SENSORY CONDUCTION STUDIES - PARAMETERS

Sensory axons are evaluated by stimulating a nerve while recording the transmitted potential (known as sensory nerve action potential or SNAP) from the same nerve at a different site. (See orthodromic and antidromic studies)
SNAPs are of much lower amplitude (measured in millivolts) than compound muscle action potentials, and they often require averaging of multiple responses. Three main parameters recorded with sensory nerve conduction studies are – latencies, amplitudes and conduction velocity.

LATENCY

Latencies reflect time taken (in milliseconds) for an impulse to travel from the point of stimulation to the recording electrode. Two types of sensory distal latencies are used – peak latency and onset latency.

SNAP AMPLITUDE
This is a semiquantitative measure of the number of sensory axons that conduct between the stimulation and recording sites. It is expressed in microvolts.

CONDUCTION VELOCITY
This requires stimulation at a single site only because unlike CMAP, SNAP is true nerve action potential. (See motor conduction studies – parameters).

Reference:

1. Aminoff, MJ. Electrodiagnosis in Clinical Neurology: Nerve conduction studies, New York: Churchill Livingston, 4th edition
2. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition