immittance audiometry
TRANSCRIPT
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
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
Common Tympanograms
Type BNo peak complianceLittle change in compliance with various pressuresMiddle ear effusion, total perforation, or impacted wax
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
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
Acoustic Reflex Threshold
“the lowest intensity at which a middle ear immittance change can be detected in response to sound”
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
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.