With summer approaching, I thought that Uhthoff’s phenomenon would be an appropriate topic for discussion in our MS Signs vs. Symptoms series.

What is the Uhthoff’s phenomenon?

Originally noted by Wilhelm Uhthoff in 1890, some patients with multiple sclerosis experience a “marked deterioration of visual acuity during physical exercise.”   Observed in 4 of 100 MS patients, temporary and reversible optic symptoms (including blurriness, double vision, and blind spots) were induced by an increase in body temperature.  This phenomenon is primarily associated with previous or ongoing optic neuritis.

Does Uhthoff’s phenomenon only involve vision?

A rise in body temperature (whether from exercise, a hot bath, illness, stress, or other causes) may cause other symptoms to appear or re-emerge in patients who have a demyelinating disease.  It is not only vision which can be affected.  For example, my legs become weak and ataxic when my body becomes overheated.  I become extraordinarily fatigued which affects my physical and cognitive abitilies.

This is often called “heat intolerance” or “heat sensitivity.”

Some sources strictly associate with Uhthoff’s phenomenon only with visual symptoms, while others expand the description to include additional temporary deficits in body functions caused by damage to the central nervous system.

What is the “hot bath” test?

In the decades before magnetic resonance imaging (MRI’s) and other tests for multiple sclerosis, doctors would observe any changes in neurological symptoms occurring during or after a patient was exposed to a hot bath.  The appearance or worsening of symptoms was used as evidence of demyelinating lesions or the presence of multiple sclerosis.  These symptoms could involve motor, sensory, or visual function.

What causes Uhthoff’s phenomenon?

The exact cause or explanation of Uhthoff’s symptom is somewhat controversial.  The rise in temperature is believed to interfere directly with the conduction of axons (which carry nerve signals), to release a chemical substance that interferes with conduction (such as serum calcium, heat shock proteins, or unidentified humoral substances), or both.  In other words, we don’t really know for certain.

A simplified way of describing what happens in my own body is that the nerve signals feel as if they SLOW DOWN when I get overheated.  When I say slow down, I really mean slow as in molasses slow.  The cause of the rise in body temperature doesn’t seem to make much of a difference.  My physical therapist says that even a rise in core body temperature of one degree Fahrenheit can cause symptoms to emerge or affect the body’s function.

If I experience Uhthoff’s phenomenon, do I have MS?