While there is now agreement among researchers that post-polio syndrome results from degenerative changes within motor units, the triggering mechanism(s) for these changes is/are not known. A favored theory to explain their onset is overuse. This theory assumes that the enlarged motor units supplying post-polio muscles cannot maintain the increased metabolic activity needed over many years (see Pathology). These giant motor units that have been long overworked begin to “wear out.” The consequence is slowly increasing weakness and rapid fatigability of muscles. Support for this theory comes from numerous clinical studies that show post-polio syndrome is most likely to occur in people who had moderately severe initial polio paralysis and yet made a relatively good recovery of useful strength and function, enough to be chronically overused in the pursuit of normal activities (Halstead & Rossi, 1987; Klingman et al., 1988; Trojan et al., 1994).
Other theories presented to explain the triggering mechanism(s) for motor unit degeneration include: premature aging changes of motor units that were involved but did not die from the original poliovirus infection (Tomlinson & Irving, 1985); an immunological abnormality that may be triggered by re-exposure to live poliovirus or by reactivity of persistent poliovirus fragments in the spinal cord (Dalakas et al., 1986; Miller, 1981; Sharief et al., 1991); scarring within the spinal cord that decreases the blood supply to motor nerve cells; environmental toxins that damage polio-involved motor nerve cells at exposure levels less than that which affect normal cells (Rea et al., 1987); chronic high levels of stress that affect immunology and circulating anti-oxidants; and lastly, a deficiency in growth hormone that precipitates age-related degenerative changes (Shetty et al., 1995).
While all of these theories are plausible and none have been completely excluded, there is not enough scientific evidence to strongly support any of them at present.
References
Dalakas, M.C., Elder, G., Hallett, M., Ravits, J., Baker, M., Papadopoulos, N., Albrecht, P., & Sever, J. (1986). A long-term follow-up study of patients with post-poliomyelitis neuromuscular symptoms. New England Journal of Medicine, 314, 959-963.
Halstead, LS., & Rossi, C.D. (1987). Post-polio syndrome: Clinical experience with 132 consecutive outpatients. In LS. Halstead & D.O. Wiechers (Eds.), Research and clinical aspects of the late effects of poliomyelitis (pp. 13-26). White Plains, NY: March of Dimes Birth Defects Foundation.
Klingman, J., Chui, H., Corgiat, M., & Perry, J. (1988). Functional recovery: A major risk factor for the development of postpoliomyelitis muscular atrophy. Archives of Neurology, 45, 645-647.
Miller, J.R. (1981). Prolonged intracerebral infection with poliovirus in asymptomatic mice. Annals of Neurology, 9, 590-596.
Rea, W.J., Johnson, AR, Fenyves, E., & Butler, J. (1987). The environmental aspects of the post-polio syndrome. In LS. Halstead & D.O. Wiechers (Eds.), Research and clinical aspects of the late effects of poliomyelitis (pp. 173-179). White Plains, NY: March of Dimes Birth Defects Foundation.
Sharief, M.K., Hentages, R, & Ciaidi, M. (1991). lntrathecal immune response in patients with the post-polio syndrome. New England Journal of Medicine, 325, 749-755.
Shetty, K.R., Gupta, K.L, Agre, J.C., Rudman, l.W., & Rudman, D. (1995). Effect of human growth hormone on muscle function in post-polio syndrome. In M.C. Dalakas, H. Bartfeld, & LT. Kurland, (Eds.), The post-polio syndrome: Advances in the pathogenesis and treatment (pp. 386-389). New York, NY: New York Academy of Sciences.
Tomlinson, B.E., & Irving, D. (1985). Changes in spinal cord motor neurons of possible relevance to the late effects of poliomyelitis. In LS. Halstead & D.O. Wiechers (Eds.), Late Effects of Poliomyelitis (pp. 57-70). Miami, FL: Symposia Foundation.
Trojan, D.A., Cashman, N.R., Shapiro, S., Tansey, C.M., & Esdaile, J.M. (1994). Predictive factors for post-poliomyelitis syndrome. Archives of Physical Medicine & Rehabilitation, 75, 770-777.