How does heat or humidity impact people with multiple sclerosis (MS)?

Heat or high humidity can cause many people with MS to experience a temporary worsening of their symptoms. Doctors believe that this occurs because heat causes nerves (whose myelin covering has been removed by MS) to conduct electrical signals even less efficiently.

For reasons that are not well understood, extremely cold temperatures and changes in temperature can also cause MS symptoms, usually spasticity (muscle stiffness), to flare.