Late changes in the motor unit after acute poliomyelitis.

Patients who have suffered an acute attack of poliomyelitis may develop a clinically evident progression of weakness later in life. A syndrome known as forme fruste amyotrophic lateral sclerosis occurs in severely affected older polio patients. Neither process is well understood. Ten subjects who had had polio at least 22 years prior to examination and had no complaints of rapid progression of weakness were studied. Single-fiber recordings were made from the extensor digitorum communis or the tibialis anterior muscle. Fiber densities were increased in all patients. Eight of 10 patients showed marked abnormalities in jitter in more than 50% of the recordings examined. There was a significant association between the percentage of recordings with abnormal jitter and the time since the attack of polio. Blocking was seen in more than 30% of the recordings examined in 6 of 10 patients. There was a significant association between the percentage of recordings demonstrating blocking and both the chronological age and the number of years since the attack of polio. These abnormalities may represent disintegration with aging in the reinnervated motor units.