Acoustic Reflex Testing

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Acoustic Reflex Testing
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But first, some revision!
Type A - Normal
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Type C Eustachian tube dysfunction
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Type B Middle ear pathology eg effusion,
cholesteatoma)
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Type As Normal can also be seen in Otosclerosis
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Type Ad Hypermobility
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The two middle ear muscles
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1 Tensor muscle 2 Stapedius Muscle
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The muscles are believed to stabilise the
mechanical system and protect the cochlea from
excessive low frequency vibration
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The Acoustic Reflex
Loud sounds make the stapedius muscle
contract This makes the middle ear system
stiffer...
and less low frequency sound can get through the
middle ear
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Acoustic Reflex Thresholds
Loud stimuli are presented, whilst the admittance
is measured
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Deflection criterion
- It is useful to study the growth with intensity
to confirm a reflex threshold
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Acoustic Reflex Decay
Threshold10 dB, 10 seconds stimulus time
0
5
10
seconds
Admittance change
No reflex decay present
Admittance change
Half-Life Time (HLT), the time after stimulus
onset when the admittance deflection has
decreased by 50. A half-life time value lt5
seconds is indicative of tumour
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Facial Nerve R
Facial Nerve L
Auditory Nerve R
Auditory Nerve L
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• Diagram of the acoustic reflex arc (after Borg
  1973)
• The pathway moves through the auditory nerve
  (CNVIII) to the ventral cochlear nucleus (VCN)
  and activates a binaural neural response through
  both superior olivary complexes (SOC) in the
  brainstem to the motor nuclei of the facial nerve
  (MNVII CNVII) and the facial nerve that
  innervates the stapedius muscle.

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Facial Nerve innervation of the middle ear

• As mentioned previously a twig of the facial
  nerve crosses the middle ear cavity. This
  innervates the acoustic reflex muscles so the
  absence of the acoustic reflex therefore may
  indicate pathology (e.g. retrocochlear, facial
  nerve as in Bells palsy).

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• The acoustic reflexes depends upon the adequate
  function of the whole reflex arc hearing in the
  stimulus ear, neurological pathways of the reflex
  arc and the motor unit innervating the stapedius
  muscle.

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Ipsilateral and Contralateral

• Ipsilateral
• Probe and stimulus in same ear
• Contralateral
• Probe in test ear stimulus in non test ear

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• Ipsi Right
• Probe and stimulus in right ear
• Contra Right
• Probe in right ear
• Stimulus in left ear

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Normal hearing reflex and pathways normal
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Right stimulus ear (cochlea) has a problem
Stimulus
X
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Left Probe ear (middle ear) has a problem
(Tympanometry matters!)
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Left Probe ear (VII facial) has a problem
(Tympanometry matters!)
probe
X
Stimulus
Stimulus
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Left stimulus ear (facial branch of VII) has a
problem
probe
X
Stimulus
Stimulus
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No sure where the problem is may be stimulus ears
or both probe ears or pathway. More information
needed
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?
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Problem may lie in the crossed reflex arc in
the brainstem (VIII)
X
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Brainstem Lesion Right
X
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• SUMMARY
• The reflexes patterns add to the tympanogram,
  audiogram and speech discrimination results and
  result in a battery of tests that define the
  status of the hearing system at the time of
  testing. Reducing the tests in the battery
  weakens the assessment.

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Sound Pathway
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The function of the middle ear
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Middle ear anatomy
Air pressure must be near equal on both sides of
the ear drum in order to optimise mobility of the
whole system
Inner ear fluids
The Eustachian tube (ET) ventilates the middle
ear and allows pressure equalisation The ET opens
when swallowing or yawning
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Testing the middle ear?
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Tympanometry

• Tympanometry is used to establish middle ear
  function
• It measures
• Volume of ear canal
• Tympanic membrane compliance
• Middle ear pressure

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The probe

• The probe has three parts
• A sound source (226Hz)
• A pressure pump
• A microphone

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Admittance - letting the sound wave in
Part of the probe tone is reflected, and some is
admitted into the middle ear The admittance is
continuously measured by a microphone In the
probe tip
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Imittance monitoring has two different diagnostic
components
Tympanometry pressure sweep according to the
normative data applied (sweep rate and
direction) Acoustic reflexes sound stimuli
according to the normative data applied
(frequency content)
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How it works

• The probe is introduced to the ear canal and a
  seal established
• The client will hear a sound of around 85dBSPL
• Pressure is then increased to 200 mmH2O and then
  decreased to 200mmH2O
• In a normally functioning ear, the eardrum will
  be most compliant at 0mmH2O pressure. As the
  pressure increases and decreases, the eardrum
  will be more stiff

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3 elements define the response to sound

• Spring (S)
• tympanic and round window membranes
• ligaments
• middle ear muscles
• air in the ear canal and middle ear
• Referred to as Compliance

F
Inner ear fluids

• Mass (M)
• ossicles
• air in the middle ear mastoid air cells

• Friction (F)
• energy loss through dissipation into heat,
  molecules in motion collide and rub against each
  other
• Referred to as Conductance

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Tympanometry
Admittance
-200 daPa
200 daPa
0
Ear canal pressure
1 3 Pressure stiffens the tympanic membrane so
the probe tone bounces back, and the sound level
in the middle ear decreases 2 When pressure is
equal on both sides, the sound level in the
middle ear is at maximum
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226 Hz tympanometric features
MEP pressure where the curve peaks (daPa) Vol
equivalent volume of the ear canal (cm3, cc or
ml) SC Static compliance of TM TW sharpness of
the tympanogram (daPa) Type curve shape
originally used in early uncalibrated equipment
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Type B

• Other cases where a type B may occur are when
• Ear canal volume is very low wax or debris
  occlusion. The ear canal needs to be cleared
  before accurate testing can be performed
• Ear canal volume is very high perforation in the
  ear drum, ie middle ear volume is measured as
  well as canal
• Vol is high patent grommet

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Impacted Wax
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Perforation or Grommet
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Different configurations of tympanometric results
_at_ 226 Hz
Tympanometry is an invaluable diagnostic tool
when combined with other audiological tests
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Acoustic Reflex features

• The reflex should be activated in both ears even
  if stimulated in only one ear
• The upper normal intensity limit for reflex
  thresholds in adults is 95 - 100 dB HL
• Reflex thresholds cannot be determined if there
  is a problem with the middle ear
• Broad Band Noise provokes a reflex at about 10 -
  15 dB lower than pure tone stimuli
• Determines type of hearing loss when compared
  with the audiogram
• Decays with neural fatigue (typically 8th nerve
  tumour)

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Sensation Levels

• Start at 85dB and work up in 5 dB steps until ART
  is established
• 80-105dB ART for 0-50dB HTL
• SL 55-85dB
• Reduced SL sensori-neural loss
• Need certain level of stimulus to activate
  threshold
• lt15dB SL for pure tones is very unlikely

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Protocol

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Different configurations of Acoustic Reflex
results
Using this kind of table makes reflex
interpretation easier
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Tympanometry in infants younger than 4 - 7 months
226 Hz probe tone
226 Hz probe tone Normal tympanogram in
abnormal ear!
The infant outer and middle ears are still
developing, and do not vibrate with sound the
same way adult ears do.
Use 1000 Hz probe tone in infants
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What is different in the infant ear?

• Developmental factors over the first 6 - 7
  months
• Size increase of external ear
• Size increase of middle ear cavity and mastoid
• Change in tympanic membrane orientation
• Fusion of the tympanic ring
• Decrease in overall mass (bone density, loss of
  mesenchyme)
• Tightening of ossicular joints
• Closer coupling of the stapes to the annular
  ligament
• Formation of the bony ear canal wall

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Middle ear assessment in infants, recommendations

• Low frequency probe tone tympanometry is
  unreliable in infants and should not be used
• 1000 Hz probe tone is preferable to 678 Hz
• A low peak, below 1000 Hz Susceptance or
  Admittance norm data range, or no peak at all is
  likely due to effusion
• Reflex testing in infants
• 1000 Hz probe tone
• ipsilateral stimulation
• broadband noise stimulus
• Combine 1000 Hz tympanometry and reflex testing

References to the above are available in the
literature study by Purdy Williams (2000),
reprinted by GN Otometrics with the kind
permission of the authors.
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