11 - AR Pathology Flashcards
What is the acoustic reflex activated by?
- loud sound
- tactile stimulation
What is the AR?
Acoustic Reflex
-a bilateral reflexive contraction of the ME muscles resulting from sound stimulation of one ear
Which muscle primarily controls the AR in humans?
The stapedius muscle
What does the reflexive contraction of the ME muscles do?
Stiffens the ME conductive mechanism
Which nerve innervates the stapedius muscle?
CN VII (Facial)
The stapedius muscle contraction exerts force on the stapes _____ to normal direction of movement, thereby stiffening the conductive mechanism
Perpendicular
The Tensor Tympani muscle may contribute to the acoustic reflex under certain situations. Name 2 things that can activate it.
- acoustic startle response at highest intensities
- vocalizations
- facial movements (chewing, yawning)
- tactile stimulation (ear, face, eye)
What nerve is the Tensor Tympani Muscle innervated by?
CN V (Trigeminal)
Where does the Tensor Tympani tendon insert and how does it pull?
Inserts at top of manubrium of malleus
-pulls malleus anteriorly medially
What are the 4 main theories as to why we have an acoustic reflex?
Interference Theory - attenuates low frequencies to reduce the “upper spread of masking” effects and improve perception of higher frequency components
Desensitization Theory - desensitization to internal body sounds during eating and vocalizations
Injury Prevention Theory - attenuates intense sounds, though this is unlikely due to delay in onset and decay with continuous sound
Multifunctional Theory - combination of all theories
The AR is not useful for “lesions” above where?
The lower (auditory) brainstem level
During AR, the Test Ear is the ear with the ______ (tone/probe)
Tone
During AR, “ipsi” refers to the the test (stimulus) ear being the ear with the ____ (earphone/probe)
Probe
What are the 4 possible AR outcomes (ie. presence and level of AR).
Present and normal threshold
Present and elevated threshold
Present and reduced threshold
Absent
Name the 2 abnormal AR threshold results possible
Absent AR
Elevated Threshold
Describe a normal AR threshold result
- threshold is usually ~85 dB HL
- upper limits for normal hearing individual are:
- 500-2 kHz = 95 dB HL
- 4 kHz = 105 dB HL
What 2 things can we infer if the AR is present?
- normal ME function
- normal - mild SNHL range
Name 2 of the 5 reasons an AR might be absent or elevated
- ME disorder
- SNHL
- VIIIth nerve pathology
- VIIth nerve pathology
CNS dysfunction (brainstem)
What does a normal AR threshold indicate?
- reflex at a normal sensation level
- normal ME and brainstem function
- abnormalities above brainstem may be present
What AR threshold results would you expect to see for a Normal Right Ear and a Left Ear with a Conductive HL?
Right Contra - absent
Right Ipsi - present
Left Contra - elevated/absent
Left Ipsi - absent
LE Conductive HL attenuates LE stimulus
LE ME pathology reduces LE ME Ya and limits AR
What problem would you expect if all four (RE ipsi and contra, LE ipsi and contra) reflexes were absent?
Conductive HL bilaterally
- bilateral conductive HL attenuates stimulus in both ears
- bilateral ME pathology reduces ME Ya and limits AR bilaterally
For Otosclerosis, what would we expect for:
- tympanogram shape
- physical change of ear bones
- baseline admittance
- AR threshold
- Tymp: As (shallow peak; usually associated with otosclerosis, but also otitis media)
- Fixation of stapes footplate
- Increased ME stiffness and reduced baseline admittance
- Absent AR (stapes muscle contraction ineffective due to footplate fixation)
What would the following results indicate for the right and left ear?
- Right Ipsi: present
- Right Contra: present
- Left Ipsi: absent or elevated
- Left Contra: absent or elevated
Right: Normal hearing
Left: Cochlear HL ( > mild)
-reduced LE cochlear activity
What kind of AR threshold results would you expect for:
- Right: normal hearing
- Left: cochlear HL (mild)
-Right Ipsi: present
-Right Contra: present
-Left Ipsi: present
-Left Contra: present
(sufficient LE cochlear activation and normal ME)
True or False: mild cochlear HL bilaterally can still result in present AR thresholds
True; there can be sufficient cochlear activation and normal ME function bilaterally
If there is >mild cochlear HL bilaterally, what might our AR thresholds show?
Right Ipsi: absent or elevated
Right Contra: absent or elevated
Left Ipsi: absent or elevated
Right Contra: absent or elevated
- Moderate and Severe SNHL bilaterally with sufficient cochlear activation and normal ME = elevated
- Severe and Profound SNHL bilaterally with insufficient cochlear activation and normal ME = absent
CN VIII pathologies: A cerebellopontine angle tumour (acoustic tumour or neuroma) grows the occupy the space between the cerebellum and pons. Would you expect this to cause abnormal results on one or both sides?
Abnormal (elevated or absent) results when the ear on the side with the pathology is stimulated
- elevated = affected side has normal or SNHL, reduced neural input, and normal ME. Unaffected side all normal
- absent = affected side has normal or SNHL, blocked neural input, and normal ME. Unaffected side all normal
Describe how CN VII pathologies affect AR threshold results and why
- abnormal AR outcomes for all probe measures on same side as lesion
- normal ME function
- lesion is on the facial nerve in the descending pathway of the AR arc
If the Left Ear had normal hearing and the Right Ear had normal hearing, but a CN VII pathology, what AR threshold results would you expect?
Right Contra: present
Right Ipsi: absent
Left Contra: absent
Left Ipsi: present
Right ear normal, except for abnormal efferent pathway
Brainstem (intra-axial) Pathologies:
If there is a lesion within the brainstem proper, what might we expect to see regarding the crossed and uncrossed pathways?
Lesions within the brainstem proper involve the CNS regions of the AR pathway, but can vary in size and location
- there may be damage to one or both crossed pathways
- uncrossed pathways may be spared
Brainstem (extra-axial) Pathologies:
Where are extra-axial pathologies found and which pathways might they affect?
- lesions are found external to the brainstem proper and can vary in size and location
- involve crossed and/or uncrossed pathways
- cortical pathologies do not influence the AR pathways
True or False: In a normal auditory system, the stapedius muscle remains contracted with constant stimulation
False: stapedius muscle gradually relaxes
- amount and rate of this reflex adaptation is directly related to the frequency of the activator signal ( >1000 Hz AR decay is considered normal)
- inversely related to the level (less AR decay as test tone intensity increases)
What is abnormally rapid reflex decay associated with?
Retrocochlear disorders
To measure AR decay, we present a stimulus at a level ____ dB above the acoustic reflex threshold for 10 seconds
10 dB above the AR threshold
How is abnormal AR decay determined?
More than 50% decay in 10 seconds
OR
50% decay in less than 5 seconds
(always retest for reliability)
At which frequencies do we test AR decay?
500 Hz and 1000 Hz
How do we record decay?
As the time taken to reach 50% decay
Why do patients with CN VIII lesions exhibit high rates of AR adaptation?
CN VIII cannot maintain neural firing rate due to pathology
Which pathology is more likely to show reflex decay: a cochlear pathology (e.g. Meniere’s) or a retrocochlear pathology (CN VIII)?
Retrocochlear lesion
