Objective Tests of the Auditory System

Objective Tests of theAuditory System

SPA 4302

Summer A, 2004

Impedance (Z) vs. Admittance (Y)

Resistance(R)

Mass Reactance(Xm)

Stiffness Reactance(Xs)

Z = (R2 + Xt2)1/2

Conductance(G)

Mass Susceptance(Bm)

Stiffness Susceptance(Bs)

Y = (G2 + Bt2)1/2

Tympanogram:

• A plot of middle ear admittance as a function of ear canal pressure

• Pressure is swept from +200 to -200 or -400 dPa

• Should see peak at point where pressures are equal

Tympanogram types:

• A: peak between +100 and -200 dPa: normal

• C: peak beyond -200 dPa: neg pressure

• B: no peak flat tymp: effusion

• As: peak but shallow: stiff: otosclerosis

• Ad: peak off scale: floppy: dysarticulation

Tympanogram Types

Static Admittance

• A measure of the height of tympanogram peak

Adults 3-10 yrsA 0.30 to 1.70 0.25 to 1.05As < 0.30 < 0.2Ad >1.70 > 1.05

(All values are in ml of equivalent volume)

The Acoustic Reflex• Stapedius contraction measured as change

in compliance

• Reflex arc:– peripheral ear,

– VIIIth n.

– Cochlear nucleus

– Superior Olivary Complexes

– Medial Nuclei of VII

– VIIth nerves to both middle ears

R L

SOC

CN

8th

IE

ME

OE

SOC

CN

8th

IE

ME

OE

7th CN 7th CN

MN 7MN 7

Clinical Tests using Acoustic Reflexes:

• A.R. Threshold: how intense sound must be to elicit the reflex?

• A.R. Decay: Is the degree of a contraction maintained throughout a 10 second stimulus?

AR Threshold:

• to pure-tones, 0.5, 1, 2, 4 kHz

• = lowest level at which change in admittance can be read

• ipsilaterally or contralaterally

• should be between 65 and 90 dB SL.

Otoacoustic Emissions (OAE)

• Low-level sounds produced by the cochlea and recordable in the external ear canal.

• Spontaneous

• Click-evoked

• Distortion Product

• Stimulus Frequency

Recording OAEs

Spectrum of Sound in Ear Canal

OAE as Measure of Cochlear Health

Auditory Evoked PotentialsRecording: Comes From:

Electrocochleography Cochlea, VIIIth nerve

Auditory Brainstem Response VIIIth nerve & brainstem

Middle Latency Response Thalamus & auditory cortex

Auditory Late Response Auditory Cortices

P300 Hippocampus area

Auditory Brainstem Response

• Electrical response from neurons to sound

• Recorded by electrodes on scalp

• Coming from VIIIth nerve and brainstem

ABR MEASURES

Time (ms)

Am

plit

ude

(μV

)

Wave V Latency

I-V IPL

I-III III-V

I IIIII

IVV

Amp V

Conductive Loss: Curve shifted to higher levels by amount of

conductive loss

Severe Cochlear Losses