Shortly after, vaccines were developed that greatly reduced its spread. Today, hardly anyone in developed countries gets polio, and the disease is well on its way to being eliminated in developing countries, thanks to massive immunization efforts with oral polio vaccine.
But for some people, some of the initial problems they had with polio are reappearing in a condition called post-polio syndrome (PPS). The cause is unknown, but new research is beginning to yield a better understanding of this complex syndrome.
Treatment is aimed at managing the signs and symptoms associated with PPS and increasing quality of life.
Signs and symptoms
Post-polio syndrome refers to a cluster of disabling signs and symptoms that appear decades — between 10 and 40 years — after the initial illness. Common signs and symptoms include:
Nobody knows exactly what causes these signs and symptoms to appear so many years after the first episode of polio. Currently, the most accepted theory regarding the cause of PPS rests on the idea of degenerating nerve cells. When poliovirus infects your body, it affects certain 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) and 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, such as when your brain sends a message to the muscles of your legs and feet to step forward.
A polio infection often leaves many of these motor neurons dead or damaged. To compensate for the resulting neuron shortage, the remaining neurons sprout new fibers and the motor unit becomes enlarged. This promotes recovery of the use of your muscles, but it 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 to begin with that they end up with fewer neurons than people who've never had polio. This may lead to the progressive weakness characteristic of PPS. Currently, little evidence suggests that people with polio lose neurons at a faster rate than normal.
Scientists have also noted inflammation in the spinal cord of people with PPS, leading some to hypothesize that the poliovirus has remained dormant and later been reactivated. Another theory is that the initial illness has created an autoimmune reaction, causing the body's immune system to attack normal cells as if they were foreign substances. But the amount of evidence surrounding these two theories is small compared with the studies supporting the more generally accepted motor neuron degeneration theory.
It's possible that some of the signs and symptoms of PPS, 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.
Factors that may increase your risk of developing PPS include:
When to seek medical advice
If you're experiencing weakness or fatigue that seems to be slowly getting worse, see your doctor. It's important to rule out other causes of your signs and symptoms that may require different therapy than what's currently advised for PPS.