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Nerve conduction studies (NCS), F wave, H reflex and repetitive stimulation
In a conduction study, amplitude tells how many axons are working, and latency and conduction velocity tell about the myelin — so axonal damage lowers amplitude, and myelin damage slows conduction, prolongs latency or produces conduction block.
Why it matters in the exam
A motor study records a CMAP from a muscle after stimulating the nerve at several points; a sensory study records a SNAP. Normal conduction velocity is roughly above 50 metres per second in the upper limbs and above 40 in the lower limbs. Demyelinating damage: marked slowing (usually below 75% of the lower limit), long latency, temporal dispersion, and conduction block — a fall of more than 50% in amplitude or area between distal and proximal stimulation. Stimulation must be supramaximal; weak stimulation mimics a fall in amplitude, and overly strong stimulation can activate a neighbouring nerve.
Key point: in a root lesion (preganglionic, for example root avulsion) the SNAP is preserved despite sensory loss, because the sensory cell body in the dorsal root ganglion is intact; in a plexus or nerve lesion (postganglionic) the SNAP falls. Low temperature slows conduction and raises amplitude, so the limb is warmed. After a nerve is cut, the distal segment keeps conducting for a few days until Wallerian degeneration develops — motor amplitude falls first and sensory afterwards. The Martin-Gruber anatomical variant (a median-to-ulnar connection in the forearm) can look like a false conduction block.
Late responses: the F wave arises from antidromic stimulation reaching the anterior horn cells and returning — it tests the proximal segment, varies from stimulus to stimulus, and in early Guillain-Barré syndrome it may be prolonged or absent while distal studies are still normal. The H reflex is the electrical counterpart of the ankle reflex (S1): stimulation of Ia sensory fibres, one synapse and a motor response in soleus; it is prolonged or absent in S1 radiculopathy and polyneuropathy. Low-frequency repetitive stimulation (2–3 Hz): a fall of more than 10% in amplitude in myasthenia gravis. In Lambert-Eaton syndrome the CMAP is low at rest and rises sharply after brief exercise or high-frequency stimulation; in botulism — a similar picture but with a more modest rise.
Key points
- Amplitude — axons; velocity and latency — myelin.
- Conduction block — a fall of more than 50% between distal and proximal stimulation.
- The SNAP is preserved in a root lesion.
- F wave — proximal segment; H reflex — S1.
- Myasthenia — decrement on slow stimulation; Lambert-Eaton — increment after exercise.
Summary table
| Finding | Axonal damage | Demyelinating damage |
|---|---|---|
| Amplitude | Falls | Relatively normal (unless there is block) |
| Conduction velocity | Normal or mildly slowed | Markedly slowed |
| Latency | Normal or mildly prolonged | Prolonged |
| Conduction block and temporal dispersion | Absent | Present |
| Examples | Diabetic polyneuropathy, severe trauma | Guillain-Barré, CIDP, focal compression |
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Sources
- AANEM: Proper performance and interpretation of electrodiagnostic studies (position statement), 2014 — Nerve conduction studies
- Dumitru's Electrodiagnostic Medicine, 2nd edition (2002) — Nerve conduction studies, late responses and repetitive stimulation
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.