Lecture 6 Flashcards
in ____, the ABR was described as a neurodiagnostic tool
1975
What are the two areas we look at with neurodiagnostics?
Demyelination and neuropathy
An ABR is a test of ____ timing
neural
Explain demyelination and myelination
- Na+ channels at nodes of Ranvier – K+ channels between… current jumps from node to node (saltatory conduction)
- If a demyelinated portion is encountered, conduction velocity is slowed
- Variable lengths of demyelinated portions will result in a loss of synchrony
What gives you very rapid conduction?
Saltatory conduction
What can MS affect?
Can affect the auditory pathways (not always the case)
- Axonal demyelination in CNS (motor, sensory, or both)
- ABR more likely to be recordable than peripheral neuropathy
What did Jean-Martin Charcot first describe?
- MS
- Charcot-Marie-Tooth (HSMN – hereditary sensory motor neuropathy), named Parkinson’s disease
What does demyelination result in?
- Loss of myelin sheath can result in halted transmission (low membrane resistance results in little spread of current—cannot reach ion channels)
- When you get a demyelinated portion of a nerve, you get a conduction blocl
- Ion channels may redistribute over time (which causes slow conduction)
- Loss of hearing and then recovery (loss of conduction followed by restored, but slow conduction)
What are some progressive changes in ABR with MS?
Loss of wave V or very late wave V
What is often seen with auditory neuropathy?
- Reverse sloping loss
- Decreased speech perception
- Absent ABR
- Normal OSEs
Auditory neuropathy affects transmission in the ____
Auditory system
What is the main symptom individuals with auditory neuropathy have?
Poor speech perception
What is another name for auditory neuropathy?
Auditory neuropathy is sometimes called auditory dys-synchrony (we don’t necessarily know that it is neural, we just know they are not getting a synchronized response).
What is the operational definition of auditory neuropathy
- OAEs / CMs without ABRs, ARs, efferent (absent ABR, ARs, and efferent responses)
What is the type of functional problem with auditory neuropathy?
Nerve problem
- Dys-synchrony (i.e. temporal deficit)
- Speech understanding poorer than thresholds
What is the type of physiological problem with auditory neuropathy?
Physiological problem
- A peripheral retrocochlear pathology
- IHCs may be involved
What happens to the auditory nerve with auditory neuropathy?
- Fewer axons (and loss of myelin sheath) compared to age-matched control
- This is showing that it is an issue with a nerve
What is febrile hearing loss?
- A neural pathology
- This child had a fever and their hearing dropped
- Hearing loss with slight increase in temperature (febrile hearing loss is associated with neural neuropathy)
- Fever is causing a temporary conduction block
- This is a clear pathology of the nerve (only happens if there is demyelination of the nerve)
What is selective IHC loss?
- A cause for AN
- You can get selective IHC loss without OHC loss (OHC give OAEs so they will still show up)
- These will show up as having neuropathy (even though it may not be)
Do OHCs or IHCs give OAEs?
OHCs
What is the mechanism of auditory neuropathy?
- Loss of myelination (variable) and/or
- Loss of fibres, IHCs and/or synapses
- All will result in poorer coding of information
What can be caused by noise exposure and aging?
Cochlear neuropathy (hidden hearing loss)
What is hidden hearing loss?
- Cochlear neuropathy
- Damage to the auditory pathway which may not always show up in thresholds (thresholds will shift and show up as normal)
- Mostly happens in mice, but research that it may occur in humans
- Need to lose 80% or more of the hair cells to get a threshold shift
Is cochlear neuropathy a demyelinating condition?
No
If speech is very poor from what their PTA is, this suggests ____
Auditory neuropathy
What 3 things are poor for people with AN?
- Frequency discrimination
- Modulation detection
- Gap detection
Explain frequency discrimination for people with auditory neuropathy
- Frequency discrimination is very poor
- Difference limen is usually very good (ability to distinguish between frequencies)
- Normal = 5 Hz to notice a difference
- AN = 100 Hz to notice a difference
____ was normal in most people with auditory neuropathy
Temporal integration
What is temporal integration?
How you integrate information over a certain amount of time (the shorter the stimulus, the higher the threshold)
____ is abnormal in auditory neuropathy
- Modulation detection (ability to differentiate between something that is steady or fluctuating)
- People with AN are very poor at detecting changes in level (ex, speech)
What happens to speech in those with auditory neuropathy?
The spectrotemporal modulations in speech would be smeared (poor detection of frequency differences and modulations at the speech rate)
What are the main causes of auditory neuropathy?
Genetic
- Hereditary motor sensory neuropathy (HMSN) / Charcot-Marie-Tooth (CMT)
- Friedreich’s ataxia
Causes of AN - Hereditary motor sensory neuropathy (HMSN) / Charcot-Marie-Tooth (CMT)
- Affects roughly 1/2500
- Various inheritance patterns
- Motor component disrupts nerve communication to legs, feet and hands - muscles tend to waste away
- May affect myelin sheath or nerve fiber
Causes of AN - Friedreich’s ataxia
- Affects roughly 1/50000
- Autosomal recessive (need both parents)
- Demyelination – producing widespread sensory and motor problems
about ____ of childhood hearing loss appears to be due to auditory neuropathy (i.e. OAEs or CMs present)
10%
What are the causes of auditory neuropathy in infants?
Perinatal
- Hyperbilirubinemia (jaundice)
- Pematurity
Causes of AN in infants - Hyperbilirubinemia (janudice)
- Very common
- Fetal hemoglobin replaced with adult
- Bilirubin produced in breakdown of red blood cells
- Body must excrete (via liver) or it is toxic
- Hepatic system is immature
- Phototherapy when not resolved
- Associated with AN
Causes of AN in infants - Prematurity
- Infants < 32 weeks have immature lungs
- Anoxia/hypoxia
- In the perinatal period, this can cause damage to PNS and CNS (appears to damage process of myelination)
- Best chance for improvement
What are the silent talkers?
A family that talked to each other with no sound
What is the main way to detect auditory neuropathy?
Alternating polarity and the CM
- Because there are cochlear responses intact, we have to be sure we aren’t picking up cochlear microphonic
- If you present a click in both polarities, the first part will invert, which is the CM
- Some people with AN have prolonged CM (some abnormally large)
CM amplitude in AN
With AN, those under 10 have a very large CM
Explain how alternating polarity works?
- With AN single polarity looks to be good (BUT when you do separate polarities, it is all inverting, which means it is CM)
- With AN, separate polarities show all CM
Explain why it is important to separately average ABR to condensation and rarefaction clicks
- As you lower the level for ABR, the wave is less clear
- With AN single polarity looks to be good (BUT when you do separate polarities, it is all inverting, which means it is CM)
- With AN, separate polarities show all CM
How do you identify CM?
- Cochlear microphonic is not affected by masking
- Masking will eliminate neural response, but not cochlear microphonic
- Cochlear microphonic latency does not change with intensity
- L-I function will clearly separate CM from neural response
- And of course, CM inverts with stimulus polarity
best to record both polarities and compare (split buffers if possible)
What are the 5 characteristics of auditory neuropathy?
- Intact OAEs or CMs (cochlear responses)
- Absent or elevated reflexes (a universal finding, Berlin, 2005, J Am Acad Audiol)
- Speech perception poorer than expected given hearing loss
- Loss may be rising configuration
- Absence of ALL peaks in ABR – possibly a very reduced wave V
- Presence of wave I would suggest tumour
Why is ABR better than MRI at detecting AN?
ABR is better than MRI at detecting AN because ABR is very time specific
Schwann cells form myelin sheath in ____
PNS
Explain 8 facts about vestibular schwannomas
- Incidence 1/100,000 (but in autopsy, may be as high as 1/1000!)
- Unilateral in 95% of cases
- Usually slow growing, benign
- (1-2 mm per year)
- Only 50% grow significantly
- Usually create high frequency loss
- Most common complaint
- VS impinges on auditory portion of nerve
- Usually no large balance problems
- Can grow out of IAM into CPA
- Brainstem involvement
- 1/10th of all intracranial primary tumours
- 90% of all near temporal bone are VS
- ABR first used to detect VS
What is the most common symptom of a VS?
Asymmetrical hearing loss
What are the stages of VS?
Stage I: IAC (about 0-10 mm)
- Unilateral hearing loss and tinnitus
Stage II: beyond IAC (10-20 mm)
Stage III: touching but not compressing brainstem and cerebellum (20-35 mm)
- May compress other nerves (e.g., facial, trigeminal—facial numbness/paralysis)
Stage IV: compression of cerebellum and brainstem (> 35mm)
- Cerebellar problems (e.g., gait, writing), danger – CSF blockage and pressure may cause hydrocephalus, coma, death
What other tumour can schwann cells cause?
Neurofibramatosis (NF1 and NF2) and meningiomas
What is NF1?
- NF1 (peripheral): prevalence 60/100,000 (i.e., 6000 / 10,000,000)
- onset 1st decade
- 5% have 8th nerve tumours
- skeletal abnormalities
- café at lait spots
- skin tumours
What is NF2?
- NF2(central): prevalence 1/10,000,000
- onset 2nd or 3rd decade
- 95% have 8th nerve tumours, usually bilateral
- 90% have hearing loss
What are meningiomas?
- most common type of primary tumour (1/3rd), usually benign
- develop in the meningothelial arachnoid cells
- often develop in the CPA (cerebellar pontine angle)
- 99% are single
- tend to involve the brainstem
- may have subtle effects
How do you detect retrocochlear tumours?
- Early radiologic techniques were poor – so only larger tumours were detected
- The ABR was found to have a hit rate of 96-100%
- When MRI became the new gold-standard, it was possible to detect smaller tumours
- gadolinium contrast (accumulates in cells of abnormal tissue), tumour appears white)
- the hit rate of the ABR poorer for small tumours
- 100% for large tumours (> 2 cm)
- 98% for tumours 1.1-2 cm
- 66-89% for smaller tumours (depending on study); Less than a cm
Why would you do an MRI or ABR?
Referral is generally on the basis of asymmetry
- other factors may influence decision to refer
- tinnitus, vestibular problems
- absent or elevated reflexes
- poor speech performance
- Rollover (suggests something neural)
- Very likely to occur with a tumour
How much asymmetry is enough to warrant referral?
Usually >15-20 dB
What two protocols had more than 90% sensitivity?
- AMCLASS A Urben (10 dB or more asymmetry at any two neighbouring frequencies)
- Mangham (10 dB or more in the average from 1 – 8 kHz)
What was the specificity of AMCLASS A Urben and Mangham?
These had specificity well below 50% (a lot of false positives; a lot of MRIs being done that don’t have to be)
Explain doing MRI for identifying VS
MRI best but unless MRI is performed on everyone(!) we need to rely on symptoms such as asymmetry
- Hit rate of asymmetry is not 100%
- If greater specificity is required, hit rate will decrease
only 85% sensitivity if specificity must be greater than 50%
- the ABR can aid in decision making in unclear cases
Explain the koors et al study
- 43 studies which met the inclusion criteria for the analysis (involving 3,314 patients)
- pooled sensitivity of the ABR was 95.6% for tumours greater than 1 cm
- 85.8% for tumours less than 1 cm
- average pooled sensitivity of 93.4%
- specificity of 82%!
Prognosis for hearing preservation is better when ABR is less ____
abnormal (ABR is far better than asymmetry)
Overall, what is the decision making process for VS?
- If criteria for MRI referral is met (i.e., asymmetry), the MRI must be done. The ABR may also be considered as it provides additional information about timing.
- In any case with suspicion that does not meet criteria for MRI referral, the ABR should be considered.
If they have enough symmetry, refer for ____
MRI
But if they don’t have enough symmetry but you suspect tumour, refer for ____
ABR
Explain the utility of the ABR
- Inexpensive technique for screening/assessment when in doubt but criteria for MRI referral not met
- hit rate poor for small tumours
- for criterion asymmetry, MRI referral is a must even if ABR is normal
- Better than MRI for detection of demyelinating dysfunction
- It provides invaluable information and should be part of the audiologists diagnostic toolkit
Consider impact of tumour pressure on nerve
- Damage may impact myelin sheath and impede conduction velocity
- Damaged fibres may delay transmission at subsequent synapse
How to use the ABR?
- Timing
- Amplitude
- Morphology
What are specific ways to use the ABR?
- interpeak latencies
- interaural wave V latency differences
- absolute V latency
- V/I amplitude ratio
- morphology considerations (esp. interaural)
What are the normal latencies?
What are the normal interpeak latencies?
What ABR norms are we most concerned with clinically?
- Upper limits of normal range
- Upper limit in Stockard and Chiappa is 2.5 Standard Deviations
I-III Interval
- Normal: 2 ms (.2 ms SD)
- Abnormal for VIIIth nerve tumours (about 85-100% of the time)
- Musiek (1986): abnormal > 2.4 ms
- Lightfoot (1992): abnormal > 2.52 ms
- Previous page values 2.48 to 2.63
- Conservative estimate would be 2.4
III-V Interval
- Normal: 1.9 ms (.2 ms SD)
- Sometimes abnormal for VIIIth nerve tumours, but likely only when tumour in CPA
Musiek:- abnormal > 2.3 ms
- consistent with studies reported in Picton
- Reflect problem in or above CN
- This is less likely to be abnormal
I-V Interval
- Normal: 3.9 – 4.0 ms (.2 ms SD)
- May be easier to detect than I-III
- abnormal > 4.4 or 4.5 ms
- see studies reported in Picton
- Does not differentiate AN from brainstem tumours (typically AN)
Absolute Wave V
- Abnormal if greater than 6.3 ms (to a click)
- Affected by hearing loss, so may be difficult to interpret in isolation
- Sometimes the only possible measure
Interaural Wave V Difference (ILD)
- Abnormal if greater than .3 ms.
- Only wave V is required
- Asymmetric hearing loss may confound
adjust presentation level for each ear as needed
Present at a higher level if there is a HL
V/I Amplitude Ratio
- Abnormal if < .75 (i.e. if wave V is less than ¾ of the amplitude of wave I)
- Only interpret if amplitude is stable (varies less than 20% between runs)
- Cannot use if near-field recordings of wave I (or horizontal recordings!)
Morphology: ABR Patterns in VS
Any of these are consistent with VS (all abnormal in different ways):
- 15% show nothing
- 15% just see a very late wave V
- 5% will just show a wave I
- 30% will just show a long wave I-V interval (no other waves)
- 30% have a prolonged I-III interval
- 5% look normal, but aren’t
Consider effects of loose vs. strict criteria
- Loose: is anything is abnormal will make a referral
- Strict: don’t want to over refer (generally we don’t do this because we don’t want to miss things)
Explain why the brainstem is more complicated than the auditory nerve
- a tumour may or may not affect auditory pathways
- hearing loss is less likely
- hit rates are correspondingly poorer
Abnormal III-V suggests ____ (especially if I-II normal)
Brainstem involvement
Neurodiagnostic recording parameters
- level
- electrodes
- filter
- average
- rate
- stimulus
- rejection
- level has little impact on interpeak latency
- increase level to help with wave I (70—
90 dB nHL are typical levels)
- increase level to help with wave I (70—
- electrodes: Cz / forehead to Mastoid (TP7 / TP9); earlobe or canal for better wave I
- filter: 100 Hz 3000 Hz (30 Hz to emphasize wave V or to record simultaneous MLR)
- average: 1000-2000 (2000 most often)
- rate: 10—20 s (not an even-integer multiple of the half-period of line-noise!)—may consider slower rate for wave I
- stimulus: 100 µS click
- rejection +/- 25 uV or less
For neurodiagnostic, you want ____ levels and ____ rates (will show a clear ____)
high, slow (will show a clear wave I and III)
Does an ABR pick up tumours well?
- ABR doesn’t pick up tumours less than a cm very well (this is why MRI is also used for detecting these tumours)
- Small tumours might be missed because they might not be affecting the nerve for the ABR
- ABR is mostly affected by the high frequency fibers – 3-4k (because the cochlea Is so slow)
- A large tumour will push on the whole nerve
- A small tumour may not push on the specific part of the ABR
What is a stacked ABR?
- First do a normal ABR
- Test specific regions (higher to progressively lower)
- Subtracting ABRs from each other to get very place specific ABRs (specific to different frequency regions and spots on the nerve)
The ABRs from more apical regions occur
later, so they don’t ____ well
Sum
What are the results of a stacked ABR?
- Wave V amplitude is then compared to a gender-specific amplitude distribution
- Abnormal if less than a criterion
- Of 54 small tumours missed by conventional ABR, 95% caught with stacked ABR (Don et al, 2005)
- A stacked ABR is almost as good as an MRI
Stacked ABR vs I-V Latency and Interaural V
Stimulus rate and latency
- To see waves I and III we go slow
- But if we go fast, we may pick up de-myelinating conditions
High-Rate ABR
- Neurodiagnostic ABR is generally conducted with ca. 10—20/s stimulus rate
- waves I and III are more difficult to elicit at faster rates
- I-III, III-V, and I-V intervals important for diagnosis
- but many clinicians use a faster rate (e.g. 50-80/s) to stress the system
- maybe a more difficult stimulus will better distinguish normal from abnormal neural processing
Might not see waves ____ with high-rate ABR
I and III
Refractory Period
- Relative vs. absolute refractory period
- neuron is less likely to fire during refractory period
- Highest rate of firing is 1000-1500 Hz
absolute refractory period
Recording at Very Fast Rates
100/s is once every 10 ms
- beyond refractory period
- but faster would cause problem (overlap!)
Maximum Length Sequences (and variants)
- Response is then convolved with same sequence – this teases all of the responses apart
- Responses can be recorded up to 500/s and higher – approaching refractory period
- Can present things really quickly, then disentangle the overlapped ABRs by doing some math
- Can also be called clad (continuous loop averaging decongilution)
MLS in Neurodiagnostics
Current research suggests that this is not better for differentiating normal and abnormal neural processing, but
- Fast rate ABRs are more likely to be abnormal with demyelination
- research is scarce
- rate effects are more pronounced in development (infants)
- ABR with 3—4 ms intervals was better at differentiating VS from normal and hearing loss
- may help identify APD (Jirsa, 2001)
High Rate Stimulation in MS
