Causes of the increased fiber density in muscular dystrophies studied with single fiber EMG during electrical stimulation.

The impulse transmission in motor end-plates and along muscle fibers was studied with single fiber EMG in 13 patients with muscular dystrophy during intramuscular nerve stimulation and direct muscle fiber stimulation. For comparison, three patients with spinal muscular atrophy (SMA) and five normal subjects were studied. At nerve stimulation, 25% of the recordings in the patients with muscular dystrophy showed "increased" jitter (> 55 microsec), 48% "normal" (10-55 microsec), and 27% "abnormally low" jitter (< 5 microsec). At direct muscle fiber stimulation, the jitter was "abnormally low" in 91% of the recordings; 9% of the recordings had normal jitter. In SMA, no abnormally low jitter values were found. Increased jitter is a sign of impaired impulse transmission in immature intramuscular nerves and motor end-plates. "Abnormally low jitter" suggests the presence of branched muscle fibers and ephaptic transmission between hyperexcitable fibers.