The influence of temperature on conduction block.

In nerve conduction studies, it may be difficult to distinguish conduction block from increased temporal dispersion. We assessed whether a better distinction is possible by warming, because this increases the number of blocked nerve fibers and decreases temporal dispersion. We investigated 11 patients with conduction block in the median or ulnar nerve. Motor conduction was studied at 25 degrees C and at 40 degrees C. The amplitude reduction and area reduction were significantly greater at 40 degrees C than at 25 degrees C. Significantly more nerve segments fulfilled criteria for conduction block at 40 degrees C than at 25 degrees C. The duration prolongation was slightly smaller at 40 degrees C. These results indicate that, in demyelinating neuropathies, an increase in temperature increases the number of blocked nerve fibers and decreases temporal dispersion. Conduction block may therefore be missed when the investigated limb is too cold. Based on our study, we also propose a criterion for conduction block that is based on the influence of temperature.