Immittance audiometry
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Immittance audiometry uses measurements of acoustic impedance and admittance to assess middle ear function. It is a non-invasive and non-behavioral test. Key measures include tympanometry to evaluate the mobility of the eardrum and ossicular chain, and acoustic reflex thresholds to assess the function of the middle ear muscles and brainstem pathways. Abnormal immittance test results can help diagnose conditions like middle ear fluid, ossicular discontinuity, or retrocochlear lesions.
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Immittance audiometry
- 2. 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 Disease John T. Jacobson
- 3. 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
- 4. Compliance vs Impedance Compliance Ease with which energy flows through a system Impedance Resistance to energy flow through a system
- 5. 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
- 6. Instrumentation Major components Probe tone oscillator and loudspeaker Monitor microphone Pressure pump and manometer Ipsilateral reflex oscillator and loudspeaker Probe tip
- 7. Auditory Immittance “ A way of assessing the manner in which energy flows through the outer and middle ear into the cochlea”
- 8. Immittance Relationships Probe tone energy passed Probe tone energy reflected Compliance High impedance Low High Low Low impedance High Low High
- 9. Basic Immittance Measures Static compliance Tympanometry Acoustic Reflex Thresholds Acoustic Reflex Threshold Decay
- 10. 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.
- 11. Tympanometry “A way of measuring how acoustic immittance of the middle ear system changes as air pressure is varied in the external ear canal”
- 12. 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
- 13. Tympanometry Concepts of immittance applied in practice
- 15. 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
- 16. What is a Normal Tympanogram? Shape Pressure: -100 mm H2O or DaPa Compliance: 0.3-1.6 cc
- 17. Common Tympanograms Normal tympanogram (Type A) Shape? Pressure? Compliance?
- 20. Common Tympanograms Type B No peak compliance Little change in compliance with various pressures Middle ear effusion, total perforation, or impacted wax
- 21. Assessing PE Tubes TYPE B (BLOCKED PE TUBE) PATENT PE TUBE
- 22. Common Tympanograms Type C Peak compliance in negative ranges often beyond -100db Eustachian tube dysfunction, inadequate ventilation of ME
- 23. 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
- 24. Basic Immittance Measures Tympanometry Acoustic Reflex Thresholds
- 25. 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
- 26. 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 10ms Acoustic 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
- 27. The Middle Ear Muscles and the Acoustic Reflex Tensor tympani muscle Stapedius muscle
- 28. Acoustic Reflex Pathways Ipsilateral Right ear Left ear Contralateral Probe right Probe left
- 29. Acoustic Reflex Threshold “the lowest intensity at which a middle ear immittance change can be detected in response to sound”
- 31. Normal Acoustic Reflex Threshold Levels
- 32. Interpretation of an Absent Acoustic Reflex Threshold Possible pathologies that might lead to an absent contralateral probe left reflex (right crossed)
- 33. 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
- 34. Basic Immittance Measures Tympanometry Acoustic Reflex Thresholds Acoustic Reflex Threshold Decay
- 35. 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.
- 36. Thank you.
Editor's Notes
- Impedance properties of the middle ear are the result of complex interaction between the ossicular chain, the intra aural muscles and ligaments and atmospheric comdition within the middle ear space, all of which are reflected by the TM
- Dynamic in nature, measuring compliance while introducing air pressure change in the external ear canal Tympanometry reflects the mobility (compliance ) of tympanic membrane when air pressure is varied from 200 to -400dPa within the ear canal Tympanometry may be considered analogous to pneumatic otoscopy, but with far greater precision
- Tympanometry yields infor Volume of ear canal ranges from 0.5 to 1ml for children
- Type A. peak compliance between 0 and -100dpa, and within a normal range of compliance
- Volume of calan ranges from 0.5 to 1mls for children and 0.6 to 2.0 for adults Peak compliance between 0 and -100dpa, and within normal range of compliance
- Peaks that are located within the normal pressure range may be overly compliant Ad with atrophic tympanic membrane or As with ossicular chain fixation or tympanosclerosis
- As with ossicular chain fixation or tympanosclerosis
- Ad with atrophic tympanic membrane
- Volumes more than 1.0 for children and 2.0mls for adults may suggest tympanic membrane perforation or presence of patent ventilation tubes
- Type C is often a precursor to type B as development of negative pressure precedes the presence of effusion
- there may be a normal tympanometric curve when the 226 Hz probe is used, even in the presence of an air conduction component, which means that the tests carried out with this probe tone have a high rate of false-negative results. Kiel et al, carvello et al,
- In a normal ear void of middle ear pathology the reflex occurs at approximately 80 -90 db HTL
- Even normal individuals may exhibit decay at higher frequencies, therefore lower frequency, 500 to 1000 Hz tones are used If response decreases to less than half within 5 seconds retrocochlear pathology may be suspected
- Acoustic reflex in ears with cochlear disorders is determined primarily by the degree of snhl. If auditory thresholds are below 50 – 55 db, reflex thresholds are normal If auditory threshold are between 55 – 80 db, the reflex thresholds are elevated in proportion to auditory thresholds If losses greater than 80 db acoustic reflexes absent
- Acoustic reflex in ears with cochlear disorders is determined primarily by the degree of snhl. If auditory thresholds are below 50 – 55 db, reflex thresholds are normal If auditory threshold are between 55 – 80 db, the reflex thresholds are elevated in proportion to auditory thresholds If losses greater than 80 db acoustic reflexes absent
- Normal acoustic reflex thresholds for broadband noise are approximately 20 – 25 db lower than reflex threshold for tones Clinically important in evaluation of malingerers because it is not physiologically possible for the paritentto truly have behavioural thresholds that are higher or worse than acoustic reflex thresholds for tones In patients with SNHL the difference between the broadband and tone thresholds is decreased inversely to that amount of hearing loss (degree of snhl in db increases, the diff in ac reflex thresholds for tones and noise decreases)
- Efferent limb of reflex is absent in ossicular chain disorders, such as otosclerosis and discontinuity and facial nerve pathology In vii paralysis, the acoustic reflex is absent or abn even when the probe is placed in ear ipsilateral to the lesion Asr can also be helpful in locating cnvii lesion either proximal to or distal to stapedeal muscle
- Acoustic reflex decay measures the ability of stapedial muscle to maintain sustained contraction, usually by presenting a signal 10db above the reflex threshold for 10s Rapid decay found in cnVIII lesions