear, the stapedius muscle contracts (Bilaterally). This immediately alters the stiffness of the ossicular chain and causes a change in the acoustic impedance of the ear which is recorded by the audiometer. As the muslce contracts bilaterally, tone is delivered to one ear and the reflex can be picked from the same or the contralateral ear.