The worsening of neurological symptoms in multiple sclerosis and similar conditions when the body becomes overheated.

 Uhthoff described it as a temporary worsening of vision with exercise in patients with optic neuritis. Later research revealed the link between neurological signs such as visual loss and increased heat production and Uhthoff's belief that exercise was the etiology of visual loss was replaced by the conclusions of these later researchers stating that heat was the prime etiology. 

Guthrie, T.C.; Nelson, D.A. (1995). "Influence of temperature changes on multiple sclerosis: critical review of mechanisms and research potential". J Neurol Sci. 129 (1): 1–8. doi:10.1016/0022-510x(94)00248-m. PMID 7751837. S2CID 12555514.