Post-polio syndrome is an illness defined by a collection of symptoms that generally occur at least 10-20 years after infection with the polio virus. The hallmark of post-polio syndrome is new muscular weakness. This may present as weakness in the arms, legs, or trunk or difficulty with swallowing, talking or breathing if the muscles that control these functions are affected. Other symptoms of post-polio syndrome include muscle pain, fatigue and cold intolerance. It is not uncommon for polio survivors to have new weakness in muscles that were previously believed to be unaffected by polio. This may be because they really weren't sure which muscles were affected many years before, or because muscles were so mildly affected at first that doctors didn't detect it on physical examination at the time of the polio virus infection.
Polio is a disease caused by an infection with the poliovirus. From the early 20th Century through the 1960's in the United States, epidemics of polio affected many people. The creation of polio vaccines has practically eliminated polio in the United States, and most of the developed nations of the world. However, people in the U.S. and elsewhere who were infected by the virus before vaccines were developed can develop post-polio syndrome.
To understand post-polio syndrome, you need to understand what happens in polio. In a small number of cases of polio (less than 10%), the virus will attack the cells in the spinal cord and cause paralysis. However, many people who were infected by the poliovirus did not develop paralysis. Fortunately, many just had typical symptoms of an infection — fever, aching muscles, fatigue, etc. — that lasted a few days, and thereafter regained complete health.
Others may have had mild muscular weakness, so mild that neither they nor their doctor noted it. Along with patients who developed obvious weakness (paralysis), patients with milder weakness are at risk for getting post-polio syndrome decades later.
It's not clear how many polio survivors will be affected by post-polio syndrome. A reasonable estimate is 60% of those who had significant paralysis during the initial illness. It's also not clear why some polio survivors develop post-polio syndrome while others do not.
One theory is overuse of the nerves and muscles that remained working after the initial infection. For example, if some of the nerves and muscles that are necessary for leg strength are damaged, the remaining nerves and muscles of the legs need to work harder to compensate. After many years of having to work harder, these nerves and muscles become exhausted. Some of them even die. This then forces the nerves and muscles that are left to work even harder, and so a vicious cycle sets in.
Polio that affects the spinal cord usually destroys many of the motor neurons (nerve cells) that control the muscles of the body. During recovery from polio, you can't build new nerve cells. However, you can create new connections between surviving nerve cells and muscles, so you can recover your muscle strength by "re-wiring" your nerve connections. This is an effective way for your nervous system to compensate for polio, but it may be temporary.
Normal aging likely also contributes to new weakness. Nervous system damaged by polio years earlier goes through a natural aging process that includes the loss of some strength.
In addition, as the years pass, nerve cells can be disabled or damaged by illness, injury, your own immune system or natural aging. Delicate connections between nerves and muscles can be lost during periods of inactivity. If you are already relying on a smaller number of nerve and muscle cells than is normal, this loss of other nerve cells over time might leave you more susceptible to weakness that you might not otherwise notice.
One theory for post-polio syndrome is that some of the poliovirus remains alive in the brain and spinal cord. This theory is controversial.
People usually are affected by post-polio syndrome during middle or late adult life, decades after they first developed polio, after a long period of stability. The new symptoms sometimes emerge after an illness or injury.