Nerve Conduction Studies and EMG
On this page
Direct answer
Nerve conduction studies and EMG are electrophysiological tests that localise nerve and muscle disease and classify it as axonal, demyelinating or myopathic. Axonal neuropathy shows reduced response amplitudes with near-normal velocities, while demyelination shows slowed conduction, conduction block and temporal dispersion. EMG adds fibrillations and motor unit changes that separate denervation from myopathy.
What you must remember
- Motor study: compound muscle action potential (CMAP) amplitude, distal latency, conduction velocity and F-wave latency, recorded from a muscle after stimulating its nerve.
- Sensory study: sensory nerve action potential (SNAP) amplitude and velocity, which fall early in length-dependent axonal neuropathy.
- Axonal pattern: reduced amplitudes with near-normal conduction velocity, as seen in diabetic and toxic neuropathies.
- Demyelinating pattern: markedly slowed velocities, prolonged distal latencies and F-waves, and temporal dispersion, as seen in Guillain-Barré syndrome and CIDP.
- Conduction block: a proximal-to-distal amplitude or area drop of about 20 per cent or more across a segment (stricter cut-offs define definite block), typical of multifocal motor neuropathy and acquired demyelination.
- Repetitive stimulation: a decrement of more than about 10 per cent at 3 Hz suggests myasthenia gravis, while facilitation after brief exercise suggests Lambert-Eaton syndrome.
- Spontaneous activity: fibrillation potentials and positive sharp waves signal denervation, and myotonic discharges suggest a myotonic disorder.
- Myopathic units: small, short, polyphasic motor unit potentials with early recruitment suggest myopathy, while large, long polyphasic units suggest chronic neurogenic change.
Reading the pattern on the trace
Electrophysiology answers three questions: is the lesion in nerve or muscle, is the nerve axonal or myelin-based, and is the process focal or diffuse. The table sets out the main patterns side by side.
| Feature | Axonal neuropathy | Demyelinating neuropathy | Myopathy |
|---|---|---|---|
| CMAP and SNAP amplitude | Reduced | Normal unless axonal loss coexists | Normal |
| Conduction velocity | Near normal | Markedly slowed | Normal |
| Conduction block or dispersion | Absent | Often present | Absent |
| Motor unit potentials | Large, long, polyphasic, reduced recruitment | Normal unless axonal loss coexists | Small, short, polyphasic, early recruitment |
A reduced amplitude with normal velocity points to axonal loss, whereas disproportionate slowing points to myelin damage. A normal nerve study with abnormal motor units points towards a muscle disease rather than a nerve one.
Worked example: a 45-year-old woman has numbness and night pain in the thumb and index finger. Median distal motor latency across the wrist is prolonged, the sensory response is slowed across the wrist, and EMG of the forearm muscles is normal, so the diagnosis is focal median neuropathy at the wrist.
Where the examiner tests the interpretation
Questions usually give a short NCS summary and ask for the pattern or the diagnosis.
- Trap: Calling a reduced amplitude with normal velocity demyelination. Correct: Reduced amplitude with near-normal velocity points to axonal loss, while demyelination produces disproportionate slowing and block.
- Trap: Reading a normal EMG as excluding neuropathy. Correct: Early or mild neuropathy can have a near-normal study, so the findings must match the clinical distribution.
- Trap: Labelling a decremental response as neuropathic. Correct: A decrement at 3 Hz suggests a neuromuscular junction disorder such as myasthenia gravis, not a nerve lesion.
Frequently asked questions
How do axonal and demyelinating neuropathies differ on nerve conduction studies?
Axonal and demyelinating neuropathies differ on nerve conduction studies by the pattern of conduction. Axonal neuropathy shows reduced response amplitudes with near-normal velocities, whereas demyelinating neuropathy shows markedly slowed velocities, prolonged distal latencies and conduction block. Temporal dispersion also favours demyelination.
What does conduction block indicate on nerve conduction studies?
Conduction block indicates a segment where nerve impulses fail to conduct, seen as a sharp amplitude drop when a nerve is stimulated proximally. It is characteristic of acquired demyelinating conditions such as Guillain-Barré syndrome and multifocal motor neuropathy. The finding helps localise a demyelinating lesion within the nerve.
What EMG finding suggests denervation?
Fibrillation potentials and positive sharp waves on EMG indicate denervation of muscle fibres. These spontaneous discharges are seen in the resting muscle, without any voluntary effort. Their presence, together with large, long motor unit potentials, suggests a neurogenic cause rather than a myopathy.
What does a decremental response on repetitive stimulation suggest?
A decremental response on repetitive nerve stimulation suggests a disorder of the neuromuscular junction, most often myasthenia gravis. The muscle response falls by more than about 10 per cent at a low stimulation rate of 3 Hz. This points to a junctional problem rather than a nerve lesion.