Types of Polio diseases

Once the virus enters the bloodstream, the virus may cause one of four types of illnesses marked by varying severity. Some infections result in abortive poliomyelitis, a mild form of the disease characterized by fever, headache, sore throat, fatigue, nausea, and vomiting. This short-lived form of the illness lasts only from hours to a few days. In more than 95 percent of cases, the disease gets no worse. Sometimes, however, the virus may invade the nervous system, causing more severe forms of the disease.

Some poliovirus infections of nerve cells, or neurons, result in nonparalytic poliomyelitis. In addition to the fever and other symptoms seen in abortive poliomyelitis, nonparalytic poliomyelitis causes pain and stiffness in the neck and back. This infection typically produces aseptic meningitis—an inflammation of the membranes that surround the brain and spinal cord. As with abortive poliomyelitis, however, symptoms from nonparalytic polio usually subside within a few days without causing permanent damage.

In perhaps 1 or 2 percent of cases a more disabling form of the disease occurs, called paralytic poliomyelitis. In this form viral infection of neurons in the spinal cord may cause temporary damage to these cells—or permanent destruction. The muscles activated by the involved neurons become painful, and muscular weakness in the arms and legs may develop, sometimes followed by paralysis.

The muscles affected and the degree of damage that occurs depend on the area of the spinal cord that has been invaded and on the number of neurons involved. Any limb or combination of limbs may be affected—one leg, one arm, or both legs and both arms. In children under 5 years of age paralysis of one leg is most common. In adults paralysis of all four limbs—a condition known as quadriplegia—is more common.

In some cases muscles that control breathing are affected, requiring that the patient be placed on artificial respiration, in which breathing is controlled mechanically. Roughly 2 to 5 percent of infants with paralytic polio die. For adults the disease is even deadlier, causing death in 15 to 30 percent of cases.

In the most serious cases of polio infection the virus attacks the brain, causing bulbar poliomyelitis. Various nerves in the head and face, including those that send signals to the ears, eyes, and the muscles controlling chewing and swallowing, may be affected. Sometimes the virus affects the part of the brain that controls breathing and heartbeat, resulting in death.