1. EMG
Theory of NCS/EMG

2. EMG Is an extension of the neurological examination.
The EMG examination is a diagnostic tool used in the evaluation of pain, weakness, sensory disturbance, fatigue and atrophy
The EMG examination includes two components: Nerve Conduction Studies (NCS) and the needle electromyographic (EMG) study.

3. Localize the Problem Nerve Root NMJ Muscle Axonal Segmental Focal
Pre-synaptic
Post-synaptic
Neuropathy
Myopathy
Upper Motor Neuron

4. NCS
In NCS or electrical studies, response amplitudes and latencies are evaluated.
Nerve Conduction Velocity (NCV) studies may be used to evaluate axonal, segmental and focal peripheral nerve problems.
Other NCS techniques may be used to evaluate problems in the neuromuscular junction (NMJ), nerve root and even central nervous system.

5. Nerve Studies NCV: MNC, SNC NCV: MNC, SNC, F-waves, MUNE, CVD
Focal and Segmental neuropathy (CTS, GB, CMT, Heavy metal poising):
NCV: MNC, SNC
Axonal (ALS, Neuropathies secondary to alcoholism):
NCV: MNC, SNC, F-waves, MUNE, CVD
Other (MG):
MNC, SNC, F-wave, Rep. Stim., H-wave, Blink, MUNE

6. MNC
Supramaximal stimulation at the Wrist produces a Compound Muscle Action Potential (CMAP) from the thenar muscles.
The distal latency (W) includes the terminal axon transmission time + the delay time at the NMJ including the time required for generation of the CMAP after depolarization of the motor end plate.

7. MNCV
Supramaximal stimulation at the Elbow produces a Compound Muscle Action Potential (CMAP) from the thenar muscles.
The nerve conduction time from the elbow to the wrist equals the latency difference between the distal latency (W) and the proximal latency (E).
The Motor Nerve Conduction Velocity (MNCV) is calculated by dividing the distance between the two cathode stimulation points by conduction time.

8. Three Basic Responses Normal or Near Normal:
Response latency is normal. Response amplitude is normal or near normal.
Delayed:
Increased latency and normal or decreased amplitude
Absent:
No response to supramaximal stimulation.
Near Normal: Most motor axons are intact. Seen in neuropraxia or axonotmesis.
Delayed: Indicates demyelination affecting the majority of nerve fibers. Amplitude loss indicates neuropraxia or axonotmesis.

9. Acute Conduction Block
Three stimulation sites: ankle, fibula head and popliteal fossa
Note the amplitude drop in the CMAP between the two proximal stimulation sites
Conduction velocity my be normal or slightly increased

10. Possible Sources of Error
Sub supramaximal stimulation.
Unreliable response amplitudes
Excessive stimulation.
Artificially decreased latencies.
Measurement errors.
Inaccurate measurements across joints, e.g.., ulnar nerve across the elbow.
Martin-Gruber Anastomosis.
Communication from the median to the ulnar nerve at the forearm.

11. Motor and Sensory Potentials
Compound Muscle Action Potential (CMAP)
Amplitude: 4-16 mV
Duration: 4-6 ms
<10% amplitude loss between stimulus sites.
Sensory Nerve Action Potentials (SNAP)
Amplitude: V
Duration: ms
20-30% amplitude loss between stimulus sites.

12. F-waves Supramaximal stimulation required.
With each stimulus, <5% of motor axons in nerve produce a F-wave.
From each stimulus a different population of motor axons produce f-waves.
F-waves are evoked single Motor Unit Action Potentials (CMAP)
Latency variation is due to variation in conduction velocity of individual motor axons.

13. H-waves
A mono-synaptic response analogous to the Achilles tendon tap reflex.
Do not vary in latency.
Must be larger than M-wave.

14. Blink Reflex

15. Repetitive Stimulation
Supramaximal Stimulation.
4 to 10 stimuli at 2-3 Hz.
Maximal amplitude drop by 4th or 5th response.
Amplitude and Area should both decrement.
Typical test sequence:
pre-exercise, 30-60s exercise,
3 s post-exercise, 2 min post-exercise, 10 min post-exercise
Movement related artifact (bottom) from changes in the muscle shape during recording.

16. Needle Exam
The needle EMG examination is used to evaluate problems in muscle, the NMJ and The Motor Unit.

17. Needle Studies
Routine needle EMG
SPA, MUP, MVA: Radicular lesions, Axonal degeneration, Muscle weakness
Quantitative EMG
QMUP, AMUP, IPA: Axonal degeneration, muscle weakness
SFEMG
SFEMG: MG, Myasthenic Syndrome, Botulinum intoxication, Tetany, Myotonia, MD, Polymyositis.

18. EMG Findings

19. Spontaneous Activity Insertion Activity: Fibrillation Potentials:
Positive Sharp Waves:
End-Plate Activity:

20. Distinctive EMG Potentials
Myotonic Discharge: Repetitive at rates of 20 to 80 Hz. The amplitude and frequency of the potentials must wax and wane.
Complex Repetitive Discharge (CRD): A polyphasic or serrated action potential that may begin or end abruptly. They are uniform in shape and amplitude. They may spontaneously change configuration.

21. More EMG Discharges
Myokymic Discharge: Three different myokymic discharges. To illustrate the firing pattern, the traces on the left are 7 s long and the ones on the right are 1s long.
Cramp Discharge: Arise from involuntary repetitive firing of the motor unit action potential at a high rate (up to 150 Hz). Each trace is 5 s long.

22. Normal EMG Activity
Recruitment Pattern: Recruitment refers to successive activation of the same and new motor units with increasing strength of voluntary muscle contraction.
Motor Unit Action Potentials (MUAPs): Action potentials reflecting the electric activity of a single motor unit.It is a compound action potential of those muscle fibers within the recording range of the electrode.

23. Upper Motor Neuron Lesion
Typical Findings
Insertional Activity:
Normal
Spontaneous Activity:
None
MUAPs:
Interference Pattern:
Reduced pattern with individual MUAPs firing at a slow rate

24. Lower Motor Neuron Lesion
Typical Findings
Insertional Activity:
Increased
Spontaneous Activity:
Fibrillation & Positive Waves
MUAPs:
Large, Polyphasic with reduced recruitment
Interference Pattern:
Reduced pattern with individual MUAPs firing at a fast rate

25. Myogenic Lesion Typical Findings Insertional Activity: Normal
Spontaneous Activity:
None
MUAPs:
Small, Polyphasic with early recruitment
Interference Pattern:
Full, low amplitude pattern at less than maximal effort