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.