Heat, in general, is the enemy of myelinated nerve impulse conduction. Nerve conduction studies and EMG's are supposed to be done in an area with a constant and somewhat cool temperature, so that the patient's actual conductivity can be discerned under optimal conditions. Too much heat or humidity actually increases the resistance of the nerves to the conduction of the electrochemical impulses.

This is a well-documented phenomenon in physics--electrical conductivity reduces with temperature. Superconductivity is a low temperature phenomenon. It has to do with keeping the atomic motion down to a low enough level that the atoms line up in a way that electrical charges can traverse them in the least resistant way possible.

People with MS have known about this for a long time. Most say their symptoms increase with heat or sweaty exertion. And cooling vets are a staple in the community.