latency, amplitude, and conduction velocity

Every wave the nerve study records is read off using three simple numbers, the same three you would care about if you were testing a courier service: how long until the package arrives, how much of the shipment got there, and how fast the truck drove. In electrodiagnosis these are latency, amplitude, and conduction velocity, and together they describe almost everything a nerve study can say.

Latency is the time from the shock to the start of the response, measured in milliseconds; a long latency means the signal was delayed, usually because the insulation (myelin) is damaged and slowing things down. Amplitude is the height of the wave, in millivolts for muscle responses or microvolts for sensory ones; it reflects how many working fibres responded, so a small amplitude usually means fibres have been lost (axon loss). Conduction velocity is calculated by stimulating at two points a known distance apart and dividing distance by the difference in arrival times; it gives the true speed over a stretch of nerve, again mainly a measure of insulation. Slow speed and long latency tell the same story from different angles.

The art of the study is reading these three together. Slow speed with preserved amplitude says the wiring is intact but poorly insulated, the demyelinating pattern. Normal speed with shrunken amplitude says the insulation is fine but wires have been lost, the axonal pattern. A focal slowing confined to one short segment localizes a compression. The whole skill of localizing a lesion and naming the type of nerve injury rests on which of these numbers is off and where.