ECochG in Third Window Disorders and Cochlear Synaptopathy Flashcards
(24 cards)
What are the clinical contributions of ECochG?
Assessment of hearing
Identify Wave I in Neurodiagnostic ABR
Confirmation and diagnosis of ANSD
Diagnosis of Meniere’s Disease (Endolymphatic Hydrops)
Intraoperative monitoring
Identification of 3rd Window Disorders (SCCD)
Cochlear Synaptopathy
Are cochlear microphonics present in all individuals with a reasonable complement of OHCs if constant polarity stimuli are delivered?
Yes
In individuals with ANSD, the CM is usually the ONLY auditory potential present
AC potential closely resembles the waveform of the acoustic stimulus
Related to synchronized variation in resistance of OHCs during stimulation
What are the different types of recording electrodes?
Foil tiptrodes
Tymptrodes (TM-trode)
Transtympanic needle
How do you prep the electrodes?
Foil tiptrode - skin prep and salt-chloride conductive gel
Tymptrode - skin prep, saline rinse/soak
Transtympanic (needle) - usually done in OR or by ENT, can be at the medial or lateral round window niches or on the promontory; no significant difference in SP/AP ratio responses despite the location placement
How does electrode type affect amplitude?
The amplitude of tymptrode measurements can be up to an order of magnitude larger than tiptrode measurements
Transtympanic amplitudes can be far more than an order of magnitude larger than those on the eardrum
What is the montage?
The preferred montage is ear to ear (horizontal recording), i.e., if stimulating the left ear the tiptrode or TM electrode would be placed in the inverting (-) jack in the amplifier and the contralateral ear would be placed in the non-inverting (+) jack
Active - high forehead
Ground - low forehead
This would result in positive deflection of response
Is ECochG useful in diagnosing ANSD?
Yes, and monitoring cochlear function of patients with suspected ANSD
Estimated up to 10% of permanent hearing loss in infants associated w/ ANSD
Combined w/ other electrophysiological tests (ABR), distinct patterns have emerged
The typical pattern results from dyssynchrony of neural firing of the auditory nerve
ABR responses from the brainstem totally dependent upon synchronous discharge of neurons
Is the diagnostic specificity of the ECochG for Menieres poor?
Yes, it is 20-65% in the literature
Variability exists due to episodic nature of the disease, differences in protocols and especially recording electrode locations
What is a third window?
Bony defects of the inner ear that enable abnormal communication with the ME and/or cranial cavity
Normally, the inner ear and middle ear are entrained via the oval and round window membranes
Third window disorders include any abnormal opening; canal dehiscence and/or other fistulae
The introduction of a pressure release point allows for a flow or wave of fluid displacement from the vestibule through the dehiscence resulting in excitation of the cupula and distention of the exposed membranous canal wall
What are obvious symptoms for third window
Hear eyelids
Voice reverberates in their ear
Footsteps can sound really loud
Singing makes them feel dizzy
What are auditory manifestations for third window disorders?
Increased sensitivity of bone conducted sound
Decreased sensitivity of air conducted sound
A-B gaps on audio with normal reflexes
Autophony, pulsatile tinnitus
What are the vestibular manifestations of third window disorders?
Sound (Tullio) or pressure induced (Hennenbert) vestibular symptoms (blow nose or valsalva)
Oscillopsia
Chronic imbalance / Disequilibrium
Abnormal cVEMP/ oVEMP
What is the diagnostic criteria for third window disorders?
Bony dehiscence on multi-planar CT exam (Stenver’s, Poeschl’s views) - absence of bone on at least 2 Stenver projections
At least one physiologic sign:
Eye movements that align within the plane of SCC with loud sound or pressure changes
Conductive HL of at least 10dB in range of 250-4000 Hz
Abnormally low VEMP thresholds and elevated amplitudes relative to the normal ear
Elevated SP/AP ratios on ECochG
Is the protocol for ECochG the same if you’re testing someone for menieres or third window?
Yes
What are the proposed mechanisms for the changes in ECochG for third window?
The explanation for elevated (SP) in ECOG with hydrops is that there is a nonlinear response in Reissner’s membrane caused by elevated endolymphatic pressure and distension
Low pressure in the perilymph could also cause a bowed Reissner’s membrane, and this is the reason that ECOG may also be positive in perilymphatic fistula and superior canal dehiscence
*Electrical or pressure problem
What is cochlear synaptopathy?
The loss of nerve connections between the sensory cells and the brain, which occurs in noise-damaged and aging ears
Often referred to as “Hidden Hearing Loss” and may represent underlying etiology of speech in noise difficulties
The main cause for hearing loss has traditionally been thought to be due to damage to the hair cells in the cochlea, particularly OHCs which amplify motion of basilar membrane and enhance frequency tuning
Research in mice and other rodents however, have shown that moderate noise exposure and aging can cause a dramatic loss of synapses between inner hair cells and auditory nerve fibers without causing permanent hair cell damage, and without affecting threshold
We refer to cochlear synaptopathy as hidden hearing loss because it may occur without affecting audiometric sensitivity and thus the damage is hidden from us by traditional methods
You can get normal hearing test even with 70% or more of auditory nerve damage
Does hidden hearing loss occur more in noise exposed ears than aging ears?
Yes
Can electrophysiological measures provide a better means of assessment of hidden hearing loss and explain why some people with normal audiometric hearing have difficulty hearing speech in background noise?
Yes, maybe
Suprathreshold signal in noise ECochG is one of the proposed methods which may help uncover underlying neural pathology and provide answers / guide treatment
What is a different way to assess hidden hearing loss?
Compound action potentials
Measured as AP of the ECochG response or Wave I of ABR
Represents firing of peripheral portion of auditory nerve
SiNAP Protocol
What was found in ECochGs and ABRs in those with hidden hearing loss?
Smaller ABR wave I amplitudes
Found in normal-hearing human ears with a greater amount of voluntary noise exposure
How do you translate the SiNAPs technique for clinical use?
Adults (18-30) with normal audiometric thresholds
QuickSIN test, pre/post DPOAEs
Chirp evokes ECochG recording with TM wick electrodes and probe mic stimulus
SiNAPs procedure - amplitude growth patterns for broadband and narrowband (1 and 4 kHz) chirps at 100 dB pSPL; simultaneous masking noise (74-78 dB SPL RMS) that was high-passed in half octave intervals between 722 and 8944 Hz
What is the signal in noise AP (SiNAP) technique?
Obtain location specific estimate of neural survival using a high pass noise masking paradigm
Systematically limits the region of auditory neurons contributing to high-intensity compound action potentials (CAPs) and assesses growth of amplitude instead of raw amplitude alone
What was found using SiNAP and gentamicin on mice?
The CM amplitude drops within minutes post-gentamicin
The CAP threshold shifts post-gentamicin
What were the results of using the SiNAP protocol?
Mean SiNAP SP/AP ratios tended to be nearly twice as high in high risk (HHL) group than those in low risk (control) group
Changes in SP amplitude, SP/AP amplitude ratios and SP/AP area ratios would imply insult to IHCs since there was no significant change in AP latency or amplitude following short term noise exposure
Since DPOAEs returned to normal post-noise exposure the presumed effect then is that OHC function returned and that cochlear synaptopathy persists
Research is ongoing
