Pulmonary function and electromyographic study of respiratory muscles in myotonic dystrophy.

Ten adult myotonic dystrophy patients underwent measurements of lung function, maximal dynamic and static ventilatory efforts, and respiratory muscle electromyography (EMG). EMG studies were performed during spontaneous breathing or when subjects breathed through high inspiratory or expiratory resistive loads. Present results show that (1) a moderate restriction of lung volumes with hypoxemia plus normocapnia is often observed; (2) patients sustain dynamic ventilatory efforts more easily than static work; and (3) abnormalities in respiratory muscle EMG exist with spontaneous expiratory and inspiratory intercostal activities during quiet breathing and changes in muscular response to resistive loads. Inspiratory loading evokes contraction of expiratory muscles, with a marked decrease in inspiratory activities. Expiratory resistive loads prolong the diaphragmatic contraction throughout the expiratory time, and in some patients, relaxation of the diaphragm does not occur during the loaded run. These EMG data suggest that the reciprocal inhibition among respiratory neurons is enhanced in myotonic dystrophy and that myotonia also occurs in the diaphragm when loads oppose its relaxation.