EMG

Theory of NCS/EMG

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.

Localize the Problem

• Nerve

• Root

• NMJ

• Muscle

• Axonal• Segmental• Focal

• Pre-synaptic• Post-synaptic

• Neuropathy• Myopathy• Upper Motor Neuron

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.

Nerve Studies

• 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

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.

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.

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.

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

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.

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: 10-100 V

Duration: 1.5-2.5 ms

20-30% amplitude loss between stimulus sites.

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.

H-waves• A mono-synaptic response

analogous to the Achilles tendon tap reflex.

• Do not vary in latency.

• Must be larger than M-wave.

Blink Reflex

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.

Needle Exam

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

Needle Studies

• Routine needle EMG

SPA, MUP, MVA: Radicular lesions, Axonal degeneration, Muscle weakness

• Quantitative EMG

QMUP, AMUP, IPA: Axonal degeneration, muscle weakness

• SFEMGSFEMG: MG, Myasthenic Syndrome, Botulinum intoxication, Tetany, Myotonia, MD, Polymyositis.

EMG Findings

Spontaneous Activity

• Insertion Activity:

• Fibrillation Potentials:

• Positive Sharp Waves:

• End-Plate Activity:

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.

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.

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.

Upper Motor Neuron Lesion

Typical Findings

• Insertional Activity:

Normal

• Spontaneous Activity:

None

• MUAPs:

Normal

• Interference Pattern:

Reduced pattern with individual MUAPs firing at a slow rate

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

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