[Effect of body size, arm length, sex and temperature on somatosensory evoked potential latency].

Beside the technical factors different biological factors contribute to the variability of the somatosensoric evoked potentials. In 30 volunteers the dependence of the latency of the spinal components N9, N13 (recorded at C7), of the cortical component N20 and of the interpeak interval N13-N20 from the body size and length of the arm was investigated. A significant dependence at the 1% level (p less than 0.001) was found for the latencies of N9, N13 and N20, whereas the interpeak interval N13-N20 was significant at the 5% level (p less than 0.05). Similarly in 20 further volunteers a significant dependence of the components N1, P1, N2, P2 and N3 from the body size at the 1% (p less than 0.001) after distal stimulation of the tibial nerve was observed. Using a covariance analysis it could be shown, that even after a body size correction, in females shorter latencies occur than in males. Furtheron we studied the effect of the temperature on the SSEP-latencies, by heating the upper and lower extremity stepwise from 31 degrees C to 37 degrees C using an infrared (DISA-Regler-System) lamp. As no significant dependence could be observed, we believe that the effect of temperature on latencies is so small, that in usual conditions no correction is needed. On the contrary a correction with body size or arm length is necessary, to avoid wrong positive or negative results.