Concept
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 demyelinating diseases when the body is overheated. This may occur due to hot weather, exercise, fever, saunas, hot tubs, hot baths, and hot food and drink. Increased temperature slows nerve conduction, but the exact mechanism remains unknown. With an increased body temperature, nerve impulses are either blocked or slowed in a damaged nerve. Once the body temperature is normalized, signs and symptoms typically reverse. Symptoms of Uhthoff's phenomenon occur when exposed to heat, and include: fatigue pain concentration difficulties urinary urgency worsen of existing optic neuropathy (although optic neuropathy may occur for the first time) muscle stiffness dizziness and unsteadiness Uhthoff's phenomenon is caused by a raised body temperature. This may be caused by: hot weather exercise fever saunas sun tanning hot tubs, and hot baths and showers hot food and drink menstruation (which may raise body temperature) sitting near a radiator The exact mechanism of Uhthoff's phenomenon is unknown. It causes a decrease in the speed of action potentials in the central nervous system (CNS). Heat may increase the time when voltage-gated sodium channels are inactivated, which delays further action potentials. This is worsened by the demyelination caused by MS. Other theories have considered the role of heat shock proteins and changes to blood flow. 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 normal daily activities 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.