Nerve Conduction Study Basics: Latency, Amplitude and Velocity
Nerve conduction studies (NCS) are the foundation of electrodiagnosis. Knowing the basic measurements and the common pitfalls makes reports much more reliable.
Motor NCS
The nerve is stimulated at two or more points and the compound muscle action potential (CMAP) is recorded over a muscle.
- Distal latency. Time from stimulation to the response.
- CMAP amplitude. Reflects the number of functioning motor axons.
- Conduction velocity. The distance between stimulation points divided by the latency difference.
Sensory NCS
The sensory nerve action potential (SNAP) is recorded directly from a sensory nerve. SNAPs are small (microvolts), so a good amplifier with low noise matters.
Interpreting patterns
| Finding | Suggests |
|---|---|
| Low amplitude, near-normal velocity | Axonal loss |
| Slow velocity, prolonged latency | Demyelination |
| Focal slowing across a segment | Entrapment (e.g., median nerve at wrist) |
| Conduction block | Acquired demyelinating neuropathy |
Late responses
- F-waves. Assess proximal conduction and are useful in radiculopathy and Guillain-Barré syndrome.
- H-reflex. The electrical equivalent of the ankle reflex, useful in S1 radiculopathy.
Technical pitfalls
- Cold limbs. Warm limbs before testing (around 32 °C or more at the skin).
- Submaximal stimulation. Always confirm a supramaximal response.
- Measurement errors. Measure distances carefully with a tape.
- Electrode placement. Place the active electrode over the motor point.
- Stimulus artifact. Reduce it by repositioning the ground and rotating the anode.
Equipment features that help
Auto-markers for latency and amplitude, built-in protocols and normal values, and split-screen comparison save time. The NEUROSTIM NS-2 includes these features in a portable USB-powered system.
RS Enterprises provides application training with every EMG installation. Contact us.
EMG 2 Channel Neurostim Machine
2 channel EMG / NCV / EP system for clinics and colleges. ₹2 lakh – ₹2.75 lakh
Frequently Asked Questions
Why must limbs be warm for NCS?
Cold slows conduction and can falsely suggest neuropathy; warm limbs before testing.
What is supramaximal stimulation?
Stimulating slightly above the intensity that gives a maximal response, ensuring all fibres are activated.
What is a CMAP?
Compound muscle action potential — the summed motor response recorded over a muscle.
This article is for general information for healthcare buyers and professionals and is not medical advice. Clinical decisions should be made by qualified practitioners.
