Acoustic Immitance (Impedance and Admittance) Lecture 9

Outline Concepts of Immitance Tympanometry Interpreting Results Admittance (compliance and Impedance) Tympanometry Equipment Procedures Interpreting Results Acoustic Reflexes

Immitance Concepts: Big Picture Wave strikes the TM Some of signal is absorbed by TM Some of signal is reflected back out the external canal.

Efficient transmission of sound Ideal condition: as much of the sound signal “admitted” or go through as possible Need a system that is highly compliant Need a system that has low impedance OE & ME structures

Condition of TM Which TM would allow more sound to be admitted (more compliant/low impedance) Which TM would provide less sound to be admitted (less compliant/more impedance)

Components of Acoustic Immitance Admittance – measures the flow of energy through the middle ear system (compliance) Impedance- measures the opposition to the flow of energy through the middle ear system

Components of Acoustic Immitance A high impedance system is heavy &/or stiff and does not accept energy well.  A system that does not readily accept energy can also be called a low admittance system. IDEALLY for HEARING : want a low impedance, high admittance system  

Tympanometry Definition: Tympanometry is a dynamic measure of the acoustic immitance in the external ear canal as a function of ear canal air pressure change Used to test the condition of the middle ear and mobility of the eardrum and the conduction bones by creating variations of air pressure in the ear canal. Purpose Provides information regarding mobility (compliance) of the tympanic membrane Guides us in determining the status of the ME Cross check assessment that aids in differential diagnosis

Tympanometry Procedure A probe will be presented to the ear canal and a seal will be formed A change in pressure will be presented that the client will feel stopped up A sound will be presented through the probe tip A microphone will record the amount of sound that is reflected back

Tympanometry Equipment

Impedance Instrumentation Introduces a pure tone into ear canal through 3-function probe tip Reflex Signal Generator (speaker) Probe Tone Generator (speaker) Probe (Microphone) Air Pressure System

Tympanometry Procedure Ear canal is sealed with a probe tip – sizes vary Air pump creates a change in pressure in ear canal Pressure changes gradually: Positive +200 daPa (forces TM inward) Negative – 200 daPa (forces TM outward) While pressure is changing, a sound transmitter sends a sound wave to the TM Wave that is reflected from the TM is then picked up by a mic in the probe The tympanometer measures energy of the reflected sound.

Tympanometric Features obtained Shapes of tympanogram Equivalent Ear Canal Volume Static Acoustic Compliance Tympanometric peak pressure Tympanometric Width

If ME space is filled with air, Concepts If ME space is filled with air, energy is absorbed by the TM, ossicles, and inner ear structures tracing will show a normal peak compliance If ME space is filled with fluid most of the sound is reflected back to the probe from the stiff TM Tracing will show a flat tympanogram If disruption of ossicles large amount of energy will be absorbed into the TM tracing will display an abnormally high peak.

1. Tympanometric Shapes Type A: Normal Type As: Normal, but shallow Type Ad: Possibly disarticulated or flaccid Type B: Abnormal, reduced eardrum mobility Type C: Abnormal, Negative Pressure Or decaPascals

Shape Type A – Normal Tympanogram The peak of the pressure curve falls between +50 and -150 daPa of pressure Peak compliance falls between .2 and 1.8 mm Results indicate the absence of middle ear pathology Intact & mobile TM with normal eustachian tube function If there is a hearing loss, it is likely to be SNHL!

Shape Type As (shallow): Abnormal Tympanogram The peak of the pressure curve falls between +50 and -150 daPa of pressure (WNL) Peak compliance very low (well below .2 cm3) Often associated with ossicular fixation or TM scarring (not middle ear effusion) May result in a fairly flat, non-fluctuating hearing loss Eustachian tube function is normal -300 -150 0 +50 +200

Shape Type Ad (disarticulation): Abnormal Tympanogram The peak of the pressure curve falls between +50 and -150 daPa (WNL) Peak compliance very high or off chart Associated with ossicular disarticulation May result in a fairly flat, non-fluctuating hearing loss Eustachian tube function is normal -300 -150 0 +50 +200

Shape Type B: TM Retracted, Poorly Mobile Peak is absent/poorly defined and at markedly negative middle ear pressure (>-200 daPa) Max compliance = below normal range Tympanometric width is outside normal limits -300 -150 0 +50 +200

Shape Type C: There is a clearly defined peak, but it falls on the negative side of the chart, indicating negative middle ear pressure Peak pressure is seen at greater than -150 mm (moved to left) Peak compliance may be normal Diagnosis: Eustachian tube dysfunction, may cause a very mild conductive loss, or hearing can be WNL

Air, Fluid or Disruption of ossicles?

2) Equivalent Ear Canal Volume Measured in cm3 Made at the beginning with +200 daPa of pressure Normal values Children .3-1.5 cm3 Adults .6-2.0 cm3

3) Tymanometric Peak Pressure Machine will automatically record the peak pressure point Observe the peak pressure value in daPa Normal Values: >-150 to –200 daPa considered abnormal

4) Static Acoustic Compliance (admittance) Most machines make this measurement automatically at the peak compliant point of the tracing Low values suggest reduced compliance High values suggests excessive compliance Normal Values: Children: .25- 1.5 ml or cm3 Adults: .3-1.7 ml or cm3

5) Ear canal pressure: Tympanometric Width Defined: interval in daPa between sides of tympanogram at one half of the peak admittance value Normal range: Children: 80-159 daPa Adults: 51-114 daPa Generally > 150 daPa associate with fluid

Discriminating Fluid from a Perforation Both will be Type B!!! Equivalent Ear canal volume is the key

Summary Tymp Interpretation Peak Ear Canal Volume Type A Present and occurs around 0 Normal Volume Type B (fluid) No Peak Type B (perf) Large Volume Type C Negative Peak

Acoustic Reflex Contraction of the stapedial muscle to loud sounds Results in reduction of sound transmission to cochlea 5-10 dB of attenuation Reflex occurs both contralaterally & ipsilaterally

Acoustic Reflex If NH, at sound levels 85 dB SL, stapedial muscle contracts, pulls stapes away from OW Contraction results in TM movement that can be measured Anatomy Physiology (loud sounds) Levels elicitted (85 dB SL) Softest level = Acoustic reflex threshold Get this even for many people with hearing loss up to severe types Ipsilateral and Contralateral

Acoustic Reflex Testing Reflexes typically tested at: 500, 1000, 2000 and 4000 Hz Begin at 85 dB HL and increase level up to 120dB HL

Set-up for acoustic reflex testing Immitance set up the same as for tympanogram testing Usually conducted immediately after tympanogram is completed Set–up to measure admittance of the eardrum as well as present a loud signal

Reflex Arc: Right Ipsi and Contra pathway

Reflex Arc: Left Ipsi and Contra pathways

RE cochlear pathology

VIIIth nerve pathology

R facial nerve pathology

Four Possible AR outcomes Present at normal levels usually 85 dB SL 80-100 dB HL Absent at limits of testing > 120 dB HL Present at lower SL < 80 dB HL Present at high SL (elevated) – >100-120 dB HL 1- example 1000 Hz Th = 10, present at 95 dB HL, SL = 85 2- obvious 3- example 1000 Hz th = 60, present at 95 dB HL, SL = 35 .. Basically normal performance because IHC intact to some degree 4- example 1000 Hz th =10, present at 125, SL = 105

Outcome …