Nobody knows exactly what causes the signs and symptoms of post-polio syndrome to appear so many years after the first episode of polio. Currently, the most accepted theory regarding the cause of post-polio syndrome rests on the idea of degenerating nerve cells. When poliovirus infects your body, it affects nerve cells called motor neurons particularly those in your spinal cord that carry messages (electrical impulses) between your brain and your muscles.
Each neuron consists of three basic components :
- A cell body
- A major branching fiber (axon)
- Numerous smaller branching fibers (dendrites)
Nerve cells communicate with adjacent nerve cells at contact points called synapses. Electrical impulses run along extended chains of these neurons until they reach their desired destination, as when your brain sends a message to the muscles of your legs and feet to step forward.
How polio affects nerve cells
A polio infection often leaves many of these motor neurons destroyed or damaged. To compensate for the resulting neuron shortage, the remaining neurons sprout new fibers, and the surviving motor units become enlarged. This promotes recovery of the use of your muscles, but it also places added stress on the nerve cell body to nourish the additional fibers. Over the years, this stress may be more than the neuron can handle, leading to the gradual deterioration of the sprouted fibers and, eventually, the neuron itself.
Normally, everyone loses some neurons through the aging process. But people who've had polio may have lost so many due to the infection that they end up with fewer total neurons than people who've never had polio. This may lead to the progressive weakness characteristic of post-polio syndrome. On the other hand, little evidence suggests that people who've recovered from polio lose remaining healthy neurons at a faster rate than normal.
Some scientists theorize that the initial illness may have created an autoimmune reaction, causing the body's immune system to attack normal cells as if they were foreign substances. But the evidence surrounding this theory is limited, compared with the studies supporting the more generally accepted motor neuron degeneration theory.
It's possible that some of the signs and symptoms of post-polio syndrome, especially joint pain, may be due to chronic overuse of muscles that apparently were undamaged by the initial phase of polio. For example, if your left leg was disabled by polio, it may be that your right leg develops complications later in life because its muscles and joints have had to overcompensate for the disabled leg.
The above information thankfully comes from the Mayo Clinic.com at the following link.