Motor end-plate jitter in myasthenia gravis at different firing rates

Twenty patients with generalized myasthenia gravis (MG) were studied to assess firing rate-dependent changes in end-plate jitter. Stimulated single-fiber electromyography (EMG) was performed in the extensor digitorum communis muscles at stimulation frequencies of 1, 2, 5, and 10 Hz for all the sampled fibers and at 20 Hz for a few ones. In all 137 end-plates were obtained, of which 43 showed blocking. Statistical analysis demonstrated significant increases in jitter and the blocking rate from 1 to 2 and 2 to 5 Hz, whereas there were decreases from 5 to 10 and 10 to 20 Hz. Further analysis showed that the decrement in jitter from 5 to 10 Hz was greater in end-plates with preexisting low safety factors. In contrast, even though the safety factor of long-duration MG end-plates was lower than that of the short-duration group, the decrement in jitter was insignificant in the long-duration MG end-plates. These results suggest that the intratetanic facilitation effect begins at 10 Hz and is more prominent in end-plates with a preexisting low safety factor. Long-lasting MG, however, eliminates this effect.