Nerve Conduction Velocity Calculator
Calculate nerve conduction velocity from electrophysiology measurements.
Why Two Stimulation Points Are Needed
Measuring conduction velocity requires subtracting one latency from another. A single distal measurement includes the delay at the neuromuscular junction and within the muscle itself — time that has nothing to do with nerve conduction.
Subtracting the two latencies cancels those shared delays, leaving only the time to travel the segment between the stimulation points. This is why distal motor latency alone cannot give a velocity.
| Nerve type | Normal NCV (m/s) | Note |
|---|---|---|
| Upper limb motor | > 50 | Median, ulnar |
| Lower limb motor | > 40 | Peroneal, tibial — longer, slightly slower |
| Sensory | > 50 | Often affected earliest |
Demyelination Versus Axonal Loss
This is the central distinction in nerve conduction studies, and the two patterns are quite different.
| Feature | Demyelinating | Axonal |
|---|---|---|
| Conduction velocity | Markedly slowed (< 70% normal) | Normal or mildly reduced |
| Amplitude | Relatively preserved | Reduced |
| Distal latency | Prolonged | Normal |
| Conduction block | May be present | Absent |
| Examples | Guillain–Barré, CIDP, CMT1 | Diabetes, toxic, most axonal neuropathies |
The logic is structural. Myelin provides saltatory conduction, so losing it slows transmission dramatically while the axons still carry signal — velocity falls, amplitude holds. Axonal loss removes conducting fibres entirely, so fewer signals arrive but the surviving fibres conduct at normal speed — amplitude falls, velocity holds.
Temperature matters more than most realise: conduction slows roughly 2 m/s per degree of cooling. A cold limb can produce apparently pathological velocities, which is why limb temperature is controlled during testing.
Worked Examples
Common Mistakes
Distal latency includes neuromuscular junction delay. Two stimulation points are required so those shared delays cancel.
Conduction slows about 2 m/s per degree of cooling. A cold limb can make normal nerves appear pathologically slow.
Mild slowing occurs with axonal loss too, because the fastest fibres are lost first. Marked slowing below 70% of normal, with preserved amplitude, indicates demyelination.
Amplitude distinguishes the two patterns as much as velocity does. Reduced amplitude with normal velocity points to axonal loss.