Uhthoff’s phenomenon (Heat Intolerance)

Many people with MS experience is a marked sensitivity to increased body temperature. Demyelinated fibers in the central nervous system can be very sensitive to even small elevations of core body temperature resulting in conduction delays or even conduction block. This sensitivity can be brought out by sunbathing, exercise, hot baths, emotion, fatigue, fever, or any other factor associated with an increase in body core temperature. Any MS symptom can present this way, and one of the most common presentations is the blurring of vision after physical activity.

Because these symptoms disappear with rest and cooling, they can be confusing to health care professionals unfamiliar with this phenomenon. Heat induced weakness presents safety concerns for people with MS because the impact of the weakness can be quick and dramatic, and they may find themselves too weak to extricate themselves from the hot conditions causing the problem. Avoidance is the best defense, and when unavoidable, minimum exposure followed by cooling strategies should be sought. Generally the effects of heat exposure are reversed with rest and cooling and do not carry a long term consequence.

Fluctuating Visual Impairment (Changes with Exercise, Stress, Fatigue, or Raised Body Temperature)
Many patients may experience a dimming or obscuring of vision associated with exercise and heat. This is relieved by rest and cooling. This symptom was first described by Uhthoff.

Key Healthcare Professionals:
Neurologist

Symptom Management:
Cooling, management of fever, if present. Symptoms may be motor or sensory, and are reversible with cooling and rest. Any MS symptom can present this way, in the presence of heat and humidity.

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