2. Peripheral nerves are stimulated with an controlled
electrical stimulus
Responses recorded
1. Compound motor action potential (CMAP)
2. Sensory nerve action potential (SNAP)
3. F wave
4. H- reflex
3. Active recording electrode placed on the center of
the muscle belly (over the motor endplate)
Reference electrode placed distally about 3-4 cm
from active electrode.
Stimulator placed over the nerve that supplies the
muscle, cathode closest to the recording electrode.
Current needed
1. 20-50 mA for motor NCS
2. 5-30 mA for sensory NCS
Supramaximal stimulation is given.
4. CMAP- biphasic potential with an initial negativity
(upward deflection).
M response
For each stimulation site: latency, amplitude,
duration, and area of the CMAP are measured.
A motor conduction velocity can be calculated after
two sites of stimulation, one distal and one
proximal.
5. If an initial positive deflection exists, it may be
due to:
1. Inappropriate placement of the active electrode
from the motor point
2. Volume conduction from other muscles or nerves
3. Anomalous innervations
6. It is the time from the stimulus to the initial
negative deflection from baseline
Made in milliseconds (ms).
In CMAP latency represents three separate
processes:
(1) the nerve conduction time .
(2) the time delay across the NMJ
(3) the depolarization time across the muscle.
7. Calculated by dividing the change in distance
(between proximal stimulation site & distal
stimulation site in mm) by the change in time
(proximal latency in ms minus distal latency in ms)
Normal values are > 50 meters/sec in
the upper limbs And > 40 meters/sec in the lower
limbs
8. Most commonly measured from baseline to the
negative peak (baseline-to-peak) and less
commonly from the first negative peak to the next
positive peak (peak-to-peak).
Reflects the number of muscle fibers that
depolarize.
Low CMAP amplitudes most often result from loss
of axons (as in a typical axonal neuropathy),
conduction block.
9. Measured from the initial deflection from baseline
to the final return
Also measured from the initial deflection from
baseline to the first baseline crossing
2nd is preferred as the terminal CMAP returns to
baseline very slowly and can be difficult to mark
precisely.
10. This is a function of both the amplitude and
duration of the waveform.
CMAP area is measured between the baseline and
the negative peak.
Differences in CMAP area between distal and
proximal stimulation sites for determination of
conduction block from a demyelinating
lesion(>50%)
11.
12.
13. A pair of recording electrodes (GI and G2) are
placed in line over the nerve at an interelectrode
distance of 3 to 4 cm, with the active electrode (G I)
placed closest to the stimulator.
Recording ring electrodes are conventionally used
to test the sensory nerves in the fingers
14. Onset latency is the time required for an electrical
stimulus to initiate an evoked potential.
Onset latencies reflect conduction along the fastest
nerve fibers
Peak latency in SNAP : it represents the latency
along the majority of the axons and is measured at
the peak of the waveform amplitude (first negative
peak).
Both latencies are primarily dependent on the
myelination of a nerve.
15. Peak latency can be ascertained in a
straightforward manner.
Some potentials, especially small ones, it may be
difficult to determine the precise point of
deflection from baseline
Peak latency cannot be used to calculate a
conduction velocity
16. SNAP amplitude -sum of all the individual
sensory fibers that depolarize.
Low SNAP amplitudes indicate a definite
disorder of peripheral nerve.
Conduction velocity-Only one stimulation site
is required to calculate a sensory conduction
velocity.
17.
18. Lesions proximal to it
(injuries to the sensory
nerve root or to the
spinal cord) preserve the
SNAP waveform despite
clinical sensory
abnormalities
This is because axonal
transport from the DRG
to the peripheral axon
continues to remain
intact.
19. Antidromic studies are performed by recording
potentials directed toward the sensory receptors
Orthodromic studies are obtained by recording
potentials directed away from these receptors.
20. Antidromic studies are easier to record a response
than orthodromic studies.
May be more comfortable than orthodromic
studies due to less stimulation required.
May have larger amplitudes due to the nerve being
more superficial at the distal recording sites.
More chances of volume conducted motor
potential.
21.
22. (SNAPs) and(CMAPs) both are compound potentials
They represent summation of individual sensory
and muscle fiber action potentials, respectively.
With distal stimulation, fast and slow fiber
potentials arrive at the recording site at
approximately the same time
With proximal stimulation, the slower fibers lag
behind the faster fibers.
23. Temporal dispersion & phase cancellation is more prominent with
SNAP than CMAP for 2 reasons:
– The CMAP duration is much longer than the SNAP
– The range of fiber conduction velocity is less spread in motor than
sensory fibers (12 m/sec vs. 25 m/sec).
SNAP
CMAP
24. For this reason a drop of 50% is considered normal
when recording a proximal SNAP.
Drop of 15% is considered normal when recording
a proximal CMAP
25. DEFINITE
> 50% drop in CMAP
amplitude with <15%
prolongation of CMAP
duration, or
> 50% drop in CMAP
amplitude and area, or
> 20% drop in CMAP
amplitude and area
over a short nerve
segment (10 cm)
PROBABLE
20‐50% drop in CMAP
amplitude with < 15%
prolongation of CMAP
duration, or
20‐50% drop in CMAP
amplitude and area
26. Most common with acute
nerve lesions
– Peroneal at fibular neck
– Radial at spiral groove
– Ulnar at elbow
Is due to segmental
internodal demyelination
Is the
electrophysiological
correlate of neurapraxia
(first degree nerve injury)
27. Is due to conduction
slowing along a
variable number of the
medium or small nerve
fibers (average or
slower conducting
axons)
Often it is associated
with focal slowing
29. Stimulation is followed by depolarization which travels
in both directions: first directly to the muscle fiber
producing the M response, and retrograde up to the
motor axon and to anterior horn, where it is re
propogated back through the axons to produce the
delayed F response.
30. Small late motor response occurring after the
CMAP.
Late response
Approximately 1–5% of the CMAP amplitude.
Supramaximal stimulation
Pure motor response
Not represent a true reflex
Usually polyphasic& varies with each stimulation
31. Amplitude 1%-5% CMAP
Measurements: Minimal, maximal latency
Chronodispersion and Persistence
Minimal latency= less than 32 in UL and <56 in LL
Chronodispersion: it’s the time delay bet. Minimal&
maximal latencies (<4ms in UL and <6ms in LL)
Persistence >50%
F estimate=2D/CVx10+1ms+DL
32. Normally peroneal F waves may be absent or
nonpersistent
F responses may be absent in sleeping or
sedated patients
F responses may be absent with low-
amplitude distal CMAPs
34. Submaximal stimulation of the afferent sensory
fiber(1A) ->orthodromic conduction to the spinal
cord->synaptic stimulation of the alpha motor
neuron->evoked H response in the muscle.
A rudimentary M response is produced when a few
motor axons are directly stimulated
35. Latency
Normal: 28–30 milliseconds
Side to side difference: greater than 0.5–1.0 ms is
significant
Above 60 years: adds 1.8 milliseconds
H/M ratio <50%
Location
Soleus muscle: tibial nerve: S1 pathway
Flexor carpi radialis: median nerve: C7 pathway
Vastus medialis : femoral n : L4 pathway
36.
37. 1. Early polyneuropathy
2. S1 radiculopathy
3. Early GBS
4. Tibial and sciatic neuropathy, sacral
plexopathy
5. Electrical correlate of ankle reflex
38. Not a true reflex
It is another late potential that often is recognized
during the recording of F responses.
Typically occurs between the F response and the
direct motor (M) response
An axon reflex is identified as a small motor
potential that is identical in latency and
configuration with each successive stimulation.
39. Axon reflexes typically are seen in reinnervated
nerves, especially when a submaximal stimulus is
given
Function
1. This waveform represents collateral sprouting
following nerve damage.
2. Also shows that stimulus is submaximal.
40.
41. Amplitude decreased
May manifest with
conduction block early
(before Wallerian
degeneration)
CV is normal or slightly
slowed(<75%)
DL is normal or slightly
prolonged(<130%)
Morphology does not
change between
proximal and distal sites.
42. •CV is markedly
slowed < 75% lower
limit of normal)
• DL is markedly
prolonged (>130%
upper limit of normal).
•Usually no change in
configuration between
proximal and distal
stimulation
43. •Marked slowing of
conduction velocity and
distal latency
•Change in potential
morphology (conduction
block/temporal
dispersion) between
distal and proximal
stimulation sites
44. Occurs in approximately 15‐20%
Fibers cross from the median to the ulnar
nerve in the forearm.
Communicating branch(es) usually consists of
motor axons that supply the ulnar‐innervated
intrinsic hand muscles,
1. first dorsal interosseous muscle
2. hypothenar muscles
3. ulnar thenar muscles
4. A combination of these muscles
45. 1. Temperature
Cooler temperature prolong time of
depolarisation
Conduction velocity slows between 1.5-
2.5m/s, distal latency prolong by 0.2 ms for
every degree drop in temperature
Higher amplitude and longer duration
Temperature to be maintained between 32-
34 degree
46. 2. Age
Conduction decrease with age
More prominent after 60 yrs
Correction factor of 0.5-4m/s for older pts.
can be used.
Sural nerve may not be ellicitable in some
47. 3. Height
Taller individual have slower conduction
velocity.
Adjustment no more than 2-4m/s below
lower limit of normal
4.Proximal vs distal
Proximal nerve segment conduct slightly
faster than distal.
48. 1. Electrical impendance
60 HZ noise made by different electrical
devices.
Identical noise at each electrode best
achieved by ensuring same electrical
impendance at both electrodes.
49. 2. Stimulus artifact
Reduced by placement of
ground between recording
and stimulator
Decrease electrical
impendance
Coaxial electrodes
Stimulator directly over
nerve
Lower stimulus
Rotate anode while
maintaining cathode
Stimulator and recording
cables do not overlap
50. 2. Cathode position
reversed
Theoretical possibility
of anodal block
Distal latency
prolonged by 0.3-
0.4ms
Slowing of sensory CV
by 10m/s
51. 4. Co-stimulation of adjacent nerves
Can be reduced by
1. Stimulator directly over nerve
2. Watch for abrupt change in waveform
3. Change in resultant muscle twitch
4. Avoid excess current
5. Co-record muscles simultaneously frm
adjacent nerve
52.
53. Temperature effect and cold limb
Sloppy measurement of distances
Anatomic abnormalities of patient
Technical factors: edema, large limbs, long limbs
Too few nerve conduction studies, lack of
comparisons
Too many nerve conduction studies: Interpretation
of non-existing abnormality