In a demyelinating disease (e.g. multiple sclerosis), symptoms worsen with hot bath and improve with decreased temperature because:

In demyelinating diseases after myelin is degraded, there is redistribution of Na+ channels along the nerve (normally present at nodes of Ranvier). Lower temperature alters the Na+ channel kinetics in a way to delay the closure, thus extra Na+ can come into the cell, which enable electronic conduction upto the point on the nerve where the next Na+ channel is. At higher temperature: there is increased leakage of charges from the cell membrane k/o increased capacitance of the nerve.