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Dr. Parag Moon
Senior resident
Dept. of Neurology
GMC Kota
 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
 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.
 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.
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
 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.
 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
 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.
 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.
 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%)
Fundamentals of nerve conduction study
Fundamentals of nerve conduction study
 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
 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.
 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
 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.
Fundamentals of nerve conduction study
 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.
 Antidromic studies are performed by recording
potentials directed toward the sensory receptors
 Orthodromic studies are obtained by recording
potentials directed away from these receptors.
 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.
Fundamentals of nerve conduction study
 (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.
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
 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
 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
 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)
 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
1. F reflex
2. H reflex
3. A reflex
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.
 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
 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
 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
1. Early AIDP
2. C8-T1, L5-S1 radiculopathy
3. Polyneuropathy
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
 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
Fundamentals of nerve conduction study
1. Early polyneuropathy
2. S1 radiculopathy
3. Early GBS
4. Tibial and sciatic neuropathy, sacral
plexopathy
5. Electrical correlate of ankle reflex
 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.
 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.
Fundamentals of nerve conduction study
 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.
•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
•Marked slowing of
conduction velocity and
distal latency
•Change in potential
morphology (conduction
block/temporal
dispersion) between
distal and proximal
stimulation sites
 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
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
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
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.
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.
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
2. Cathode position
reversed
 Theoretical possibility
of anodal block
 Distal latency
prolonged by 0.3-
0.4ms
 Slowing of sensory CV
by 10m/s
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
Fundamentals of nerve conduction study
 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
Thank u
 Electromyography and neuromuscular
disorders; D. Preston, B.Shapiro: 3rd edition:
pg 19-90
 Nerve condution studies: essentials and
pitfalls: A Malik et al: J Neurol Neurosurg
Psychaitry 2005:76:pg 23-31
 AANEM guidelines for nerve conduction
studies

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Fundamentals of nerve conduction study

  • 1. Dr. Parag Moon Senior resident Dept. of Neurology GMC Kota
  • 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%)
  • 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.
  • 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.
  • 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
  • 28. 1. F reflex 2. H reflex 3. A reflex
  • 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
  • 33. 1. Early AIDP 2. C8-T1, L5-S1 radiculopathy 3. Polyneuropathy
  • 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
  • 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.
  • 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
  • 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
  • 55.  Electromyography and neuromuscular disorders; D. Preston, B.Shapiro: 3rd edition: pg 19-90  Nerve condution studies: essentials and pitfalls: A Malik et al: J Neurol Neurosurg Psychaitry 2005:76:pg 23-31  AANEM guidelines for nerve conduction studies