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Electrodiagnosis 2
1. WHAT IS NCS ?
A nerve conduction study (NCS) : is a test used to
evaluate/ detect the function of motor and sensory
nerves of the human body.
Nerve conduction velocity (NCV): is a common
measurement made during this test.
2. The nerve conduction studies most commonly
performed are compound muscle action potentials
(CMAPs) for motor nerves, sensory nerve action
potentials (SNAPs) for sensory nerves, compound
nerve action potentials (CNAPs) for mixed (sensory
and motor) nerves and late responses (primarily F-
waves and H-reflexes).
7. NCS
Latency Latency – time interval between the
Velocity onset of a stimulus and the onset of a
amplitude response (can also be referred to as a
motor latency or a sensory latency).
Amplitude – the maximal height of
the action potential.
Conduction velocity – how fast the
fastest part of the impulse travels (can
also be referred to as a motor
conduction velocity or a sensory
conduction velocity).
8. NORMAL VALUES
LATENCY AMPLITUDE VELOCITY
UL
MOTOR N. < 4.0 ms 5-10 mV 50-60m/s
SENSORY N. <3.2-3.4 ms 5-50 µV 50m/s
LL
MOTOR N. <5.2ms 2-10mV 40-50m/s
SENSORY N. <3.5-4.0 ms 5-40µV 35-40m/s
11. ELECTRODE PLACEMENT
ACTIVE ELECTRODE : placed on the motor point of
the muscle
REFERENCE ELECTRODE : placed nearby tendon or
bone
GROUND ELECTRODE : in between active and
reference electrode
14. DIRECTION OF CONDUCTION
Orthodromic conduction
Antidromic conduction
Orthodromic – when the electrical impulse travels in
the same direction as normal physiologic conduction
(e.g., when a motor nerve electrical impulse is
transmitted toward the muscle and away from the
spine or a sensory impulse travels toward the spine).
Antidromic – when the electrical impulse travels in the
opposite direction of normal physiologic conduction
(e.g., conduction of a motor nerve electrical impulse
away from the muscle and toward the spine).
15.
16.
17. M-WAVE
Orthodromically stimulated motor response known
as CMAP
Supramaximal stimulus
MNCV =
distance between 2 stimulus sites
difference between 2 latency
18.
19.
20.
21. USES :
When all motor fibers undergo Demyelination
long latency
Slow NCV
Partial demyelination
Normal latency and velocity
Reduced amplitude
Well marked demyelination
Absence of conduction
Axonal degeneration
Reduced amplitude
Neuropraxia
Distal to the lesion – normal NCV
Proximal to the lesion – reduced conduction velocity
22. F-WAVE
A compound muscle action potential evoked by
antidromically stimulating a motor nerve from a
muscle using maximal electrical stimulus. It
represents the time required for a stimulus to travel
antidromically toward the spinal cord and return
orthodromically to the muscle along a very small
percentage of the fibers
23. F wave latency :
median nerve – 22.34 ms
ulnar nerve – 23.32ms
peroneal nerve – 40.56ms
tibial nerve – 38.58ms
25. USES :
A prolonged asymmetric F waves suggest a proximal root lesion.
Clinical application best for plexopathy.
Quite prolonged in demyelination and mild prolongation in
axonal injury.
Assesses proximal neuropathology
Eg:
GBS
Thoracic outlet syndrome
Charcot’s Mary-tooth disease
proximal nerve entrapment
26. SNCV
It is performed by electrical stimulation of a peripheral
nerve and recording from a purely sensory portion of
the nerve, such as on a finger.
14 cm standard distance is used
Threshold level stimulus is required
sensory latencies are on the scale of milliseconds(ms).
sensory amplitude are on microvolt range.
2 TYPES :
ORTHODROMIC TESTS
ANTIDROMIC TESTS
27. An Idealized Sensory Waveform
S = Stimulus point, T = Takeoff point, P = Peak
The time (latency) from S to T is typically about 3 milliseconds.
The amplitude would be measured in microvolts (μV).
28.
29. H-REFLEX
A compound muscle action
potential evoked by orthodromically
Stimulating sensory fibers, synapsing at the
spinal level and returning Orthodromically
via motor fibers. The response is thought to
be due to a Monosynaptic spinal reflex
(hoffmann reflex) found in normal adults
in the Gastrocnemius-soleus and flexor
carpi radialis muscles.
30. 4. H - REFLEX
Assesses continuity and function of sensory and
motor monosynaptic pathway of 1st sacral nerve
root
Submaximal stimulus/LONG LATENCY-
29.8ms
Disappears during supra maximal stimulus
H-reflex study uses stimulation of a nerve and
recording the reflex electrical discharge from a
muscle in the limb.
33. Normal latency : 29.8 millisecond
( got long latency)
Implication : used in the
diagnosis of S1 and C7 root lesions
the study of proximal nerve segments in either
peripheral or proximal neuropathies.
Its absence or abnormal latency on one side strongly
indicates disease
34.
35.
36. REPETITIVE STIMULATION
Repeated electrical stimulus applied to the motor
neuron at a rate of 3-5 / second , the amplitude of the
recorded muscle response is constant
Decrement of more than 10% is abnormal
Supramaximal stimulus
Used to find NMJ abnormalities
Eg: myaesthenia gravis
Lambert Eaton syndrome
MMEP is measured
37. REPETITIVE STIMULATION
A repetitive nerve stimulation study
demonstrating a 61 percent decrement in
area and a 54 percent decrement in
amplitude from the first to the fourth
stimulation.
