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BASICS OF NCS/EMG
BY
DR ABDUL ALI.
PGR NEUROLOGY DEPARTMENT BMCH QUETTA
Electrodiagnostic studies (EDX)
• An extension of neurological examination.
• Includes
• Nerve conduction studies (NCSs),
• Needle electromyography (EMG)
• Late responses,
• Blink reflexes,
• Some other specialized examinations. NCSs and
needle
• EMG form the core of the EDX studies.
Cont..
• NCSs and EMG to diagnose
• peripheral nervous system diseases
• Primary motor neurons (anterior horn cells),
• Primary sensory neurons (dorsal root ganglia),
• Nerve roots,
• Brachial and lumbosacral plexuses,
• Peripheral nerves,
• Neuromuscular junctions (NMJs), and muscles.
Basic mechanism.
• Neurophysiology of Axon.
• The specialized axonal membrane
• Always impermeable to large negatively charged
anions(inside axon).
• Relatively impermeable to sodium in the resting
state.
• Membrane + active Na+/K+ pump = move sodium
outside in exchange for potassium, leads to
concentration gradients across the membrane =
resting membrane potential.
Cont..
• RMP = -70 to -90mV.
• axonal membrane = voltage-gated sodium
channels
• controlled by a voltage sensor = responds to the
level of the membrane potential.
• current is injected into the axon, Voltage gated Na+
chennals open (i.e., the axon becomes more
positive internally).
• Threshold = 10 to 30mV = Action potential.
Recording NCS/EMG
• NCS = surface electrodes over the skin,
• EMG potentials = needle electrode placed within
the muscle
• Both intracellular electrical potentials are
transmitted through tissue to the recording
electrodes.
• volume conduction . The process of an intracellular
electrical potential being transmitted through
extracellular fluid and tissue .
Volume conduction recording.
Near field potential
• Close to the source of
current
• Wave character =
distance b/t source of
current and rec
electrodes (A.P)
• Routine NCS EMG are
Volume Conducted near
field potential.
Far field potential
• Electrical potentials
that are distributed
widely and instantly.
• Two recording
electrodes, one closer
and the other farther
from the source,
essentially see the
source at the same
time.
Volume Conducted near field potential
• Produce a characteristic triphasic waveform as an
advancing action potential approaches and then
passes beneath and away from a recording
electrode.
• Sensory and mixed nerve studies.
• Initial positive phase = Negative phase = Trailing
positive tive phase.
• If a volume-conducted, near-field action potential
begins directly under the recording electrode, the
initial deflection will be negative = Biphasic
waveform= negative phase followed by positive
phase.
Principles of stimulation
• Supramaximal stimulation
current intensity is slowly increased until the
amplitude of the recorded potential reaches a
plateau. The current intensity then is increased an
additional 20%–25% to ensure that the potential no
lon-ger increases. It is only at this point that
supramaximal stimulation is achieved.
• Optimize the stimulation site
• placement of the stimulator at the optimal location
directly over the nerve, which yields the highest
CMAP amplitude with the least stimulus intensity
Motor conduction studies.
• Range of several millivolts (mV),
• [sensory and mixed nerve responses are in the
microvolt (μV) range].
• Gain usually is set at 2–5 mV per division.
• Active electrode = on belly of muscle
• Reference electrode = tendon of muscle.
• Stimulator then is placed over the nerve that
supplies the muscle, with the cathode placed
closest to the recording electrode
Cont...
• Duration of the electrical pulse = 200 ms.
• Most normal nerves = 20–50 mA to achieve
supramaximal stimulation.
• Summation of all underlying individual muscle fiber
action Potential = compound muscle action
potential (CMAP).
• CMAP = biphasic potential with an initial negativity
followed by positivity if the recording electrodes
have been properly placed.
Latency. (Motor)(Time period)
• Time from the stimulus to the initial CMAP
deflection from baseline.
• Represents three separate processes:
• (1) the nerve conduction time from the stimulus
site to the neuromuscular junction (NMJ),
• (2) the time delay across the NMJ,
• (3) the depolarization time across the muscle.
• Latency measurements usually are made in
milliseconds (ms) and reflect only the fastest
conducting motor fibers.
Amplitude (motor)(# of N. fiber)
• CMAP amplitude measured from baseline to the
negative peak., or negative to positive peak.
• Reflects the number of muscle fibers that
depolarize.
• Low CMAP amplitudes = loss of axons (as in a
typical axonal neuropathy),
• Other causes of a low CMAP amplitude
• 1 = conduction block from demyelination located
between the stimulation site and the recorded
muscle,
• 2 = some NMJ disorders
Area
• CMAP area = area above the baseline to the
negative peak.
• A negative peak CMAP area is another measure
reflecting the number of muscle fibers that
depolarize.
• Differences in CMAP area between the distal and
proximal stimulation sites take on special
significance in the determination of conduction
block from a demyelinating lesion.
Duration
• CMAP duration usually is measured from the initial
deflection from baseline to the first baseline
crossing.
• Duration is primarily a measure of synchrony, i.e.,
the extent to which each of the individual muscle
fibers fire at the same time.
• Duration characteristically increases in conditions
that result in slowing of some motor fibers but not
others (e.g., in a demyelinating lesion).
Conduction velocity
• The speed of the fastest conducting motor axons in
the nerve being studied,
• C V = distance traveled by impulse ÷ time.
• Tow stimulation sites Proximal and distal.
• Area, amplitude and duration same in Proximal
and Distal stimulation sites.
• Latency (time) is more in Proximal site than distal .
• Conduction velocity = (distance between the
proximal and distal stimulation sites) divided by
(proximal latency minus distal latency).
• Measured in meters per second (m/s).
Sensory conduction studies.
• Range in microVolts.
• Gain usually is set at 10–20 μV per division.
• Electrical pulse of 100 or 200 ms in duration.
• Normal sensory nerves require a current in the
range of 5–30 mA to achieve supramaximal
stimulation.
• Sensory nerve action potential (SNAP), is a
compound potential that represents the
summation of all the individual sensory fiber action
potentials.
• Biphasic or triphasic.
Latency
• Onsets latency
• Time from initial
stimulus to negative
deflection from
baseline or initial
positive peak.
• Conduction time in
Large sensory fibers in
nerves being studied.
• Peak latency
• Time from the stimulus
to the midpoint of the
first negative peak.
• No interindividual
variation in its
determination.
• No C.V.
• No population of
N.fibers can be
determined.
Amplitude (SNAP)
• SNAP amplitude = baseline to negative peak, but it
can also be measured from the first negative peak
to the next positive peak.
• Reflects the sum of all the individual sensory fibers
that depolarize.
• Low SNAP amplitudes indicate a definite disorder of
peripheral nerve.
Duration
• SNAP duration is usually measured from the onset
of the potential to the first baseline crossing.
• Relatively shorter than CMAP duration
(1.5 vs 5—6)
• Duration helps identify a potential as a true nerve
potential rather than a muscle potential
Conduction velocity (SNAP)
• Sensory conduction velocity determined with one
stimulation, simply by dividing the distance
traveled by the onset latency.
• Speed of the fastest, myelinated cutaneous sensory
fibers in the nerve being studied.
• Stimulus = AP = runs in both directions in sensory
nerves
• Can be calculated by antidromic and orthodromic
methods
Antidromic. Orthodromic.
• Stimulating toward the
sensory receptor.
• Amplitude is higher and
directly proportional to
the proximity of the
recording electrode to
the underlying nerve.
• Small potentials, often
pathological.
• But CMAP= difficulty to
identify latency.
• Stimulating away from
the sensory receptors.
• Amplitude is lower
because recording
electrode is away from
underlying nerve.
• No CAMP, only specific
sensory latency.
Temporal Dispersion and Phase
Cancellation
• Within any sensory nerve, there are large, medium,
and smaller myelinated fibers, which depolarize
and conduct at slightly different velocities.
• Larger, faster fibers depolarize before smaller, and
slower ones. leading to increased duration and
temporal dispersion of the waveform
• Large fibers = neg duration = 0.5 ms
• Small fibers = negative duration 1.3 ms
Cont...
• After the first 0.5 ms, the trailing positive phase of
the fastest potential overlaps with the leading
negative phases of the slower fibers, phase
cancellation occurs resulting in a smaller summated
potential.
• Results in decrease in area and amplitude.
Axonal loss pattern
• Reduced amplitude is the primary abnormality
associated with axonal loss.
• Conduction velocity is normal, provided that the
largest and fastest conducting axons remain intact.
( 65 m/s)
• Conduction velocity decrease to lower normal if
large and fast conducing fibers are damaged with
slower fibers intact. ( 35m/s).
• Latency normal or slightly increased , not more
than 130% of normal upper limit.
• Normal NCS if wallerian degeneration not
Demyelination pattern
• Demyelination = marked slowing of conduction
velocity (slower than 75% of the lower limit of
normal).
• Marked prolongation of distal latency (longer than
130% of the upper limit of normal).
• Any motor, sensory, or mixed nerve conduction
velocity that is slower than 35 m/s in the arms or
30 m/s in the legs signifies unequivocal
demyelination.
• Temporal dispersion.
Conduction block.
• Decrease in CMAP amplitude of more than 20%
denotes conduction block.
• More than 50% drop in area between proximal and
distal stimulation sites should be used to define
electrophysiologic conduction block.
Basic overview of nerve conduction studies
Basic overview of nerve conduction studies

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Basic overview of nerve conduction studies

  • 1.
  • 2. BASICS OF NCS/EMG BY DR ABDUL ALI. PGR NEUROLOGY DEPARTMENT BMCH QUETTA
  • 3. Electrodiagnostic studies (EDX) • An extension of neurological examination. • Includes • Nerve conduction studies (NCSs), • Needle electromyography (EMG) • Late responses, • Blink reflexes, • Some other specialized examinations. NCSs and needle • EMG form the core of the EDX studies.
  • 4. Cont.. • NCSs and EMG to diagnose • peripheral nervous system diseases • Primary motor neurons (anterior horn cells), • Primary sensory neurons (dorsal root ganglia), • Nerve roots, • Brachial and lumbosacral plexuses, • Peripheral nerves, • Neuromuscular junctions (NMJs), and muscles.
  • 5. Basic mechanism. • Neurophysiology of Axon. • The specialized axonal membrane • Always impermeable to large negatively charged anions(inside axon). • Relatively impermeable to sodium in the resting state. • Membrane + active Na+/K+ pump = move sodium outside in exchange for potassium, leads to concentration gradients across the membrane = resting membrane potential.
  • 6. Cont.. • RMP = -70 to -90mV. • axonal membrane = voltage-gated sodium channels • controlled by a voltage sensor = responds to the level of the membrane potential. • current is injected into the axon, Voltage gated Na+ chennals open (i.e., the axon becomes more positive internally). • Threshold = 10 to 30mV = Action potential.
  • 7. Recording NCS/EMG • NCS = surface electrodes over the skin, • EMG potentials = needle electrode placed within the muscle • Both intracellular electrical potentials are transmitted through tissue to the recording electrodes. • volume conduction . The process of an intracellular electrical potential being transmitted through extracellular fluid and tissue .
  • 8. Volume conduction recording. Near field potential • Close to the source of current • Wave character = distance b/t source of current and rec electrodes (A.P) • Routine NCS EMG are Volume Conducted near field potential. Far field potential • Electrical potentials that are distributed widely and instantly. • Two recording electrodes, one closer and the other farther from the source, essentially see the source at the same time.
  • 9. Volume Conducted near field potential • Produce a characteristic triphasic waveform as an advancing action potential approaches and then passes beneath and away from a recording electrode. • Sensory and mixed nerve studies. • Initial positive phase = Negative phase = Trailing positive tive phase. • If a volume-conducted, near-field action potential begins directly under the recording electrode, the initial deflection will be negative = Biphasic waveform= negative phase followed by positive phase.
  • 10.
  • 11. Principles of stimulation • Supramaximal stimulation current intensity is slowly increased until the amplitude of the recorded potential reaches a plateau. The current intensity then is increased an additional 20%–25% to ensure that the potential no lon-ger increases. It is only at this point that supramaximal stimulation is achieved. • Optimize the stimulation site • placement of the stimulator at the optimal location directly over the nerve, which yields the highest CMAP amplitude with the least stimulus intensity
  • 12. Motor conduction studies. • Range of several millivolts (mV), • [sensory and mixed nerve responses are in the microvolt (μV) range]. • Gain usually is set at 2–5 mV per division. • Active electrode = on belly of muscle • Reference electrode = tendon of muscle. • Stimulator then is placed over the nerve that supplies the muscle, with the cathode placed closest to the recording electrode
  • 13. Cont... • Duration of the electrical pulse = 200 ms. • Most normal nerves = 20–50 mA to achieve supramaximal stimulation. • Summation of all underlying individual muscle fiber action Potential = compound muscle action potential (CMAP). • CMAP = biphasic potential with an initial negativity followed by positivity if the recording electrodes have been properly placed.
  • 14. Latency. (Motor)(Time period) • Time from the stimulus to the initial CMAP deflection from baseline. • Represents three separate processes: • (1) the nerve conduction time from the stimulus site to the neuromuscular junction (NMJ), • (2) the time delay across the NMJ, • (3) the depolarization time across the muscle. • Latency measurements usually are made in milliseconds (ms) and reflect only the fastest conducting motor fibers.
  • 15. Amplitude (motor)(# of N. fiber) • CMAP amplitude measured from baseline to the negative peak., or negative to positive peak. • Reflects the number of muscle fibers that depolarize. • Low CMAP amplitudes = loss of axons (as in a typical axonal neuropathy), • Other causes of a low CMAP amplitude • 1 = conduction block from demyelination located between the stimulation site and the recorded muscle, • 2 = some NMJ disorders
  • 16. Area • CMAP area = area above the baseline to the negative peak. • A negative peak CMAP area is another measure reflecting the number of muscle fibers that depolarize. • Differences in CMAP area between the distal and proximal stimulation sites take on special significance in the determination of conduction block from a demyelinating lesion.
  • 17. Duration • CMAP duration usually is measured from the initial deflection from baseline to the first baseline crossing. • Duration is primarily a measure of synchrony, i.e., the extent to which each of the individual muscle fibers fire at the same time. • Duration characteristically increases in conditions that result in slowing of some motor fibers but not others (e.g., in a demyelinating lesion).
  • 18.
  • 19. Conduction velocity • The speed of the fastest conducting motor axons in the nerve being studied, • C V = distance traveled by impulse ÷ time. • Tow stimulation sites Proximal and distal. • Area, amplitude and duration same in Proximal and Distal stimulation sites. • Latency (time) is more in Proximal site than distal . • Conduction velocity = (distance between the proximal and distal stimulation sites) divided by (proximal latency minus distal latency). • Measured in meters per second (m/s).
  • 20.
  • 21. Sensory conduction studies. • Range in microVolts. • Gain usually is set at 10–20 μV per division. • Electrical pulse of 100 or 200 ms in duration. • Normal sensory nerves require a current in the range of 5–30 mA to achieve supramaximal stimulation. • Sensory nerve action potential (SNAP), is a compound potential that represents the summation of all the individual sensory fiber action potentials. • Biphasic or triphasic.
  • 22. Latency • Onsets latency • Time from initial stimulus to negative deflection from baseline or initial positive peak. • Conduction time in Large sensory fibers in nerves being studied. • Peak latency • Time from the stimulus to the midpoint of the first negative peak. • No interindividual variation in its determination. • No C.V. • No population of N.fibers can be determined.
  • 23.
  • 24. Amplitude (SNAP) • SNAP amplitude = baseline to negative peak, but it can also be measured from the first negative peak to the next positive peak. • Reflects the sum of all the individual sensory fibers that depolarize. • Low SNAP amplitudes indicate a definite disorder of peripheral nerve.
  • 25. Duration • SNAP duration is usually measured from the onset of the potential to the first baseline crossing. • Relatively shorter than CMAP duration (1.5 vs 5—6) • Duration helps identify a potential as a true nerve potential rather than a muscle potential
  • 26. Conduction velocity (SNAP) • Sensory conduction velocity determined with one stimulation, simply by dividing the distance traveled by the onset latency. • Speed of the fastest, myelinated cutaneous sensory fibers in the nerve being studied. • Stimulus = AP = runs in both directions in sensory nerves • Can be calculated by antidromic and orthodromic methods
  • 27. Antidromic. Orthodromic. • Stimulating toward the sensory receptor. • Amplitude is higher and directly proportional to the proximity of the recording electrode to the underlying nerve. • Small potentials, often pathological. • But CMAP= difficulty to identify latency. • Stimulating away from the sensory receptors. • Amplitude is lower because recording electrode is away from underlying nerve. • No CAMP, only specific sensory latency.
  • 28. Temporal Dispersion and Phase Cancellation • Within any sensory nerve, there are large, medium, and smaller myelinated fibers, which depolarize and conduct at slightly different velocities. • Larger, faster fibers depolarize before smaller, and slower ones. leading to increased duration and temporal dispersion of the waveform • Large fibers = neg duration = 0.5 ms • Small fibers = negative duration 1.3 ms
  • 29. Cont... • After the first 0.5 ms, the trailing positive phase of the fastest potential overlaps with the leading negative phases of the slower fibers, phase cancellation occurs resulting in a smaller summated potential. • Results in decrease in area and amplitude.
  • 30.
  • 31. Axonal loss pattern • Reduced amplitude is the primary abnormality associated with axonal loss. • Conduction velocity is normal, provided that the largest and fastest conducting axons remain intact. ( 65 m/s) • Conduction velocity decrease to lower normal if large and fast conducing fibers are damaged with slower fibers intact. ( 35m/s). • Latency normal or slightly increased , not more than 130% of normal upper limit. • Normal NCS if wallerian degeneration not
  • 32.
  • 33. Demyelination pattern • Demyelination = marked slowing of conduction velocity (slower than 75% of the lower limit of normal). • Marked prolongation of distal latency (longer than 130% of the upper limit of normal). • Any motor, sensory, or mixed nerve conduction velocity that is slower than 35 m/s in the arms or 30 m/s in the legs signifies unequivocal demyelination. • Temporal dispersion.
  • 34. Conduction block. • Decrease in CMAP amplitude of more than 20% denotes conduction block. • More than 50% drop in area between proximal and distal stimulation sites should be used to define electrophysiologic conduction block.