Pathology is the study of abnormal biological structure (anatomy) and abnormal function (physiology) that results from disease. Post-polio syndrome is theorized to result from pathologic changes in motor nerves and muscles from the acute paralytic poliomyelitis infection (see Poliomyelitis, Acute). After the poliovirus invades the central nervous system, it can damage or destroy anterior horn cells. During the recovery phase after acute polio, strength increases in three ways. Some nerve cells are only temporarily damaged and over a few weeks recover their function. Other muscle fibers connected to dead nerve cells atrophy (see Biopsy, Muscle) and are replaced by fibrous tissue and fat, while surviving muscle fibers hypertrophy (enlarge) in response to exercise. Third, the nerve cells that survived the poliovirus infection begin to compensate for the loss of neighboring nerve cells by growing new “sprouts” from the terminal axon branches. These sprouts make new connections with (reinnervate) muscle fibers that lost their connection with a viable nerve cell. Due to this compensatory process, surviving nerve cells may innervate three to eight times as many muscle fibers as normal (Wiechers, 1985), and reach up to 10,000 or more muscle fibers per motor unit. The result is giant motor units.
These compensations can result in apparent full recovery of normal strength. During the years of stable strength and endurance, polio survivors reach a steady state (equilibrium) of terminal sprout drop-off (denervation) and new sprouting (reinnervation). When this steady state is disrupted after many years, a critical threshold is crossed and new weakness occurs, marking the onset of post-polio syndrome (Halstead, 1998).
While there is no agreement on the cause, or trigger, for the new degeneration of giant motor units (see Theories), it is now known that changes in post-polio motor units occur at several levels and account for the symptoms of new weakness and increased muscle fatigability. First, the terminal axon sprouts deteriorate due to a tendency of giant motor units to revert to a more normal size. This process is called fragmentation or peripheral disintegration. Second, insufficient acetylcholine (the neurochemical that is released at the junction when the electrical impulse from the neuron reaches the junction) causes defects of electrical transmission, resulting in a short circuit of the nerve’s message to the muscle (Trojan et al., 1993). Third, muscle fibers may become smaller (disuse atrophy) and become less able to stay enlarged and/or contract as strongly. Also, a decrease in the flow of the nourishing neurotrophic factors from the cell body down the axon may negatively influence the muscle’s metabolism.
References
Halstead, L.S. (Ed.). (1998). Managing post-polio: A guide to living well with post-polio syndrome. Washington, DC: NRH Press.
Trojan, D.A., Gendron, D., & Cashman, N.R. (1993). Anticholinesterase-responsive neuromuscular junction transmission defects in post-poliomyelitis fatigue. Journal of the Neurological Sciences, 114, 170-177.
Wiechers, D.O. (1985). Pathophysiology and late changes of the motor unit after poliomyelitis. In L.S. Halstead & D.O. Wiechers (Eds.), Late Effects of Poliomyelitis (pp. 91-94). Miami, FL: Symposia Foundation.