Electromyography (EMG)
Electromyograms test motor units and peripheral sensory units. It is useful in the differential diagnosis of conditions and diseases of the anterior horn, peripheral nerve, neuromuscular junction, and muscle.
It is comprised of two complimentary tests.
- nerve conduction studies
- needle examination
Nerve Conduction Studies
Nerve conduction is evaluated by placing an electrode over a peripheral nerve, stimulating the nerve, and measuring the resulting action potential. Measurement is done distally for motor nerves and proximally for sensory nerves.
Sensory nerve action potential (SNAP) and compound muscle action potential (CMAP) are calculated to assess nerve function. These tests can show decreased amplitude in axonal degenration or conduction slowing in demyelination.
Repetitive stimulation studies can be used to pick up neuromuscular junction problems, such as myesthesia gravis, botulism, and Lambert Eaton syndrome.
Needle Examination
In needle examination, an electrode is inserted into a muscle fibre to measure electrical activity. Normal muscle is silent at rest. Spontaneous activity during relaxation is called fibrilation in a single fibre and fasciculation in a muscle group. This can suggest myotonic disorders, inflammatory myopathies, or muscle denervation.
Denervation results in reinnervation by other motor neurons, resulting in characteristic patterns of increased amplitude, longer duration, and polyphasic form.