MYOTONIA OF THE RESPIRATORY MUSCLES IN MYOTONIC-DYSTROPHY

Myotonic dystrophy is a muscle disorder in which there is a tendency t o rapid shallow breathing and a reduced ventilatory response to chemic al stimuli. Respiratory failure may occur when respiratory muscle weak ness is not marked. One explanation proposed for these observations is that myotonia of the respiratory muscles reduces the compliance of th e chest wall. However, direct electrical evidence of myotonia in the r espiratory muscles with breathing is lacking. In 11 patients with myot onic dystrophy the scalene, sternocleidomastoid, a parasternal muscle, and a lateral intercostal muscle were studied using intramuscular bip olar wires. Five of the 11 patients had an elevated Pa(CO2). All patie nts 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 pa tients demonstrated no myotonia. Myotonia on insertion of the needle w as seen in four patients. Myotonia was rare in an isolated respiratory cycle during quiet breathing, and repetitive myotonia with consecutiv e breathing cycles was never noted. Voluntary big breaths produced myo tonia in five patients, whereas chemically stimulated larger breaths p roduced myotonia in seven patients. In two of these seven patients, my otonic 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 a ccount for the respiratory failure and tachypneic breathing pattern fo und in myotonic dystrophy. Its occurrence at higher levels of ventilat ion may contribute to the reduced ventilatory response to chemical sti muli.