Uhthoff's phenomenon (also known as Uhthoff's syndrome, Uhthoff's sign, and Uhthoff's Symptom) is the worsening of neurologic symptoms in multiple sclerosis (MS) and other neurological, demyelinating conditions when the body gets overheated from hot weather, exercise, fever, or saunas and hot tubs. It is possibly due to the effect of increased temperature on nerve conduction. With an increased body temperature, nerve impulses are either blocked or slowed down in a damaged nerve but once the body temperature is normalized, signs and symptoms may disappear or improve.
This phenomenon was first described by Wilhelm Uhthoff in 1890 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.
Clinical significance 
Many patients with MS experience increased fatigue and other symptoms such as pain, concentration difficulties, and urinary urgency when exposed to heat. As a result, many patients with MS tend to avoid saunas, warm baths, and other sources of heat or wear ice or evaporative cooling apparel in the form of vests, neck wraps, arm/wrist bands, and hats.
Peripheral nerve studies have shown that even a 0.5°C increase in body temperature can slow or block the conduction of nerve impulses in demyelinated nerves. With greater levels of demyelination, a smaller increase in temperature is needed to slow down the nerve impulse conduction. Exercising and performing activities of daily living can cause a significant increase in body temperature in individuals with MS, especially if their mechanical efficiency is poor due to the use of mobility aids, ataxia, weakness, and spasticity. However, exercise has been shown to be helpful in managing MS symptoms, reducing the risk of comorbidities, and promoting overall wellness.
Taking advantage of the cooling properties of water may help attenuate the consequences of heat sensitivity. In a study done by White et al. (2000), exercise pre-cooling via lower body immersion in water of 16-17°C for 30 minutes allowed heat sensitive individuals with MS to exercise in greater comfort and with fewer side effects by minimizing body temperature increases during exercise. Hydrotherapy exercise in moderately cool water of 27-29°C water can also be advantageous to individuals with MS. Temperatures lower than 27°C are not recommended because of the increased risk of invoking spasticity.
- Davis SL, Frohman TC, Crandall CG, et al. (March 2008). "Modeling Uhthoff's phenomenon in MS patients with inte rnuclear ophthalmoparesis". Neurology 70 (13 Pt 2): 1098–106. doi:10.1212/01.wnl.0000291009.69226.4d. PMID 18287569.
- Flensner, G.; Ek, A.C., Soderhamn, O., & Landtblom, A.M. (2011). "Sensitivity to heat in MS patients: a factor strongly influencing symtomology-an explorative survey". BMC Neurol 11: 27. PMID 21352533.
- W. Uhthoff: Untersuchungen über die bei der multiplen Herdsklerose vorkommenden Augenstörungen. Archiv für Psychiatrie und Nervenkrankheiten, Berlin, 1890, 21: 55-116 and 303-410.
- 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. PMID 7751837.
- White, A.T.; Wilson, T.E., Davis, S.L., & Petajan, J.H. (2000). "Effect of precooling on physical performance in multiple sclerosis". Mult Scler 6 (3): 176–180. PMID 10871829.
- White, L.J.; Dressendorfer, L.H. (2004). "Exercise and multiple sclerosis". Sports Med 34 (15): 1077–1100. PMID 15575796.