Increased visual impairment after exercise (Uhthoff's phenomenon) in multiple sclerosis: therapeutic possibilities.

The Uhthoff symptom, a transient impairment of visual function after exercise, is demonstrated in 2 multiple sclerosis patients. Following exercise, impairment of visual function, as documented most clearly by the testing of contrast sensitivity, was less obvious after body surface cooling and after treatment with orally administered 4-aminopyridine. It is hypothesized that both treatment modalities improve the nerve conduction safety factor and thereby prevent the occurrence of a conduction block, which is believed to be the mechanism underlying the Uhthoff symptom.