Immittance Battery & Otto Metz

Immittance Battery Focus

• Includes tympanometry and acoustic reflexes.
• Provides rich information about the middle ear and auditory pathways.

Otto Metz and Immittance Audiometry

• Otto Metz, a Jewish German doctor, contributed to distinguishing sensorineural from conductive hearing loss and researched sound absorption.
• The Danish resistance helped Metz escape the Nazis to Sweden in 1943.
• Metz continued his work in Lund before returning to Copenhagen in 1945.
• He published the basic principles of immittance audiometry in 1946 and the first work on acoustic reflexes five years later.
• Immittance audiometry aids in learning about the middle ear by measuring its impedance to sound.

Understanding Impedance and Admittance

• Acoustic impedance measures how much sound energy is NOT absorbed.
• Acoustic admittance measures how much sound energy IS absorbed and is the inverse of impedance.
• Immittance is a general term referring to both impedance and admittance.
• In the middle ear, low impedance means the ossicles move well with sound.
• Acoustic impedance can be determined by listening for reflection; higher reflection indicates higher impedance.

Components of Acoustic Impedance

• Three components include mass, stiffness, and resistance.
• Resistance is friction.
• Mass reactance increases as frequency goes up, interfering with high frequencies.
• Stiffness reactance decreases as frequency goes up, interfering with low frequencies.
• Mass and stiffness effects are collectively called "Reactance."
• Mass reactance and stiffness reactance are inversely related to frequency
• Total Reactance = Xm - Xs.

Middle Ear System and Acoustic Impedance

• Stiffness and mass are balanced at around 800-1200 Hz, which is the resonant frequency of the middle ear.
• Changes in middle ear impedance are primarily changes in its stiffness.
• Lower frequencies are not transmitted well due to the high stiffness of the system and low mass
• Impedance is clinically measured at 226 Hz, where mass reactance is negligible and changes are mostly due to stiffness.
• Clinical impedance measures are essentially stiffness measures.
• For a 226 Hz tone, the air in a 1 cubic centimeter cavity has approximately 1 millimho of admittance, which is why 226 Hz it is used.

Acoustic Admittance

• Acoustic admittance = sound flow (volume velocity) / sound pressure which is measured in m³/s / pascals (N/m²).
• Acoustic admittance can be estimated by measuring reflected sound.
• Low admittance (high impedance) can indicate the TM/ossicular chain cannot be set in motion easily.
• More volume in the ear canal relates to more admittance.

Tympanometry

• Tympanometry provides rich information about the middle ear.
• Measuring middle ear admittance involves finding a way of subtracting the outer ear impedance:
  • Tightening the eardrum until there is zero middle ear admittance enables the calculation of outer ear admittance.
  • This can be achieved by adding or removing pressure to the ear.
• The device used is called a tympanometer
  • This involves varying the pressure of air in the ear canal with the use of a pump
  • A speaker plays a 226 Hz tone.
  • A microphone measures the reflected sound
  • The system automatically controls gain and keeps sounds level in the ear canal

Tympanometry: Anatomy of a Tympanogram

• Volume: Admittance with positive pressure; indicates external ear canal admittance.
• Peak: Maximum admittance; indicates when TM and ossicles are maximally set in motion due to equal pressure on both sides of the membrane.
• Static Compliance: Peak admittance minus ear canal admittance which determines the admittance of the middle ear
• Peak Pressure: Pressure at maximal compliance ; measured relative to x axis (daPa).
• Width: Measured relative to x axis which determines the slope of the graph.

Types of Tympanograms

• Non-Compensated and Compensated
• Terminology for interpretation: - Tympanometric peak pressure (TPP) is the pressure at which peak compliance is found.
  • Equivalent ear canal volume (VEC) is the admittance with maximum positive pressure.
  • Middle ear admittance or ‘static compliance’ is the peak admittance - external ear admittance
  • Tympanogram width (TW) is the width of the tympanogram at 50% of peak.
• Standard values: - TPP: -100 to +50 daPa - VEC : 0.6 to 2 cm³ - Middle ear admittance : 0.3 to 1.7 mmho - Tympanogram width: 51 to 114 daPa
• Common classifications: - Type A: Normal - Type B: Flat, indicating fluid or perforation - Type C: Negative pressure, indicating Eustachian tube dysfunction - Type As: Shallow peak, indicating stiffness - Type Ad: Deep peak, indicating hypermobility

Acoustic Reflexes

• Otto Metz discovered that loud sounds trigger a reflex that reduces admittance.
• The stapedius muscle contracts at thresholds between 70 and 100 dB HL.
• Stiffens the ossicular chain which reduces the intensity of low-frequency sounds, aiding the auditory and facial nerves, lower brainstem.
• The acoustic reflex tests the VIIIth (vestibulocochlear) and VIIth (facial) nerves, and lower brainstem.
• Vestibular Schwannomas (tumors) can affect the reflex.
• Stimulation of one ear causes a reflex in both ears.

Acoustic Reflex Pathway

• The auditory pathway consist of
  • Ventral Cochlear Nucleus
  • Superior Olivary complex - Motor nucleus of CNVII from lesion
  • CN VIII
  • Cochlea

Acoustic Reflex: Testing and Terminology

• Ear canal pressure is adjusted to peak pressure for best sound transmission.
• Reflex is always measured by the ear of sound presentation (e.g., right ipsilateral means sound went to the right ear).
• Average reflex threshold expectations: varies by activator frequency
  • For people with no hearing loss ~ 85 dBs for pure tones and ~ 65 dBs for broadband noise stimulus
• Threshold is confirmed by checking if the level increases +5 dB.
• Reflex is labeled by the ear of sound presentation, not the measurement ear.
• “Probe” is the reflex measurement device (i.e., the tympanometry probe)
• The stimulus can be presented by the probe or by a plain earphone on the other side of the head from the probe
• Reflex terminology: - Within norms= between the 10th and 90th percentile on the Gelfand norm - Elevated indicates a neural problems which is higher than you would expect based on hearing loss. - Absent is not there at highest levels tested, indicates neural problem - Abnormal involvise absent and/or abnormal findings which indicates a neural problem

Acoustic Reflex Effect - Patterns

• VIIIth nerve disorder: abnormal reflex on stimulus side.

  • Could have elevated or absent right reflexes (ipsi and contra) for a right-side tumour

• VIIth nerve disorder will have abnormal reflex on probe (measurement)side

• Brainstem can eliminate or make abnormal both contralateral reflexes but preserve both ipsilateral or both contras and one ipsi (depending on location).

• Cochlear loss affects reflex when sound is presented to that ear but should be within norms; overcome the loss, the elevated level should be account for when you look at gelfland norms

• Conductive loss can eliminate reflex on the probe (measurement) side because you generally can't measure the reflexion in the ear with conductive loss. Furthermore, in elevation of stimulation to stimulate the reflect but with the additional conductive loss it has gone over the 90th percentile

Acoustic Reflex: Decay

• Test: Present a tone 10 dB above reflex threshold at 500 and 1000 Hz (if safe).
• Measure half-life time, the time after stimulus onset when admittance deflection decreased by 50%.
• If that reaches before 10 s, considered to be abnormal and indicates adaptation.

Summary of Acoustic Reflex & Tympanometry

• Acoustic Reflex provides detailed site-of-lesion information; differentiate conductive vs. sensorineural and indicate the side and position of a neural lesion.
• Tympanometry provides detailed information about the middle ear and classifies types (A, B, C, Ad, As).