Transient paresis in myotonic syndromes. A surface EMG study

In order to study transient paresis, force and surface electromyograms (EMG) were recorded from the biceps brachii in 3 patients with recessive myotonia congenita, in 8 with myotonic dystrophy and in 3 controls. Epochs of 0.34 s EMG signal were stored every 0.44 s and processed to determine the integrated EMG (IEMG), the power spectrum and the average muscle fibre conduction velocity (MFCV). The protocol consisted of 5 maximal voluntary contractions lasting 10.3 s and 13 s recovery. During the first 1-1.5 s the MFCV and median frequency (Fmed) of the power spectra of the controls increased (mean increase 12.5% and 14%, respectively) in all 5 consecutive contractions. Subsequently, MFCV, Fmed and force declined as a consequence of fatigue. Both the initial force and MFCV declined with successive contractions. The results in myotonic dystrophy were not different from the controls, except that the changes during fatigue were far less pronounced. These events are the reverse of the changes found in myotonia congenita in which an initial loss of force (maximal by 61-79%) and a decline of the IEMG (maximal by 79-92%) was found during the first contraction. This transient paresis was accompanied by a dramatic fall in the MFCV concomitant with a shift of the power spectrum to the lower frequencies. The first MFCV measurement of the 5 contractions was always normal. The decline in MFCV was maximal after 1.5-2.5 s and varied for the 3 patients from 32-52%. In general, the decline in force, IEMG and MFCV lessened with each successive contraction (warming-up phenomenon), though sudden deteriorations were sometimes observed during later contractions. The same results were found for brachioradialis and abductor digiti minimi. The results provide evidence that transient paresis is of clinical relevance in myotonia congenita and that it is caused by alterations in the muscle membrane. These membrane changes result in a strong decline of the muscle action potential conduction velocity and consequent depolarization block of the muscle fibres. Our method did not show the presence of transient paresis during voluntary contraction in myotonic dystrophy.