Post polio syndrome (PPS) affects people who have had the poliomyelitis virus, or polio, anywhere from 10 to 40 years before. Of the 300,000 polio survivors in the US, one-quarter to one-half will have symptoms of PPS. If the initial bout with polio was severe, there is a greater chance of developing post polio syndrome. There is also a greater chance of developing more severe PPS symptoms.

What is going on in the body?

Individual nerve terminals in the motor units die, causing deterioration. Muscles that were already weakened by the first bout with the polio virus become even weaker.

PPS is usually not life-threatening unless breathing is impaired. Post polio syndrome is not a recurrence of polio. It is a response to polio, rather than a part of it.

What are the causes and risks of the disease?

The cause of PPS is the death of individual nerve terminals in the motor units that remain after a bout of polio.

The deterioration may actually result from the process of recovery from polio. During recovery, surviving motor neurons sprout new endings. This allows the body to regain function. The motor units become large and add stress to the body. The person may have normal function for quite some time, even years. But the body may not be able to meet the metabolic demands of all the new sprouts. A slow deterioration can develop. Nerves may be restored again, but eventually the nerve terminals are destroyed. The body develops permanent weakness.

This hypothesis fits with the unpredictable course of post polio syndrome. Post polio syndrome progresses very slowly and there are long periods of stability.