Poliomyelitis

Poliovirus is a single-stranded ribonucleic acid (RNA) enterovirus. There are three immunologically defined serotypes of poliovirus (serotypes 1, 2, and 3), all of which cause paralytic disease. Natural polio infection occurs through ingestion of the virus, which initially replicates in the mucosa of the oropharynx and gastrointestinal tract. The virus enters the central nervous system via either the bloodstream or alternatively through afferent neural pathways into motor neurons of the anterior horn, motor nuclei of the brainstem, and Betz cells of the motor cortex.

The earliest cellular change in the motor neurons is dissolution of the cytoplasmic Nissl substance (chromatolysis). Infected neurons that have only a mild degree of chromatolysis survive and continue to support motor units. In contrast, neurons that have severe chromatolysis become necrotic and cannot support motor units. As a result, permanent loss of function occurs in the muscle groups innervated by the motor unit. Historically, poliovirus infections have been divided into a minor illness and a major illness. The minor illness is characterized by fever, myalgias, nausea, and diarrhea. A major illness can be associated with the minor illness or follow the minor illness by a few afebrile days and is characterized by increasing signs of meningeal irritation, headache, and stiff neck. When the illness progresses to the paralytic form, muscle soreness is prominent, particularly in the back and neck. Patients who develop paralysis usually do so on the second to fifth day after meningeal signs and fever develop. The weakness is generally an asymmetric flaccid muscle weakness, and the legs are involved more often than the arms.

During the polio epidemic in the past century, the diagnosis was based on the clinical syndrome of fever with paralysis and lower motor neuron weakness. Today, poliovirus is the least common cause of an asymmetric flaccid lower extremity weakness. The other enteroviruses (the coxsackieviruses, the echoviruses, and the numbered enteroviruses), and the flaviviruses, most notably the West Nile virus, are the much more common etiologic agents of a flaccid paralysis.

When poliovirus infection is suspected, at least two stools specimens and two throat swabs should be obtained 24 hours apart. As with all suspected enteroviral infections, acute and convalescent serology should be sent 4 weeks apart to detect a fourfold increase in immunoglobulin G (IgG). Spinal fluid analysis demonstrates a lymphocytic pleocytosis, a normal glucose concentration and enteroviral RNA by polymerase chain reaction (PCR).

The treatment of poliovirus infection is primarily supportive, and prevention with mass vaccination of all children is essential.

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