بسم الله الرحمن الرحيم

1. بسم الله الرحمن الرحيم

3. Basic nerve conduction study By Mohammad Hassan Abu-Zaid Ass. lect. of Rheumatology & Rehabilitation

4. MOTOR CONDUCTION STUDIES • The CMAP is a biphasic potential with an initial negativity, or upward deflection from the baseline • For each stimulation site:the 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. • It’s called M response

5. CMAP

6. If an initial positive deflection exists, it may be due to: • Inappropriate placement of the active electrode from the motor point • Volume conduction from other muscles or nerves • Anomalous innervations

7. CMAP electrode placement. I: Over the endplate region. II: Off the endplate region.

8. The active recording electrode is placed on the center of the muscle belly (over the motor endplate), and the reference electrode is placed distally about 3:4 cm • The stimulator then is placed over the nerve that supplies the muscle, with the cathode placed closest to the recording electrode.

9. As current is slowly increased from a baseline: more of the underlying nerve fibers are brought to action potential, and subsequently more muscle fiber action potentials are generated. • most nerves require a current in the range from 20 to 50 mA to achieve supramaximal stimulation.

10. The recorded potential, known as the compound muscle action potential (CMAP), represents the summation of all underlying individual muscle fiber action potentials. • When the current is increased to the point that the CMAP no longer increases in size, one presumes that all nerve fibers have been excited and that supramaximal stimulation has been achieved. The current then is increased by another 20% to ensure supramaximal stimulation.

11. Latency • Latency measurements usually are made in milliseconds (ms). • The latency is the time from the stimulus to the initial negative deflection from baseline

12. 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.

13. The only major difference between CMAPs produced by proximal and distal stimulations is the latency.

14. Conduction Velocity • It’s measurement of the speed of the fastest conducting nerve axons • It is calculated by dividing the change in distance (between proximal stimulation site & distal stimulation site in mm) by the change in time (proximal latency in msec minus distal latency in ms)

15. Conduction Velocity

16. Normal values are > 50 meters/sec in the upper limbs And > 40 meters/sec in the lower limbs.

17. Amplitude • it is 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).

18. CMAP amplitude reflects the number of muscle fibers that depolarize. Although low CMAP amplitudes most often result from loss of axons (as in a typical axonal neuropathy), other cause of a low CMAP amplitude include conduction block from demyelination located between the stimulation site and recorded muscle • average CMAP amplitude 3 mv

19. Duration • This is measured from the initial deflection from baseline to the final return, also is measured from the initial deflection from baseline to the first baseline crossing& it’is preferred as a measure of CMAP duration because when CMAP duration is measured from the initial to terminal deflection back to baseline, the terminal CMAP returns to baseline very slowly and can be difficult to mark precisely.

20. It depends on the summation and rate of firing of numerous axons. • Duration characteristically increases in conditions that result in slowing of some motor fibers (e.g., in a demyelinating lesion). • average CMAP duration 14: 16.5 msec

21. Area • 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 are significant in the determination of conduction block from a demyelinating lesion

22. SENSORY CONDUCTION STUDIES • 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

23. A sensory nerve study represents the conduction of an impulse along the sensory nerve fibers • The SNAP usually is biphasic or triphasic in configuration with initial negative deflection . • sensory fibers usually have a lower threshold to stimulation than do motor fibers

24. SNAP CMAP

25. Latency • Onset latency is the time required for an electrical stimulus to initiate an evoked potential. • onset latencies reflect conduction along the fastest nerve fibers

26. 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.

27. Advantages of peak latency measurements in SNAPs • The 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 • N.B peak latency cannot be used to calculate a conduction velocity

28. Amplitude • The SNAP amplitude reflects the sum of all the individual sensory fibers that depolarize. • Low SNAP amplitudes indicate a definite disorder of peripheral nerve.

29. Duration • Similar to the CMAP duration

30. Conduction Velocity • Only one stimulation site is required to calculate a sensory conduction velocity.

31. SNAP & localizing lesion • It can also be useful in localizing a lesion in relation to the dorsal root ganglion (DRG). • The DRG is located in the intervertebral foramen and contains the sensory cell body.

32. 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.

33. Technical Considerations • Antidromic and Orthodromic Recording • 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.

34. 1.Antidromic stim.2.orthdromic stim of same n

35. Temporal dispersion and phase cancellation • (SNAPs) and(CMAPs) both are compound potentials, representing the summation of individual sensory and muscle fiber action potentials, respectively. • In each case, there are fibers that conduct faster and those that conduct more slowly • 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.

36. Temporal Dispersion • This reflects the range of conduction velocities of the fastest and slowest nerve fibers. • It is seen as a spreading out of the waveform with proximal compared to distal stimulation.

38. Phase Cancellation • When comparing a proximal to distal stimulation, a drop in amplitude and increase in duration occurs, mostly with a (SNAP) because of its short duration. • When the nerve is stimulated, the action potentials of one axon may be out of phase with neighboring ones. The negative deflections of one axon can then cancel the positive deflection of another, reducing the amplitude. • The summation of these axons creates an action potential that appears as one long prolonged wave.

40. 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

41. Patterns of nerve conductionNormal study of median nerve. • Note the normal median distal latency (DL) 3 ms, amplitude >4 m V, and conduction velocity (CV) >49 mls.

42. Axonal loss. • amplitudes decrease • CV is normal or slightly slowed. • DL is normal or slightly prolonged. • The morphology of the potential does not change between proximal and distal sites.

43. Demyelinationassociated with inheritedconditions (uniform) • CV is markedly slowed < 75% lower limit of normal) • DL is markedly prolonged (>130% upper limit of normal). • However, there usually is no change in configuration between proximal and distal stimulation

44. Demyelination with conduction block usually acquired as GBS • Marked slowing of conduction velocity and distal latency, but also with change in potential morphology (conduction block/temporal dispersion) between distal and proximal stimulation sites,

45. CMAP amplitude and conduction block location. Late responses may be abnonl

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47. In both patients, there is a drop in amplitude across the spiral groove • In patient 1, block. Conduction block signifies demyelination; therefore, the prognosis is good. • In patient 2, This implies significant axonal loss. Although there is some conduction block across the spiral groove & most of patient's weakness is secondary to axonal loss, which implies a longer and possibly less complete recovery process.

48. Late Responses • F wave • H reflex • A wave

49. F wave • 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 the neuron, where it is re propogated back through the axons to produce the delayed F response.

50. The F-wave is a small late motor response occurring after the CMAP. • It represents a late response from approximately 1–5% of the CMAP amplitude. • It is produced using supramaximal stimulation • The F-wave is a pure motor response and does not represent a true reflex because there is no synapse along the nerve pathway being stimulated