Uhthoff’s phenomenon is when the vision problems associated with multiple sclerosis worsen temporarily following an increase in body temperature from activities such as exercising or bathing in hot water.

Multiple sclerosis (MS) is a disease that affects the protective covering of the nerves, called the myelin sheath. The myelin sheath conducts electrical signals and protects the nerves.

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Uhthoff’s phenomenon causes the symptoms of optic neuritis to temporarily worsen.

MS causes scarring on the myelin sheath, resulting in plaque and lesions. This scarring causes nerve fiber damage and loss of certain necessary body functions, including vision. Uhthoff’s phenomenon is when changes in body temperature make a person’s existing MS symptoms worse.

Neuro-ophthalmologist Wilhelm Uhthoff first identified the condition in 1890, noting that the phenomenon occurred in those with optic neuritis.

MS causes nerve damage and nerves relay information to and from the brain. Optic neuritis occurs when the optic nerve, which is responsible for relaying information from the eye to the brain, is damaged.

Common MS symptoms include:

  • muscle weakness
  • changes in vision
  • balance and coordination changes
  • numbness and tingling
  • memory changes

Uhthoff’s phenomenon causes the symptoms of optic neuritis to worsen temporarily. These symptoms can affect one or both eyes, and include:

  • blurred vision
  • worsening vision
  • eye pain when moving the eye
  • visual disturbances, such as flashes of light

The worsening of vision can occur quickly, over the course of a few hours, or may take up to a week.

A 2011 study found that heat sensitivity could also make other symptoms of MS worse, including:

  • fatigue
  • pain
  • difficulty concentrating
  • urinary problems

Heat sensitivity affects upwards of 60-80 percent of people diagnosed with MS.

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Sudden changes in temperature, including bathing in hot water, could trigger symptoms.

Activities that may increase the incidence of overheating include:

  • exercise
  • sunbathing or being outdoors in hot weather
  • bathing in hot water
  • strong emotional responses or experiences
  • fever
  • use of a hairdryer
  • hormonal changes in women
  • hot flashes during menopause

Uhthoff’s phenomenon is a temporary condition and typically resolves once the body’s temperature has returned to normal. There is no treatment specifically for Uhthoff’s phenomenon, but there are many steps a person can take to manage symptoms.

A person should try not to panic, as the symptoms of Uhthoff’s phenomenon are temporary. It may help to call a loved one, especially if a person’s vision is impaired. The visual symptoms will improve with rest and time.

The best way to prevent an episode of Uhthoff’s phenomenon is to avoid overheating and remain cool.

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Trying to stay in the shade and protecting the eyes from the sun will help to prevent Uhthoff’s phenomenon.

A person can cool down by:

  • drinking cold liquids
  • sucking on ice cubes
  • taking a cool bath or shower
  • finding shade when possible
  • using a fan or air conditioner
  • wearing special body-cooling garments
  • avoiding hot environments, including warm houses, stores, saunas, and hot tubs
  • wearing light colored, loose fitting clothes in the summer
  • wearing a wide-brimmed hat to shield the eyes

Some people with MS do not experience Uhthoff’s phenomenon or any changes in their symptoms after a change in body temperature. If a person is not sensitive to heat, they will not need to make any of the preventative changes above.

A person with MS or optic neuritis should speak to a doctor if their symptoms worsen after a change in body temperature, especially if it is the first time.

Someone who has experienced Uhthoff’s phenomenon and who knows what to expect and how to cool down may not require medical care.

A person experiencing Uhthoff’s phenomenon should also tell a doctor if:

  • the symptoms do not improve with time
  • there are accompanying symptoms not related to MS
  • they have not been diagnosed with MS