Electromyography and electroneurography
Electromyography (EMG) and electroneurography (ENoG, also called nerve conduction studies (NCS)) are both neurophysiological studies. EMG measures the electrical activity of muscles in response to neural stimulation, while NCS records the conduction of action potentials along the nerve fibres.
These investigations are sensitive but not disease specific. They provide information as to whether the patient’s problem are due to problems with the muscle, nerve, or the neuromuscular synapse, but cannot say what disease is causing the problem. They complement each other and are often performed in the same setting.
Indications
- Differentiate neuropathic from myopathic muscle weakness
- Evaluation of myopathy and neuropathy
Nerve conduction study is useful in the evaluation of peripheral nerves, plexuses, and nerve roots. A variant of the nerve conduction study is the repetitive nerve stimulation (RNS) study, which is useful in the evaluation of disorders of the neuromuscular junction.
Electromyography is useful in evaluation of both myopathy and neuropathy. A variant of EMG, called single fibre EMG, is useful for neuromuscular junction disorders as well.
Procedure
During EMG, either surface or intramuscular needle electrodes are used. Needle electrodes are inserted into different sites of the muscle. The electrical activity of the muscle is measured both during rest and during active movement. For regular EMG, concentric needle electrodes are used, which measure motor unit action potentials (MUAPs). For single fibre EMG, a special single fibre electrode is used.
During NCS, surface electrodes are placed at two or more points along a nerve or muscle, which measure compound muscle action potentials (CMAPs) or sensory nerve action potentials (SNAPs). One of these electrodes applies an electrical stimulus to the nerve, and the others measure various parameters of the resulting conducted action potential.
EMG findings
| Findings | Disorder |
|---|---|
| Spontaneous fibrillation and positive sharp waves | Denervation |
| Complex repetitive discharges | Denervation |
| Fasciculations | Denervation |
| Myokymia | Denervation |
| Myotonic discharge | Myotonia |
| Increased jitter (only in single-fibre EMG) | Reduced NMJ tramission time (myasthenia gravis) |
| Short, small, polyphasic MUAPs | Myopathy |
NCS findings
| Finding | Disorder |
| Reduced amplitude (of CMAP or SNAP) | Axonal degeneration |
| Reduced conduction velocity | Demyelination |
| Temporal dispersion | Demyelination |