EMG and Nerve Conduction Studies | The Massachusetts General Hospital Handbook of Neurology

Authors: Flaherty, Alice W.; Rost, Natalia S.

Title: Massachusetts General Hospital Handbook of Neurology, The, 2nd Edition

> Table of Contents > Adult Neurology > EMG (Electromyography) and Nerve Conduction Studies

EMG (Electromyography) and Nerve Conduction Studies

Table 7. EMG findings in neuromuscular diseases.

	Conduction Velocity	Amplitude	F & H Latency	Distal Latency	Fibrillations
Axonal damage	>70%		~	nl
PNS demyelination	<50%	~		~	Variable
LMN dz	>70%	motor	~c	nl
UMN dz	nl	nl	nl	nl	None
Radiculopathy	>80%	~		nl
NMJ dz	nl	nl/	nl	nl	None
Myopathy	nl	~	nl	nl	None

A. Ordering tests

Always give tentative dx; specify muscles, nerves of interest.

B. Nerve conduction studies (NCS)

Measure electrical conduction along a nerve orthodromically (stimulate near cell bodies, record near terminals) or antidromically (vice versa).

1. Motor NCS: Stimulate motor nerve and record muscle CMAP (compound muscle action potential), i.e., the sum of all axons directed to the muscle. CMAP depends on axon, NMJ, and muscle fibers.

P.41
- a. Amplitude: Reflects the number of conductive muscle fibers (therefore axons). It is decreased by damage to motor unit (axon + muscle fibers).
- b. Distal latency: Time between stimulus and recording, depends on conduction of axons (not NMJ or muscle). Slowed in demyelination.
- c. Conduction velocity: Divides the distal latency between two locations along the same nerve by the distance between them. Conduction block is the hallmark of demyelination.
2. Sensory NCS: Stimulate a nerve and record its sensory nerve action potential (SNAP) from purely sensory portion of the nerve. The sural nerve is purely sensory.
3. Late responses:
- a. F-wave: Motor stimulus travels both ortho- and antidromically. Antidromic stimulus reflected through neuron and back; creates small F-wave seen after first (M) wave. F-wave delayed in proximal neuropathies (e.g., DM, GBS) and also in motor neuron dz and radiculopathy.
- b. H-reflex: CMAP from stimulation of sensory nerve, via monosy naptic reflex arc. Always tested in tibia: dorsal and ventral S1 roots. H-reflex delayed and small if proximal sensory or motor axon damage.
4. Repetitive stimulation: Abnormalities are hallmark of NMJ dz.
- a. Myasthenia: Baseline CMAP normal, but slow repetitive 3-Hz stimulation results in decrement of CMAP if pt. is at rest. With exercise, CMAP can improve for first min, but then worsen in 4-6 min. Abnormal in only half of myasthenics. Single-fiber EMG is more sensitive but less specific.
- b. Lambert-Eaton syndrome: Baseline CMAP small and gets smaller with repetitive 3-Hz stimulation. With exercise, CMAP size greatly increases in a few sec but then worsens in 2-4 min.
- c. Botulism: Baseline CMAP very small; gets smaller with repetitive 3-Hz stimulation; no change with exercise.

C. Electromyography (EMG)

AKA single-needle study. Evaluates individual motor units by inserting electrode into muscle.

1. Insertional activity: Brief burst of AP following insertion into resting muscle. Increased in early denervation, some myopathies (especially inflammatory myopathies), myotonia. Decreased in muscle fibrosis, fatty replacement, or periodic paralysis during the paralysis.
2. Spontaneous activity: Normal resting muscle has no spontaneous activity except end-plate noise.
- a. Configuration = duration, amplitude, number of phases
  - 1) Duration: Long in LMN and neuropathy due to axon sprouting. Short in myopathies and early reinnervation.
  - 2) Amplitude: High in chronic neurogenic dz.
  - 3) Polyphasic: Neurogenic and myopathic dz.
- b. Recruitment: Effort recruits additional motor units. Low in LMN or nerve dz. Early recruitment in myopathies. Poor activation has brain cause, e.g., stroke, poor effort.
- c. Fibrillations: Spikes, positive sharp waves. Seen 2-3 wk after denervation.
- d. Fasciculations: Discharge of an entire MU. Seen in LMN dz.
- e. Myotonia, myokymia: Complex repetitive discharges.