Diagnostic Audiology

Diagnostic Audiology 307

Anatomy & Physiology

Divisions of the Ear

Outer/External Ear - Transmission of Acoustic Energy; sound waves from the ambient ether
- Auricle (Pinna)
- External Auditory Meatus
  - Curved, irregularly shaped tube
  - 25-30mm in length
  - 8mm diameter in adults
  - bony portion
    - lateral 1/3
  - cartilaginous portion
    - medial 2/3
    - receives distortion from mandible movment
  - isthmus is the junction between bony and cartilaginous
    - gradation between bone and cartilage
Middle Ear - Transmission from acoustic to mechanical to hydraulic energy; a liquid moving in a confined space under pressure
- Tympanic Membrane
  - 65 mm^2
  - displaced inward 2mm in center
    - cone-shaped
  - 14g mass
  - periphery thickened to form annulus
  - three layers of tissue
    - thin, outer cutaneous layer
    - fibrous middle layer largely for compliance (stiffness)
      - Compliance - the property of a material of undergoing elastic deformation when subjected to an applied force; it is equal to the reciprocal of stiffness.
    - internal serous (mucous) membrane
  - Pars Flaccida
    - located superiorly, has few fibers
  - Pars Tensa
    - Stiff portion
    - vibrates efficiently in response to incoming air pressure variations
  - Impedance Matching
- Ossicles
  - incus, malleus & stapes
  - occupy most of the inner ear space
  - smallest bones in the body
  - deliver sound vibrations to the middle ear
- Middle Ear Cavity
Inner (Internal) ear - Transduction from hydraulic energy to neural impulses that are sent afferently through CN VII (facial)
- Cochlea & Vestibular Structures; semi-circular canals
Eustachian Tube
- 35-38mm in length
- Directed inferiorly, anteriorly and medially
- Tensor Veli Palatini & Levator Palatini muscles dilate
- Normally in closed position
Tympanic Muscles
- encased entirely in the bony canals with only their tendons (a flexible but inelastic cord of strong fibrous collagen tissue attaching a muscle to a bone.)
- contraction increases the stiffness of the ossicular chain
  - reduces the transmission of low frequency sounds (<1-2kHz)
  - elicited by loud sounds (>75 dB above threshold), vocalization, tactile stimulation of the head, or general body movement
- Tensor Tympani
  - draws malleus medially and anteriorly
  - innervated by the trigeminal (CN V) and glossopharyngeal (CN IX) cranial nerves
- Stapedius
  - contraction draws the head of the stapes posteriorly
  - innervated by the facial (CN VII) nerve
Pressure Gain
- Concha, external auditory meatus (canal) and TM are a complex acoustic cavity
- Increase or decrease sound pressure at different frequencies
- broad 15-20 dB peak at 2.5 kHz
Impedance of the Cochlea
- approximately the same as sea water
- means about 1% of the incident wave energy would be transmitted; 99% reflected
- acoustic threshold would become worse by about 30 dB
Middle Ear Transfer Function
- transforms sound pressure variations of the EAM to sound pressure variation in the scala vestibuli
- transfer curve is a bandpass filter in shape and peaks around 1kHz and slowly declines at higher frequencies

Basic Principles of Acoustic Immittance

Acoustic Immittance is a measurement of the amount of sound flowing through a system
Three major elements that offer impedance
- stiffness
- mass
- friction
Impedance of the Ear
- Mass - smaller the mass, smaller the force is needed to keep the same velocity
  - Pars Flaccida
  - Ossicles
  - Perilymph in the cochlea
- Spring - stiffer the spring, the larger the force is required to act upon
  - ligaments
  - tendons
  - tympanic membrane
  - air enclosed in the canal and middle ear behave like spring elements
- Friction
  - viscocity of the perilymph
  - viscocity of the mucous lining of the middle ear
  - narrow passages between middle ear and the mastoid
- Impedance of the ear contains both acoustic and mechanic elements
  - Elastic tympanic membrane - mechanical spring
  - Ossicular Chain - Mechanical mass
  - Frication
- Acoustic Impedance
  - Acoustic Frication
    - collision of air molecules
  - Acoustic Spring
    - outer and middle ear as one where air molecules are compressed like a mechanical spring
  - Acoustic Mass
    - a quantity of air is moved as a unit as in a constriction or narrow tube
- Impedance (Z)
  - defined as the rejection of energy/opposition to the flow of energy
  - a complex ration between pressure and velocity
  - measured in ohms (Ω)
  - Z = P/U
    - P is sound pressure
    - U is volume velocity
    - can be specified in polar notion (rectangular)
    - Z = R + jX
    - Mass relationship to P & U is 90 degree phase lead of force relative to velocity
    - Spring relationship to P & U is 90 degree phase lag of force relative to velocity
    - Friction relationship to P & U is force and velocity are in phase
      - R = Resistance
        R = |Z| cos Φ
        X = |Z| sin Φ
      - X = Reactance (Xc + Xm)
      - j is the imaginary sqrt -1
- Admittance (Y)
  - defined as the case with which acoustic energy passes through a system
  - reciprocal of impedance (1/Z)
  - Y = U/P
    - U is volume velocity
    - P is sound pressure
  - measured in mho
  - Y = G + jB
    - G = conductance
    - B = susceptance - ease of energy flow through the system
    - Y = G + j(Bc + Bm)
      - Bc - positive susceptance of spring element
      - Bm - negative susceptance of mass element
      - G = |Y| cos Φ
      - B = |Y| sin Φ
  - Total Admittance of the Ear
    - 1/Za = 1/Z1 + 1/Z2
    - Ya = Y1 + Y2
      - Za = total impedance
      - Z1 = impedance of the outer ear
- Middle Ear Admittance
  - mass of three middle ear ossicles
  - stiffness of the ossicular ligaments and muscles
  - stiffness of tympanic membrane and round window membrane
  - stiffness of the air contained in the tympanum
  - mass and frication that result from the air movement within the tympanum
  - total immitance offered by the cochlea at the oval window
- Middle Ear Impedance or Admittance
  - the contribution of the ear canal is determined by taking acoustic immittance measures with substantial air pressure in the ear canal
  - if the ear canal pressure is changed appreciably from ambient, the acoustic impedance at the probe tip increases
  - It effectively stiffens the tympanic membrane and middle ear transmission system
  - the acoustic immittance at the probe tip represents that portion of the total acoustic immittance contributed by the volume of the air within the ear canal alone.
  - measure independent of ear canal size and the distance between the tip and TM
- Two assumptions
  - The outer and middle ears are two parallel systems
  - middle ear impedance can be driven to be infinitely high, when high sound pressure is applied
    - impedance of the outer ear alone can be estimated
    - middle ear impedance can be estimated by subtracting outer ear impedance from the total impedance

Tympanometry

Tympanometry is an objective measurement
- sensitive in detecting middle ear disorders
- used as a cross check to pure tone audiometry
Normal ears operate most efficiently at atmospheric pressure
Clinical interest is to measure ME function at greater and lesser pressures compared to ambient pressure for diagnostic purposes
A graphic representation of middle ear function at various pressure shows admittance versus ear canal air pressure
- least admittance to maximum admittance to least admittance
Pressure change: tympanic membrane and ossicular chain stiffen
Most efficient point is that EAM pressure = middle ear pressure
The peak height
- is the static admittance
- it is the amount of acoustic energy that flows into the middle ear
- it is the most common tympanometric measurement
Procedure
- instruct the patient that they will feel a slight amount of pressure and hear a tone for a short amount of time, remain still, do not talk, swallow, yawn, sneeze or chew
- perform otoscopy to:
  - ensure external canal is clear from debris and otorrhea (ear discharge)
  - determine the status of the TM
  - determine the tip size
- select tip size and fit onto probe
- place probe into the external ear canal
  - pull pinna superiorly and posteriorly
  - put tip in canal and twist anteriorly
  - let go of pinna and then the probe
  - ensure that there is a hermetic (air-tight) seal
Probe stimulus
- pure-tone or click
Pressure change
- air pump/manometer
Admittance measurement
- microphone
- ampere-meter
Probe-tip equipment
- probe tone loudspeaker
- monitor microphone
- pressure pump and manometer
- ipsilateral reflex loudspeaker
Pressure is varied from ambient pressure to positive pressure to negative pressure
- As the pressure is varied in the sealed ear canal, admittance is plotted as a function of ear canal pressure on a tympanogram
Interpretation
- judging tympanic shape provides an overall impression
  - normal vs abnormal
  - classification of type
- Four basic tympanometric measurements
  - Equivalent ear canal volume (Vea)
    - unit is cc or mL
      - 1 cc or 1 mL of trapped air equals the acoustic admittance of 1 mmho in a hard-walled cavity
    - factors that effect; cerumen, foreign bodies, drainage, TM perforation, collapsed canal
    - generally if TM is visible, cerumen blockages of less than 50% do not affect ECV
      - at extreme pressures, ME is sufficiently stiff to cause a decrease of the admittance close to zero
        extreme positive: +200 daPa
        extreme negative: -400 daPa
    - then, the admittance measured at the probe tip could be attributed only to the air trapped in the ear canal itself
      - assuming the walls are rigid
  - Static-compensated acoustic admittance (Ytm)
  - Tympanometric peak pressure (Tpp)
  - Tympanometric width (TW) or gradient
Tympanograms are normally asymmetric
- negative tail slightly below the positive tail
- volume estimate at negative tail is typically lower than at the positive tail
  - lower conductance at extreme negative pressures than at extreme positive pressures
  - physical volume changes when positive pressure pushes inward on the TM
  - negative pressure pulls outward on the TM
- cases of collapsed canal
  - negative tails sharply descends below positive tail, approaching 0 cc
  - most frequently in infants and newborns
What finding is consistent with a flat tympanogram and a large ECV?
- TM perforation
- Patent PE tube
What finding is consistent with a flat tympanogram and and a low ECV?
- blockage
- cerumen impaction
- probe tip against wall
What finding is consistent with a flat tympanogram and a normal ECV?
- otitis media with effusion (not always consistent)
- perforation
Static Admittance
- equivalent ear canal volume
  - ECV = Ycanal (226Hz)
    - static acoustic compliance of the outer and middle ear combined Ytotal
  - the static compliance of the middle ear then is based on: Ytm = Ytotal - Ycanal
Tympanometric Shape
- Liden-Jerger Classification
  - A - normal height and peak pressure (0.3 - 1.5ml, 0)
  - Ad - peak higher than normal (high volume, 3.0ml) normal peak pressure
  - As - peak shallower than normal, normal peak pressure
  - B - flat, no peak
  - C - normal height, negative pressure peak
  - D - double peaked
Tympanometric Width
- a measurement of how sharp or how rounded the peak is; sharp is normal
- the width of the tympanogram in daPa measured at 50% of its static acousitc admittance values
  - from the positive tail (point of reference), draw a vertical line from the peak
  - at 50% of the vertical line, draw a horizontal line through the tympanometric peak
  - where the
  - the uncompensated static admittance is at the peak
  - the compensated static admittance is the difference between the peak and the tail (positive or negative)
  - the ear canal volume is where the tail meets the y-axis; in cm3

Eustachian Tube Function Tests

Normally the Eustachian Tube is closed

opens during actions of chewing, swallowing and yawning
when it opens, air comes in and equalizes the pressure

Eustachian tube is blocked, there is negative pressure in the middle ear

extreme pressure changes; flying, diving
barotrauma
- TM perf, CHL, fistula of oval window (rare)

Tympanometric Peak Pressure

We know the pressure on the outer side of the eardrum because it is generated and measured using the air pump and manometer connected to the probe tip. In addition, we have learned that the tympanogram peak occurs when the same pressure exists on both sides of the tympanic membrane. Consequently, the ear canal pressure corresponding to the tympanogram peak is also an estimate of the pressure within the middle ear. Increasingly negative tympanometric peak pressures are shown moving to the left of 0 daPa. Even though "tympanometric peak pressure" and "middle ear pressure" are often used interchangeably, we distinguish between the two terms here to point out that they are not always the same, especially when the patient has a flaccid tympanic membrane.

Abnormally negative tympanometric peak pressures are associated with Eustachian tube disorders, which can occur either with or without the presence of middle ear fluid. The amount of negative pressure needed to consider the tympanometric peak pressure abnormally negative is not clearly identifiable in the literature. Lower pressures suggest the possibility of Eustachian tube dysfunction. Unfortunately, there does not seem to be a particular tympanometric peak pressure cutoff value that successfully distinguishes between the presence and absence of middle ear effusion.

One can sometimes follow the course of recovery from a case of otitis media as tympanometric pressures that become progressively less negative over time.

Unlike the situation for negative middle ear pressure, the significance of abnormally high positive peak pressures is not clear. In spite of the extensive literature on tympanometry and middle ear pathology, only a few papers have reported positive pressures in some cases of otitis media. In addition, positive peak pressure has also been associated with non-pathologic cases such as rapid elevator rides, crying, or nose blowing.

The Acoustic Reflex

Presenting a sufficiently intense sound to either ear results in the contraction of the stapedius muscle in both ears and it is called the acoustic or stapedius reflex. This reflexive muscle contraction stiffens the conductive mechanism via the stapedius tendon, and therefore changes the ear's immittance. The acoustic reflex is easily measured because the immittance change is picked up by the probe tip and displayed on the immittance device meter

Acoustic Reflex Arc

Let us follow this pathway assuming that the right ear was stimulated. The afferent (sensory) part of the arc involves the auditory (eighth) nerve from the right ear, which goes to the right (ipsilateral) ventral cochlear nucleus. Neurons then go to the superior olivary complexes on both sides of the brainstem. The right and left superior olivary complexes send signals to the facial (seventh) nerve nuclei on their respective sides. Finally, the efferent (motor) legs of the acoustic reflex arc involve the right and left facial nerves, which direct the stapedius muscles to contract in both ears. Notice that the acoustic reflex involves the stapedius muscles. While the tensor tympani muscles do respond to extremely intense sounds, this is actually part of a startle reaction and the accumulated evidence reveals that the acoustic reflex in humans is a stapedius reflex. Certain kinds of non-acoustic stimulation also elicit contractions of the stapedius muscles or of both middle ear muscles. These reflexes can be used in advanced diagnostic methods.

Acoustic Reflex Tests

The basic acoustic reflex testing procedure involves presenting a sufficiently intense tone or noise to activate the reflex, and observing any resulting immittance change, which is usually seen as a decrease in the ear's admittance. The immittance change caused by the contraction of the stapedius muscle is measured in the ear containing the probe tip, which is called the probe ear. The ear receiving the stimulus used to activate the reflex is called the stimulus ear. either ear can be the stimulus ear because the stimulus can be delivered from the receiver in the probe tip or the earphone on the opposite ear. The ipsilateral or uncrossed acoustic reflex is being measured when the stimulus is presented to the probe ear which is the same ear in which the immittance change is being monitored. In contrast, the contralateral or crossed acoustic reflex is being measured when the probe tip is in one ear and the stimulus goes to the opposite ear.

It is easy to identify whether the right or left ear is being tested for the ipsilateral reflex because the reflex is activated and monitored in the same ear. However, there can be confusion about which ear the stimulus and probe are in opposite ears. In fact, both ears (and the reflex pathway between them) are really being tested with the contralateral reflex. The convention is to identify a contralateral acoustic reflex according to the stimulated ear. Hence, a "right contralateral acoustic reflex" means that the stimulus is in the right ear (with the probe in the left ear) and a "left contralateral acoustic reflex" means that the stimulus is in the left ear (with the probe in the right ear). Another way to avoid confusion is to describe the test results as, for example, "stimulus right" or "probe left." The usual reflex testing order is to do the left contralateral and right ipsilateral reflexes while the probe is in the right ear, and then to reverse the headset and do the right contralateral and left ipsilateral reflex test while the probe is in the left ear.

A variety of acoustic reflex tests are regularly used in clinical assessment. The two basic measurements are discussed here: the acoustic reflex threshold which is the lowest stimulus level that produces a reflex response and acoustic reflex decay which is a measure of how long the response lasts if the stimulus is kept on for a period of time.

Acoustic Reflex Threshold

Acoustic reflex threshold (ART) testing involves finding the lowest level of a stimulus that causes a measurable change in acoustic immittance.The immittance changes attributed to the reflex are associated in time with the stimulus presentations and that the magnitude of the reflex response increases as the stimulus is raised above the ART. Hence, we may also say that the ART is the smallest discernible immittance changes that is associated in time with the presentation of a stimulus, and that responses should also be present (and generally larger) at higher stimulus levels. Clinical ARTs are usually obtained using pure tone stimuli at 500, 1000 and 2000 Hz. Even though some clinicians also use 4000 Hz, it is not recommended because even young, normal hearing persons experience elevated ARTs at this frequency due to rapid adaptation Clinically, pure tone ARTs are obtained by changing the intensity of the stimulus in 5 dB steps while watching for admittance changes caused by the stimuli. These admittance changes are observed by watching for deflections on the admittance device meter, and the ART is considered to be the lowest intensity causing a deflection that can be distinguished from the background activity on the meter.

Normal ARTs with a 226Hz probe tone occur between 85 and 100 dB SPL for pure tones and 20 dB lower when the stimulus is broadband noise. Most clinical measurements involve pure tone ARTs

Acoustic Reflex Decay

In addition to the ART, it is also common practice to test for acoustic reflex decay (or adaptation) which is a measure of whether a reflex contraction is maintained or dies out during continuous stimulation. Reflex decay is tested at both 500 and 1000 Hz. Higher frequencies are not tested because normal people can have rapid reflex decay above 1000 Hz. The test involves presenting a stimulus tone continuously for 10 seconds at a level 10 dB above the reflex threshold. The magnitude of the reflex response will either stay the same or decrease over the course of the 10 second stimulus The central issue is whether the response decays to half of its original magnitude. If the magnitude of the reflex response does not decrease to 50% of its original size during the 10 second test period then the outcome is considered negative. The test is considered positive if the magnitude of the reflex response does decay by 50% or more within this time period.

Conductive Hearing Loss

Conductive hearing losses cause acoustic reflexes to be either "elevated" or "absent." By "elevated" we mean that the ART is higher than normal, that is, it takes more intensity to reach the reflex threshold than would have been needed if there was no conductive loss. An "absent" reflex mean that a reflex response cannot be obtained, even with the most intense stimulus available (which is usually 125 dB HL on most modern immittance devices). The effects of conductive loss can be summarized by two basic rules:

Probe-ear rule - The presence of conductive pathology in the probe ear causes the acoustic reflex to be absent. Here, even though the stapedius muscle itself may actually be contracting, the presence of the pathology prevents us from being able to register any change in acoustic admittance that can be picked up by the probe tip. In fact, it has been found that the chances of having a measurable acoustic reflex fell to 50% when the air-bone gap in the probe ear was only 5dB
Stimulus-ear rule - A conductive loss in the stimulus ear causes the ART to be elevated by the amount of the conductive impairment. This occurs because the amount of the stimulus that actually reaches the cochlea will be reduced by the amount of the air-bone gap. For example, suppose an otherwise normal patient develops otitis media with a 25 dB air-bone gap. The 25 dB air-bone gap causes the signal reaching the cochlea to be 25 dB weaker than the level presented from the earphone. If the ART would normally have been 85 db HL (without the conductive loss) then the stimulus would now have to be raised by 25 dB to 110 dB HL to reach the cochlea at 85 dB HL. Hence, the ART would now be elevated to 110 dB HL. In addition if the air-bone gap is large enough then the ART will be elevated so much that the reflex will be absent. This occurs because the highest available stimulus level cannot overcome the size of the air-bone gap and still deliver a large enough signal to the cochlea. The chances of having an absent acoustic reflex reached 50% when the conductive loss was 27 dB in the stimulus ear. This occurred because the (otherwise normal) average ART of ~85dB HL plus an average air-bone gap of 27 dB is more than 110 dB HL, which was the highest stimulus level available at that time. Modern immittance instruments allow testing up to 125 dB HL so that the 50% point for absent reflexes is not reached until there is a 42 dB air-bone gap in the stimulus ear.

The following acoustic reflex configurations result from the two principles just described: For unilateral conductive losses, contralateral acoustic reflexes tend to be

absent when the probe is in the pathological ear
elevated or absent when the probe is in the normal ear

Contralateral acoustic reflexes tend to be absent in both ears when there is a bilateral conductive impairment. Ipsilateral acoustic reflexes are affected by both principles at the same time; that is, the air-bone gap reduces the effective level of the stimulus that actually reaches the cochlea and the conductive pathology prevents an immittance change from being monitored even if the reflex is activated. Consequently, ipsilateral acoustic reflexes tend to be absent when testing an ear with a conductive disorder, regardless of the condition of the opposite ear.

Sensorineural Hearing Loss

Acoustic reflex threshold depend on hearing sensitivity in a rather peculiar way. In this context "hearing sensitivity" represents a continuum going from normal hearing through various magnitudes of sensorineural hearing loss due to cochlear disorders.

It is well established that patients with retrocochlear pathologies have acoustic reflexes that are elevated, often to the extent that the reflex is absent. However, the decision about when an ART is "elevated" must account for the fact that the ARTs depend on the magnitude of the hearing loss in patients who do not have retrocochlear involvement. The 90th percentiles provide us with upper cutoff values for ARTs that meet this need. In fact, many prior inconsistencies about the diagnostic usefulness of the reflex were resolved by the introduction of 90th percentiles that account for the degree of hearing loss.

In practice, the patient's ARTs are compared with the respective 90th percentiles that apply to his hearing thresholds for the frequencies tested. If an ART falls on or below the relevant 90th percentile, then it is considered to be essentially within the normal and/or cochlear distribution. However, ART's that fall above the applicable 90th percentiles are considered elevated because only a small proportion of normal and/or cochlear impaired ears have ARTs that are so high. If the abnormally elevated or absent reflexes are not attributable to a conductive disorder, then the patient is considered to be at risk for eighth nerve pathology in the ear that receives the stimulus. In contrast, many patients with functional impairments have ARTs that are below the 10th percentiles.

We have seen that acoustic reflex abnormalities such as elevated/absent reflexes and/or positive reflex decay are associated with retrocochlear pathologies in the ear receiving the stimulus tone. Testing both ARTs and reflex decay and considering a positive result on either or both tests as the criterion for suspecting retrocochlear pathology has been referred to as acoustic reflexes combined.