Somatotopy of perceptual threshold to cutaneous electrical stimulation in man

Neurological testing tools for measuring and monitoring somatosensory funct ion lack resolution and are often dependent on the clinician testing. In th is study we have measured perceptual threshold (PT) to electrical stimulati on of the skin and compared it with two-point discriminative ability (TPDA) in 12 control subjects. Tests were made on both sides of the body at Ameri can Spinal Injury Association (ASIA) key points on seven spinal dermatomes (C3 (neck), C4 (shoulder), C5 (upper arm), C6 (thumb), T8 (abdomen), L3 (kn ee), L5 (foot)) and in the mandibular (chin) and maxillary (cheek) fields o f the trigeminal (V) nerve. Electrical stimulation (0.5 ms pulse width; 3 H z) was applied via a self-adhesive cathode and an anode strapped to the wri st or ankle. The stimulus intensity was adjusted and PT was recorded as the lowest current at which the subject reported sensation. Sites were tested in random order. Indices for both TPDA and PT differed according to the der matome tested but there was no correlation between TPDA and PT for any derm atome. There was good correlation between results from equivalent dermatome s on left and right sides for both PT and TPDA. Women frequently had lower mean (+/- S.E.) PTs and better TPDA than men; differences were significant (P < 0.05) for PT on the knee (women, 1.31 +/- 0.15 mA; men, 2.05 +/- 0.26 mA) and the foot (women, 2.90 +/- 0.19 mA; men, 4.13 +/- 0.28 mA) and for T PDA on the thumb (women, 3.8 +/- 0.2 mm; men, 7.8 +/- 1.3 mm) and the knee (women, 17.8 +/- 1.6 mm; men, 27.1 +/- 4.0 mm). Four subjects repeated the experiment on another day and the results correlated well with the first te st for PT (r(2), 0.62) and TPDA (r(2), 0.48). PT differs between dermatomes in a predictable way but does not relate to TPDA. PT is easy to measure an d may be a useful assessment tool with which to monitor recovery or deterio ration in neuropathies, neurotrauma or after surgery.