Without impulses from these nerve cells, a muscle cannot function. Fortunately, some anterior horn cells survive the polio and send out new nerve connections to the orphaned muscle cells in an attempt to take over the function of the nerve cells that were destroyed. This process enables the patient to regain use of muscles and recover. However, after many years, these overburdened nerve cells may begin to fail, resulting in new muscle weakness. While researchers don't fully understand the process, many now believe that PPS symptoms result, at least in part, from the unusual stress placed on surviving nerve cells.
Some research suggests that the normal aging process also plays a role.
After the age of 60, most people have a decrease in the number of anterior horn cells in their spinal cords. Persons who have not had polio can lose a considerable number of anterior horn cells as they age without experiencing any serious muscle weakness. However, among polio survivors who have already lost a considerable number of anterior horn cells, this age-related loss may further contribute to muscle weakness. Researchers have found no conclusive evidence that PPS is related to reactivation of a polio virus. However, as viral fragments have been found in some individuals with PPS, researchers are looking at whether the body's immune response to these fragments may play some role in triggering symptoms in some individuals.