Poliomyelitis

Poliomyelitis (po-lee-o-my-e-LY-tis), or polio, is a viral infection that attacks nerve cells and causes deterioration of muscles and sometimes paralysis.

Iron Lungs

Remarkable photographs from the mid twentieth century show people—sometimes a large roomful—enclosed in metal tanks being attended to by nurses. The tanks were called iron lungs. In the 1940s and 1950s, these machines breathed for patients whose breathing had been either temporarily or permanently disabled by poliomyelitis. Once the most feared of childhood diseases because of its potential to cripple people for life, polio is now almost unknown in the United States. Elsewhere in the world, however, the disease still strikes thousands of people each year.

What Is Poliomyelitis?

Polio is an infection caused when a person swallows water or food contaminated with poliovirus. Poliovirus particles are found in feces, and the virus can be spread when people touch contaminated objects and then put their fingers in their mouths or handle food. It is not spread by coughing or sneezing. Once inside the body, the virus multiplies in the throat and intestines. It then either passes harmlessly from the gut or travels in the blood to all parts of the body. In a very small number of

people, about 1 to 2 percent, the virus invades the central nervous system, that is, the brain and spinal cord, where it can cause paralysis. Depending on where the virus goes in the spinal cord, a person may also have difficulty breathing, talking, or swallowing.

Poliovirus is special in that it attacks only the nerve cells that control muscles—called motor neurons—and leaves other nearby nerve cells unaffected. Nerve cells are like roads connecting our muscles to the central nervous system, and the virus is able to travel along these roads to the spinal cord and brain. The virus infects more than 95 percent of the motor neurons in the spinal cord and many other cells in the brain. The neurons that are destroyed cannot be replaced, and paralysis is the result. But many neurons survive, and when they recover, they form new connections to the muscles. In fact, for reasons that are not clear, a nerve cell that survives the virus sprouts many more connections to muscle cells than it did originally. So even if many neurons are destroyed by the virus, often the remaining neurons are able to make up for the loss.

Signs and Symptoms

In more than 90 percent of cases, people do not know that they have caught the poliovirus. When symptoms do occur, they may be similar to those of a cold or the flu. Although paralysis is a devastating result of poliomyelitis, most people with polio do not become paralyzed. And many patients who do develop paralysis recover much of the use of their muscles weeks to months after infection. About two-thirds of patients with paralytic polio have long-lasting effects from the disease.

How Do People Know If They Have Polio?

Usually polio can be diagnosed by the kind of paralysis it causes—more on one side of the body than the other. A person cannot move his or her legs, arms, or even the muscles required for breathing. Tests identify cells in the fluid around the brain and spinal cord that are not supposed to be there. To confirm the diagnosis, samples from a throat swab and a stool culture are tested to see whether they contain poliovirus. There is an even more rapid and sensitive test, called polymerase (pol-IM-erase) chain reaction, that is used in some cases.

How Is Polio Prevented?

There is no treatment for polio infection, but it can be prevented by vaccination. Vaccination is a way of protecting a person from a disease by introducing the body to a harmless form of the disease-causing organism. Then if the body encounters the disease-causing germ, it is able to recognize it and fight against it. In 1952 there were 57,879 cases of poliomyelitis in the United States—the highest number of cases ever. But the introduction in 1955 of an injectable vaccine developed by Jonas Salk and in 1961 of an oral vaccine developed by Albert Sabin ultimately eradicated poliovirus in the western hemisphere. The last cases in the United States occurred in 1979, among religious groups that had declined to be vaccinated. At present, the only cases of polio reported in the United States are due to the oral vaccine itself. Five to 10 such cases are reported each year. Most of the other cases develop in close contacts of these patients who have not been properly vaccinated or who have weakened immune systems.

Of 6,241 polio cases reported from 51 countries in 1994, more than 7 of every 10 were from India, Pakistan, and Bangladesh. There are several reasons for these outbreaks: Many people live in unhealthy, crowded conditions, and they may be improperly vaccinated or not vaccinated at all.

Surviving Polio

People who become paralyzed by the poliovirus have major adjustments to make. Survivors must receive physical therapy to keep their muscles from deteriorating, and they may need leg braces to walk or have to use a wheelchair. Others may need help breathing for the rest of their lives. Some people overcome early paralysis only to suffer from post-polio syndrome decades later. But most survivors of paralytic polio have normal childhoods and go on to lead fulfilling lives. The violinist Itzhak Perlman, who caught the virus at the age of 4 and wears leg braces, is one example. Franklin D. Roosevelt (see box), who got polio at age 39, went on to become one of our greatest presidents.

What Is Post-Polio Syndrome?

Neurons are constantly losing and making new connections, but in a balanced way, so that no matter how many connections are wearing out, there are always enough to keep the muscles working. But in the late 1970s, some of the people who had survived paralytic polio in the 1930s and 1940s began to complain of muscle and joint pain and of new muscle weakness. At first, physicians did not believe the complaints were real, but the number of patients kept increasing. In the 1980s, the term "post-polio syndrome" was coined to describe the condition. Post-polio syndrome is not caused by reinfection with the virus. Researchers believe that in these individuals, over time, the core, or axon, of their motor neurons may just wear out and can no longer make new connections. Or nerves may fail for other reasons. People who are affected by post-polio syndrome experience weakness, tiredness, muscle twitching, and pain. In addition, muscles affected by polio 20 to 30 years earlier begin to lose strength again. Unfortunately, no test exists to detect post-polio syndrome. The disease progresses very slowly, and treatment consists of exercise and aids such as braces or a wheelchair and assisted breathing when needed.