Early and late losses of motor units after poliomyelitis.

Motor unit number estimation was employed to assess muscle innervation in 76 patients with prior poliomyelitis. Of the 68 patients who were < 70 years of age, new musculoskeletal symptoms had appeared in all but four; the mean latent interval was 38.0 +/- 10.1 years. As expected, there was a high incidence of muscles exhibiting denervation in previously affected limbs (87%). However, the incidence in supposedly unaffected limbs was also high (65%). Significant differences in the degree of denervation were found between muscles of the same hands and feet. Judged on the basis of their potential amplitudes, the surviving motor units in partially denervated muscles tended to be enlarged. The enlargement was proportional to the extent of the denervation and was comparable to that found in amyotrophic lateral sclerosis. In some muscles, possibly those innervated by failing motor neurons, motor-unit enlargement was not present. Needle examination confirmed the high incidences of denervation in affected and allegedly unaffected limbs. Of the 188 muscles with EMG features of chronic denervation, only nine exhibited fibrillations or positive sharp waves (4.8%). Ninety-five muscles of 18 patients were studied a second time after an interval of 2 years. Overall, there was a 13.4% reduction in motor-unit number and a 18.4% diminution in M-wave amplitude (P < 0.001). The rate of motor-unit loss was twice that occurring in healthy subjects aged > 60 years. Analysis of individual patients indicated that some were deteriorating more rapidly than others. These studies confirm that denervation progresses in patients with prior poliomyelitis in both clinically affected and unaffected muscles, and indicate that this progression is more rapid than that occurring in normal ageing.