Myotonia of the respiratory muscles in myotonic dystrophy

Myotonic dystrophy is a muscle disorder in which there is a tendency to rapid shallow breathing and a reduced ventilatory response to chemical stimuli. Respiratory failure may occur when respiratory muscle weakness is not marked. One explanation proposed for these observations is that myotonia of the respiratory muscles reduces the compliance of the chest wall. However, direct electrical evidence of myotonia in the respiratory muscles with breathing is lacking. In 11 patients with myotonic dystrophy the scalene, sternocleidomastoid, a parasternal muscle, and a lateral intercostal muscle were studied using intramuscular bipolar wires. Five of the 11 patients had an elevated PaCO2. All patients had a FVC greater than 70% of predicted. Myotonia was sought with needle insertion, quiet tidal breathing, voluntary large breaths, and involuntary larger breaths with chemical stimulation. Two of the 11 patients demonstrated no myotonia. Myotonia on insertion of the needle was seen in four patients. Myotonia was rare in an isolated respiratory cycle during quiet breathing, and repetitive myotonia with consecutive breathing cycles was never noted. Voluntary big breaths produced myotonia in five patients, whereas chemically stimulated larger breaths produced myotonia in seven patients. In two of these seven patients, myotonic activity with consecutive respiratory cycles was seen at higher levels of ventilation. In conclusion, the rare occurrence of myotonia with tidal breathing would suggest myotonia in the muscles does not account for the respiratory failure and tachypneic breathing pattern found in myotonic dystrophy. Its occurrence at higher levels of ventilation may contribute to the reduced ventilatory response to chemical stimuli.