This document provides an overview of nerve conduction studies (NCS) and needle electromyography (EMG). It discusses the basics of neurophysiology and how electrical signals in nerves and muscles are recorded through surface electrodes during NCS and needle electrodes during EMG. It describes how motor and sensory nerve conduction studies are performed, including stimulation techniques and measurements of latency, amplitude, area, duration, and conduction velocity. Patterns of results are discussed for axonal loss, demyelination, and conduction block neuropathies.

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.

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).

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).

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.

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.

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

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.