Immittance audiometry

NONBEHAVIORAL ASSESSMENT MEASURES

IMMITANCE AUDIOMETRYIMMITANCE AUDIOMETRY

The Functions of Immittance Audiometry

Detection of middle ear pathology

Differentiating cochlear from retrocochlear pathology

Estimate sensitivity of hearing loss

Cross referencable with pure tone results

The Role of Immittance Audiometry in Detecting Middle Ear DiseaseJohn T. Jacobson

Acoustic immitance

◦ Impedance:◦ Resistance to the flow of acoustic energy

◦ Useful diagnostic tool to detect presence of fluid in the middle ear, evaluate EU tube function, help evaluate the facial nerve and help predict audiometry

Admittance◦ Ease of which acoustic energy flows

Immitance is a term derived from the terms for two inversely related processes for assessing middle ear function◦ Impedance◦ Admittance

Compliance vs Impedance Compliance

Ease with which energy flows through a system

Impedance

Resistance to energy flow through a system

Advantages of Immittance Audiometry

“Immittance is a physical characteristic of all mechanical vibratory systems, of which the middle ear is one example”

Non-invasive

Non-behavioral

Instrumentation Major components

Probe tone oscillator and loudspeaker

Monitor microphonePressure pump and manometer

Ipsilateral reflex oscillator and loudspeaker

Probe tip

Auditory Immittance

“ A way of assessing the manner in which energy flows through the outer and middle ear into the cochlea”

Immittance Relationships

Probe tone energy passed

Probe tone energy reflected

Compliance

High impedance

Low High Low

Low impedance

High Low High

Basic Immittance Measures

Static compliance

Tympanometry

Acoustic Reflex Thresholds

Acoustic Reflex Threshold Decay

Static compliance Measure of ear canal volume under two specific physical condition

200mH20 of positive air pressure is applied to ear canal and a volume is read

Second volume reading occurs at a pressure value of maximum eardrum compliance

Under normal mddle ear conditions, maximum eardrum compliance occurs when atmospheric pressure is equal on both sides of TM (0mmH2O)

Two volumes are subrtracted from one another and remaining volume represents the static compliance of the middle ear

Because wide variety of middle ear pathologies produce overlapping compliance values, static compliance is least applicable measure of immitance test battery.

Tympanometry

“A way of measuring how acoustic immittance of the middle ear system changes as air pressure is varied in the external ear canal”

Tympanometry

Yields information about◦ Air pressure status of the middle ear◦ Static acoustic immitance (establishing parameters of stiffness or flaccidity at

the ear drum)◦ The integrity and mobility of the eardrum and ossicular chain◦ Resonance point of the middle ear system

Tympanometry

Concepts of immittance applied in practice

Normal Tympanogram

Impedance as Equivalent Volume

When the amount of reflected probe tone pressure is high, it’s as if the volume has decreased. As volume increases, sound pressure decreases

What is a Normal Tympanogram?

Shape

Pressure: -100 mm H2O or DaPa

Compliance: 0.3-1.6 cc

Common Tympanograms

Normal tympanogram (Type A)Shape?Pressure?Compliance?

Common Tympanograms

Type As

Shape?Pressure?Compliance?

Common Tympanograms

Type Ad

Shape?Pressure?Compliance?

Common Tympanograms

Type BNo peak complianceLittle change in compliance with various pressuresMiddle ear effusion, total perforation, or impacted wax

Assessing PE Tubes

TYPE B (BLOCKED PE TUBE) PATENT PE TUBE

Common Tympanograms

Type CPeak compliance in negative ranges often beyond -100dbEustachian tube dysfunction, inadequate ventilation of ME

Tympanometry in very young children

0 – 6 months

High frequency probe required

1000Hz for babies younger than 3months

3 – 9 months, initially use 1000hz, if fails repeat tone with 226hz probe

Tympanometry with 226 and 1000 Hertz tone probes in infants 2012 Luciana Macedo de ResendeI; Juliana dos Santos FerreiraII; Sirley Alves da Silva CarvalhoIII; Isamara Simas OliveiraIV; Iara Barreto BassiV

Basic Immittance Measures

Tympanometry

Acoustic Reflex Thresholds

ASR Defined as the lowest intensity required to elicit a stapedial muscle contraction

Neural connection located in lower brainstem, with influences of higher CNS structures on the reflex via the olivocochlear bundle

Afferent portion of the reflex is the ipsilateral eight nerve to cochlear nuclei

Efferent limb is the facial nerve which innervates stapedial muscle

Contraction of the stapedial muscle tilts the anterior stapes away from the oval window and stiffens the ossicular chain and results in increased impedence, which is measured as small decrease in compliance by the ear canal probe

ASR 3 primary acoustic reflexes characteristics commonly evaluated

◦ 1. presence or absence of the stapedial reflex◦ 2.acoustic reflex threshold◦ 3 acoustic reflex decay or adaptation

The time delay of acoustic reflex is thought to be 10msAcoustic reflex thresholds for tones in patients with normal hearing are usually

70 to 80 db above their tone thresholds and about 5db greater for the contralateral threshold

The Middle Ear Muscles and the Acoustic Reflex

Tensor tympani muscle

Stapedius muscle

Acoustic Reflex Pathways

IpsilateralRight earLeft ear ContralateralProbe rightProbe left

Acoustic Reflex Threshold

“the lowest intensity at which a middle ear immittance change can be detected in response to sound”

Instrumentation for Acoustic Reflex

Thresholds

Normal Acoustic Reflex Threshold Levels

Interpretation of an Absent Acoustic Reflex Threshold

Possible pathologies that might lead to an absent contralateral probe left reflex (right crossed)

CN VIII lesions Demonstrate absent acoustic reflex when stimuli presented to affected ear

Acoustic reflexes differ from cnVIII lesion versus cochlear lesion◦ cnVIII refles will be absent or abnormal regardless of degree of hearing loss◦ Cochlear lesion usually dependent on degree of hearing loss

Abnormal reflexes also recorded when stapedial muscle function is altered by myopathic disease such as Myasthenia Gravis and Eaton – Lambert syndrome or hyperthyroidism

Basic Immittance Measures

Tympanometry

Acoustic Reflex Thresholds

Acoustic Reflex Threshold Decay

Time Course of the Acoustic Reflex Threshold Decay

The test is carried out by presenting a 10 s signal at 10 dB above the ART.

Thank you.