The Influence of Temperature on Conduction Block.

Franssen H, Wieneke G, Wokke J (Departments of Clinical Neurophysiology and Neurology, Rudolf Magnus Institute for Neuroscience, Utrecht, The Netherlands), Muscle Nerve. 1999;22:166-173.

The purpose of this study was to distinguish between conduction block and temporal dispersion of demyelinated nerves when the nerve temperature is raised. The study used nerve conduction studies at 25 [degrees] C and 40 [degrees] C to examine these differences. The examiners defined conduction block as a tailed propagation of a nerve action potential at a given site along a myelinated nerve fiber with an intact neuron. They differentiated conduction block from delayed conduction that often results in temporal dispersion of action potentials in affected nerve fibers. Conduction block and delayed conduction are usually caused by focal demyelination, and both mechanisms may reduce compound muscle action potential (CMAP) amplitude or area.

According to the authors, differentiating between both mechanisms is important "because conduction block explains lack of function and because the finding of conduction block is required for the diagnosis of definite multifocal motor neuropathy." Abnormal amplitude reduction was defined as a reduction of at least 30% of the CMAP alter proximal stimulation was compared to distal stimulation.

The study subjects were 11 patients, 4 women and 7 men, who had an abnormal amplitude reduction in at least one median or ulnar nerve segment recorded by routine electromyography. Seven patients had multifocal motor neuropathy, 2 had chronic inflammatory demyelinating neuropathy, and 2 had entrapment neuropathy of the ulnar nerve at tire elbow. In each subject, the influence of temperature was assessed in the nerve segment with the abnormal amplitude reduction and in at least one other segment. This arrangement permitted the examiners to study different nerve segments with varied amplitude reductions over a continuous range. Overall, 31 nerve segments were studied.

Each arm was immersed in water at 25 [degrees] C for 45 minutes. It was then taken out of the water so that skin temperature and nerve conduction measurements could be performed. The arm was then warmed in water at 40 [degrees] C lot 45 minutes, and the skin temperature and nerve conduction measurements were repeated. The amplitude, duration, and area of the negative phase of each CMAP were measured for each nerve segment. The reduction in amplitude or area of the CMAP was calculated. A Student t test was used to compare values at 25 [degrees] C and 40 [degrees] C; a Spearman's rank test, to determine the correlations, and McNemar's test, to compare the numbers of nerve segments that fulfilled criteria for conduction block or temporal dispersion.

The results of the study indicated an increase in temperature that produced several well-known effects on all nerve conduction variables. The amplitude, area, and duration of the CMAP were significantly smaller at 40 [degrees] C than at 25 [degrees] C. The nerve conduction velocity was significantly greater at 40 [degrees] C than at 25 [degrees] C. When these results were compared to criteria from the literature, it was found that significantly more nerve segments met the criteria for conduction block at 40 [degrees] C compared with 25 [degrees] C. Duration of prolonged CMAPs also decreased an average of 8% at the higher temperature; this change, however, was not statistically significant.

According to the investigators, the results indicated that, in demyelinating neuropathies, an increase in temperature increases the number of blocked fibers and decreases temporal dispersion. The authors believed that conduction block might be missed when the assessed limb is too cold. They recommended warming a limb to 40 [degrees] C when conduction block is suspected but cannot be proved.

Raymond E Hogue, PhD, P77 ECS Joplin, Mo