| WHAT CAUSES THE LATE SYMPTOMS?
Postpolio syndrome occurs in polio survivors who had injury to their central nervous system, generally the anterior horn cells in the spinal cord, during the initial infection. The cause of the late symptoms is not well understood but is believed to involve attrition of motor neurons during aging.4 Other theories abound, however, and the etiology is likely multifactorial.
When motor neurons are lost in acute polio, the surviving motor neurons sprout collateral fibers that reinnervate the denervated muscle fibers (FIGURE 1). The resulting motor units are larger than normal, and there are fewer of them than before. Therefore, the burden on each of these remaining motor neurons is higher than under normal conditions.
With age, we all gradually lose some motor neurons.5 Polio survivors may be more affected by this loss of motor neurons because they have fewer to begin with.
Another theory is that insufficient levels of acetylcholine are released at the neuromuscular junction, resulting in diminished muscle contraction.5
Maselli et al6 noted reduced amplitudes of miniature end plate potentials and structural abnormalities of the neuromuscular junction, such as reduced diameter of nerve terminals, but these changes were not noted in all postpolio syndrome patients.
Some have found ongoing immune activation and defective viral particles in the spinal fluid,7 although the significance of these is unclear.
General fatigue may also have a central cause an abnormality in the reticular activating system in the brain that occurred during the acute polio episode.5,7
Page 1 2 (3) 4 5 Previous Page - Next Page
|
