Uhthoff's phenomenon

Uhthoff's phenomenon is the worsening of neurologic symptoms in multiple sclerosis after periods of exercise and increased body heat.

History
First described by Wilhelm Uhthoff in 1890 as a temporary worsening of vision with exercise in patients with optic neuritis, later work revealed the link between exercise and increased heat production. The phenomenon can occur with any neurologic deficit caused by demyelination and is possibly due to the effect of increased temperature on nerve conduction.

Clinical significance
Many patients with MS experience increased fatigue and other symptoms related to MS when exposed to heat. As a result, many patients with MS avoid saunas, warm baths, and other sources of heat.