Dr Sankalp Mohan , SR Neurology GMC Kota

1. Dr Sankalp Mohan
Senior Resident
Neurology
GMC, Kota
Basics of RNST,VEP ,BAER

2. Repititive Nerve Stimulation
Test
 Variant of the Nerve conduction Study
 First Described by German Neurologist Jolly in
1895
 Harvey and Masland(1941) reported electrical
decremental muscle response on repetitive motor
nerve stimulation.
 electrical stimulation is delivered to a motor
nerve repeatedly several times per second-
observing the change in the CMAP

3. Definitions
 Quantum.- A quantum is the amount of Ach
packaged in a single vesicle (5000-10000
molecules )
 Miniature EPP –Presynaptic terminals
spontaneously release Ach quantum Causing
MEPP
 End plate potential -EPP is the potential
generated at the postsynaptic membrane
following a nerve action potential and
neuromuscular transmission

4.  Muscle action potential (MAP) – If EPP
exceeds threeshold generated MAP
 CMAP – Sum of MAPs generated by no of fibres
 Safety Factor –Amplitude of EPP above
threshold needed to generate MAP

6. Physiology of RNS
 Ach stores: immediately available (primary)
store and secondary (or mobilization) store
 Primary or immediately available store
1000 quanta- beneath presynaptic nerve terminal
membrane.
 Secondary or mobilization store
10,000 quanta- supplies the primary stores after few
seconds.
 Tertiary or reserve store.
More than 10,000 quanta –in the axon and cell body

7.  - Low Rate RNS – (<5 Hz) – Progressive decline of Ach
Quanta from Primary store
 EPP falls in amplitude but -normal safety factor it remains
above the threshold to Generate muscle action potential with
each stimulation.
.- After first few seconds secondary mobilization store
replaces depleted quanta
-- Rapid RNS
It takes 100 msec for ca2+ to diffuse back out of the
presynaptic terminals
- If RNS is rapid enough so that new ca2+
influx occurs before previously infused ca2+ Causing
increased release–
.

8. S
A- Normal
B- Postsynaptic slow RNs
C- Presynaptic Slow RNS
D. – Presynaptic – Rapid RNS

9. Decremental response:
 The decrement is usually calculated by
comparing the lowest CMAP amplitude or area to
the baseline CMAP.
 (lowest CMAP divided by baseline CMAP).
 With 3 Hz stimulation the lowest CMAP is
usually the 4th or 5th

10. Variables affecting RNS
Age- Newborns CMAP is 30 – 50 % of adults
Temperature – cooling decreases and warming
increases the CMAP decrement. Temp
recommended 26-33 degree.
Muscles tested - Proximal or distal .ADM -7 %
Deltoid- 13 %
Exercise can result in Facilitation/Exhaustion

11. Technique
 Anticholinesterases withheld 24 hrs prior to study
 Recording Electrodes placed in Belly Tendon
Montage
 Immobilization of Electrodes and Limb
 Muscles –1. Deltoid Highest diagnostic yield (78%) ,
Trapezius (65%).
2. ADM Technically easier less diagnostic
3. Facial nerve – Nasalis /Orbicularis Oculi .
CMAP amplitudes are small . Immobilization
difficult .
- Stimulation Frequency for Low Rate RNS – 2to 3 Hz
- Number of Stimulations – Train of 5 to 10 Pulses

12. Exercise testing or Tetanic
Stimulation
 Maximum Voluntary contraction of Involved
muscle for 10 seconds, F/b 3 Hz RNS – Post
exercise Facilitation
 Maximum voluntary exercise for 1 min
Slow RNS at 1,2,3,4 Min – Post Exercise
Exhaustion
Psuedofacilitation – seen in normal individuals
Following exercise .CMAP amplitude increased but
area same . Does not exceed >40%

13. Protocol For Evaluating Disorder
Of NMJ
 Warm the extremity (33 degree centigrade)
 Immobilize the muscle as best as possible
 Perform Routine NCS first to ensure that the nerve is
normal
 Perform RNS at rest. After making sure that the stimulus
is supramaximal, perform at 3 Hz RNS, normally there is a
less than 10% decrement b/w the first and the fourth
response.

14.  If more than 10% decrement occurs and is
consistently reproducible Patient Has patient
perform maximal voluntary exercise.
Immediately repeat 3 Hz RNS post exercise
 If less than 10% decrement or no decrement:
Has patient perform maximal voluntary
exercise for 1 min and perform 3 Hz RNS
immediately and at 1,2,3 and 4 mins.

15.  Perform RNS on one distal and one proximal
muscles especially the weak muscles.
 If no decrement is found with a proximal limb
muscle, a facial muscle can be tested.
 If the compound muscle action potential is low
at baseline, have patient perform 10 sec
exercise, then stimulate the nerve
supramaximally immediately post exercise

16. Rapid RNS
 optimal frequency is 20–50 Hz,for 2–10 seconds
 brief (10-second) period of maximal voluntary
isometric exercise has,the same effect as rapid
RNS
 Depletion of quanta vs calcium accumulation
 Incremental response in LEMS

17.  MG is considered to be a reproducible 10%
decrement in amplitude when comparing the
first stimulus to the forth or fifth, which is
found in at least 1 muscle.
 Abnormality in LEMS is considered to be a
reproducible postexercise increase in
amplitude of at least 100% as compared to
preexercise baseline value.

18. Myasthenia Gravis
 Classic Findings
 1. Normal CMAP
 2. Decremental Response at low rate RNS
 3. Normal or minimal post exercise facilitation
 4.Normal or decremental response at High rate
RNS
 5. Post Exercise or post tetanic Exhaustion

19. LEMS
 Classic Findings
 Low Normal CMAP
 Decremental Response at Low Rate RNS
 Post exercise facilitation
 High Rate RNS 100 % increment in two muscles
,400 % increment in one muscle

20. Utility of RNS
 Most commonly used test, easy.
 RNS is relatively insensitive,10-50% in ocular
myastenia,75% in generalised MG
 RNS is relatively specific(90%)
 SFEMG is Most sensitive.(90% in ocular,95%
in MG)

21. Visual Evoked Potentials

22. Evoked Potential
Electrical potentials that occur in the cortex after
stimulation of a sense organ which can be
recorded by surface electrodes is known as
Evoked Potential.
eg. SEP, BAER and VEP

23. VEP
 VEPs are electrophysiologic responses to
stimulation by either patterned or unpatterned
visual stimuli.
 Stimulation at a relatively low rate (up to 4/s) will
produce “transient” VEPs
 Stimulation at higher rates (10/s or higher) persist
for the duration of the stimulation and are referred
to as “steady-state” VEPs.
 Responses evoked by patterned stimuli are
“pattern” VEPs
 Responses evoked by unpatterned stimuli are
“flash” VEPs

24. Choice of Stimulus
 Patterned visual stimuli elicit responses that have
far less intra- and interindividual variability
 greater sensitivity and accuracy
 Checkerboard pattern reversal is the most widely
 Unpatterned stimuli are generally reserved for
patients who are unable to fixate or to attend to
the stimulus

25. Physiologic basis
 The generator site for VEPs is believed to be the
peristriate and striate occipital cortex

26. Pretest Evaluation
 Test should be explained
 Ability to fixate important throughout
 Avoid Hair Spray or Oil
 Cycloplegics generally should not be used
 Subjects with refractive errors should be tested
with appropriate corrective lenses

27. Electrode Placement
 Standard Disc EEG electrodes usede
 Active/Recording Electrode Placed at Oz in
midline 4cm above Inion
 Reference Electrode FPz 12 cm above Nasion.
 Ground Electrode placed at vertex Cz

29. Pattern Reversal Visual Evoked
Potential Testing
 Negative and positive polarities are designated N
and P, respectively.
 Peak latencies are expressed in milliseconds
 Peaks N75, P100, and N145 are recorded over
the occiput
 Wave Nl00 is recorded from the midfrontal region
 N145 is highly variable and is not used for
standard test interpretation
 Type of pattern.- Checkerboard ,Bar and
sinusoidal grating stimuli

30. Stimulus field types
 pattern that extends equally to both sides of the
fixation point is referred to as a full-field stimulus ‘
 A pattern presented to one side of the fixation
point in one-half – Half field stimulus
 pattern presented to a small sector of the visual
field is designated a partial-field stimulus
 half-field or partial-field stimuli are used, the
fixation point should be displaced to the
nonstimulated visual field by a small amount, to
prevent stimulation of both retinal hemifields

31. Test Protocol for Full-Field
Stimulation
 Full-field PVEP testing is most sensitive in detecting
lesions of the visual system anterior to the optic
chiasm
 should be performed monocularly,
 black-and-white checkerboard pattern,
 at a reversal rate of 4/s or less.
 The subject should be placed no closer than 70 cm to
the stimulus screen.
 Small checks (12—16‟) and small fields (2-4˚)
selectively stimulate central vision. These responses
are particularly sensitive to defocusing and decreased
visual acuity
 Recommended recording time window (ie, the sweep
length) is 250 msec; 50-200 responses are to be
averaged. A minimum of 2 trials should be given,

32. Electrode placement-
 Montages – International federation of Clinical
Neurophysiology (IFCN) recommends 2 channels
minimum
 Channel1 – Oz – Fpz
 Channel 2 – Oz – Linked ear
Four Channel montage
 Channel 1 : Oz –Fpz
 Channel 2- Pz- Fpz
 Channel 3 – L5-Fpz
 Channel 4 –R5 -Fpz

33. Factors Affecting VEP
 The size of the checks
 Pupillary size
 Gender (women have slightly shorter P100 latencies),
 Age: below 1 yr of age P100 may be 160ms, & above
60 yrs. also it get delayed- upto 120
 Sedation and anesthesia abolish the VEP.
 Visual acuity deterioration up to 20/200 does not alter
the response significantly .
 Drugs.

34. Waveforms
(The NPN complex)
 The initial negative peak (N1 or N75)
 ِA large positive peak (P1 or P100)
 Negative peak (N2 or N145)
N75
P100
N145

36. Clinically Significant Abnormality
 changes in latency, amplitude, topography, and
waveform
 P100 latency prolongation is the most reliable
indicator
 Waveform abnormalities are generally subjective
in nature and difficult to quantify
 Amplitude affected by technical Factors wide
individual variation – Hence interoccular
amplitude ratio used
 P100 is 110 milliseconds (ms) in patients younger
than 60 years .

37. Flash Visual Evoked Potential
Testing
 limited to: (1) subjects with severe refractive errors or
opacity of ocular media
 subjects who are too young or too uncooperative
 results should demonstrate reproducible peak positive
responses to flash stimulation
 consist of up to six major peaks in the first 250 ms
after flash
 Unpatterned visual stimuli commonly consist of brief
flashes of light with no discernible pattern or contour
 (LED) board can be viewed from a distance or LED
goggles can be placed directly over the eyes.
Goggles have the advantage of producing a very
large field of stimulation that minimizes the effect of
changes in direction of gaze

38. Clinical Applications of VEP
 VEPs are most useful for testing optic nerve
function and less useful for assessing
postchiasmatic disorders
 Non Specific for etiology
 Partial-field studies may be useful for
retrochiasmatic lesions; however, they are not
performed routinely
 VEP may be abnormal ( low amplitude ) in
Refractive error severe ,Retinal diseases

39.  Optic neuritis-MS – P100 latencies prolonged
with or without amplitude loss
 NMO – unrecordable P100 waveform with
reduced amplitude more likely
 Ischemic optic neuropathy – Attenuation of
amplitude earlier than latency
 Vit B12 deficency – Bilateral asymmetric
prolonged p100 latencies
 Papilledema only – VEP not affected

40. VEP in cortical blindness
 Some reports suggest that VEP may show a
varied result
OR normal VEP
 other reports suggest prognostic importance of
VEP with absent VEP response foretelling poor
prognosis
 INCONSISTENT PATTERN

41. Brainstem Auditory Evoked
Responses

42. BAER
 BAER are recorded from the ear and vertex in
response to brief auditory stimulation
 Actually a misnomer as responses from responses of
the auditory nerve, brainstem, and, perhaps, higher
subcortical structures

43. AEP
 Short latency AEP
 Middle latency AEP
 Long latency AEP
 The short latency AEP include peak of up to 10 msec
and amplitude of about 0.2uv, they are generated in
brainstem.
 The middle latency AEP have several variable peaks
with latency of 10-50 msec and with amplitude of
about 1 uv, they probably reflect early cortical
excitation.
 The long latency AEPs beginning after 50sec and
having peak of 1-10uv, represent later cortical
excitation

44. Auditory pathway

46. Electrode placement
 Typically are placed at the vertex (location Cz of
the International 10–20 System) and at both ear
lobes (Ai and Ac)
 Electrodes at the mastoids (Mi and Mc) may be
substituted, although wave I tends to be smaller
because of muscle noise

47. Montages Cz-Ai
Ac- Ai
May assist in the identification of wave

48. Stimulus
 “broad-band” clicks, the acoustic energy of
which is spread over a wide range of audio
frequencies
 100usec rectangular pulse (single
monophasic square wave)
 Stimulus Polarity -clicks in which the first and
major acoustic wave applies negative pressure in
front of the earspeaker diaphragm are referred to
as rarefaction clicks
 first and most prominent acoustic wave applies a
positive pressure in front of the earspeaker
diaphragm are referred to as condensation clicks

49.  Stimulus Rate –10 -70 times/s . Most common 11-
31 Hz
 Stimulus Intensity- about 70 db
 Click should be delivered monaurally, i.e., to one
ear at a time
 contralateral (nonstimulated) ear be masked by
white noise at 60 dB SPL to eliminate “crossover”
responses, i.e., bone-conducted responses

50. Waveform components
 Wave I - wave I response -in the distal portion of
cranial nerve (CN) VIII
 Prominent initial upgoing peak
 Wave II – Poorly defined in some adults and most
neonates. More prominent in contralateral channel
.Proximal VIII nerve /Cochlear nucleus

51.  Wave III: Prominent peak followed by trough .Arises
from Superior Olivary nucleus
 Wave IV: The ABR wave IV, which often shares the
same peak with wave V, arise from pontine third-
order neurons mostly located in the Lateral
lemniscus, but additional contributions may come
 Wave V: Most prominent peak appearing 5.5 ms
after stimulus. Wave v may fuse with wave IV .
Origin – Inferior colliculus
 Wave VI and VII: Thalamic (medial geniculate body)
and cortical region. Not clinically significant

53. Abnormalities of BAER
 Absence of waveforms
 Abnormal absolute or interpeak latencies
 Amplitude ratio abnormality
 Right to left asymmetry latencyof > 0.5 ms
- Wave II is difficult to distinguish ,Wave IV absence
may not be pathological

54. Abnormalities of Wave I
 Reflect peripheral auditory dysfunction, either
conductive or cochlear, or pathology involving the
most distal portion of the eighth nerve
 Poorly formed or absent wave I but a clear wave
V may reflect high-frequency hearing loss.
 May reflect intracranial pathology because the
cochlea receives its blood supply from the
intracranial circulation via the internal auditory
artery

55. Abnormalities of the I–III Interpeak
Interval
 Prolongation reflects an abnormality within the
neural auditory pathways between the distal
eighth nerve on the stimulated side and the lower
pons . Upper limit- 2.5 ms
 Seen in acoustic neuromas, demyelinating
disease, brainstem tumors, or vascular lesions of
the brainstem,meningitis ,Sub

56. Abnormalities of the III–V Interpeak
Interval
 Reflects an abnormality between the lower pons and
the midbrain most often, although not always,
ipsilateral to the lesion.Upper limit- 2.4 ms
 Prolongation not an abnormality if the I–V interpeak
interval is normal.
 Seen in a variety of disease processes involving the
brainstem, including demyelination, tumor, and
vascular disease

57. Abnormalities of the I-V Interpeak
latencies
 Upper limit – 4.5 ms
 Variety of disorders demyelination ,ischemia
 Wave V to I amplitude ratio –
 If ratio less than 50 % suggests central impairment
 Very high ratio suggests Peripheral impairment

58. Clinical Application
 Most Important is for Hearing assessment in
newborns and children
 Neurologic Conditions- Multiple Sclerosis ,
Stroke,Coma
 Retrocochlear Hearing loss – Acoustic Neuroma

59. Clinical Application
- Usually non specific and should be correlated
clinically and with other investigations
CP angle tumor –
 1. Unrecordable BAER
 2. Only wave I recordable
 3. Prolongation of I –III and I-V Interpeak
latencies
 4. Right to left asymmetry in wave V latency
- MRI is gold standard for evaluating retrocochlear
hearing loss
 BAER – 71 % sensitivity ,74 % specificity

60. Multiple Sclerosis –
1. Most frequent – Absence of Wave V
2. Prolongation of III-V interpeak latency
3. Prolongation of I-V interpeak latency
4. Reduction of V/I amplitude ratio
Diagnostic yield is higher in definite MS- 67%,Probable-
41% ,
Higher in those with brainstem signs
- Diagnostic yield Lower compared to VEP
- Can be used to detect silent brainstem lesions
,follow up and response to treatment

61. Coma and Brain Death
 Prognostic predictor of coma
 BAER is normal in toxic or metabolic cause of
coma
 Absence of III or IV waves associated with
vegetative state
 Better Predictor of outcome following head injury
than GCS
 In brainstem stroke abnormal BAEP correlated
with unstable clinical course

62. Role of BAER in pediatrics
 Newborn Hearing Screening – Hearing loss
occurs in 1/1000 births .Early detection to
improve language skills
 Hyperbilirubinemia/kernicterus
 Children with intellectual impairment
 Spastic Cerebral Palsy

63. THANK YOU

64. References
 Electromyography and Neuromuscular disorders –
David Preston ,Barbara Shapiro -3rd Edition
 Clinical Neurophysiology – UK Mishra & J Kalita – 3rd
Edition
 Practice parameter for repetitive nerve stimulation –
AANEM
 Guidelines on Visual Evoked Potentials 2008
American Clinical Neurophysiology Society (ACNS)
 ACNS guidelines for Auditory evoked potentials