Definition of Polio

Polio: Abbreviation for poliomyelitis, an acute and sometimes devastating disease caused by a virus. Man is the only natural host for polio virus. The virus enters the mouth and multiplies in lymphoid tissues in the throat and intestine. Small numbers of virus enter the blood and go to other sites where the virus multiplies more extensively. Another round of viremia (virus in the bloodstream) leads to invasion of the central nervous system (CNS), the spinal cord and brain, the target sites struck by the virus.

In polio, there is inflammation of the central nervous system, especially the anterior horn cells of the spinal cord and the brainstem (the portion of the brain between the cerebral hemispheres and spinal cord).

Polio can be a minor illness, as it is in 80-90% of clinical infections, chiefly in young children, and not involve the CNS. Symptoms are slight fever, malaise, headache, sore throat, and vomiting 3-5 days after exposure. Recovery occurs in 24-72 hours. This is termed the abortive type of polio.

As a major illness, polio may or may not be paralytic. Symptoms usually appear without prior illness, particularly in older children and adults, 7-14 days after exposure. Symptoms are fever, severe headache, stiff neck and back, deep muscle pain, and sometimes areas of hyperesthesia (increased sensation) and paresthesia (altered sensation). There may be no further progression from this picture of viral meningitis ("aseptic" meningitis) or there be loss of tendon reflexes and weakness or paralysis of muscle groups.

Recovery is complete in the abortive and nonparalytic forms of polio. In paralytic polio, about 50% of patients recover with no residual paralysis, about 25% are left with mild disabilities, and the remaining patients have severe permanent disability. The greatest return of muscle function occurs in the first 6 months, but improvement may continue for up to 2 years. Physical therapy is the most important part of treatment of paralytic polio during convalescence. The ideal strategy with polio is clearly to prevent it by immunization against poliovirus.

U.S. President Franklin D. Roosevelt (1882-1945) had polio and worked with the National Foundation/March of Dimes to raise money to combat this once-fearsome scourge. Polio (myelitis) is also called infantile paralysis.