Respiratory muscle function and ventilatory control. I in patients with motor neurone disease. II in patients with myotonic dystrophy.
We have investigated the severity and functional consequences of weakness of the respiratory muscles in 12 patients with motor neurone disease and 19 patients with myotonic dystrophy. We have also evaluated the control of ventilation and have assessed the influence of both overall respiratory muscle involvement and specific weakness of the diaphragm on various indices of ventilatory control. Measurements were made of chest wall motion, static and dynamic lung volumes, maximum voluntary ventilation, maximum static respiratory (mouth) pressures, mixed venous PCO2 and various indices derived from the ventilatory response to rebreathing CO2. In 19 patients (eight motor neurone disease, 11 myotonic dystrophy) direct measurements of transdiaphragmatic pressure were made during maximum static inspiratory efforts and during a full inspiration. 
 
Respiratory muscle weakness was found in almost all the patients. In motor neurone disease this correlated in general with the overall severity but in myotonic dystrophy its degree was often unsuspected clinically. 
 
In both groups of patients reductions in the ventilatory response to CO2 were common and could be attributed to weakness. There was no evidence of disproportionate involvement of the diaphragm but the most severely affected patients with motor neurone disease had hypercapnia and evidence of severe diaphragmatic weakness. In myotonic dystrophy the presence of CO2 retention did not correlate well with the ventilatory response to CO2, and could not be clearly attributed to weakness of the diaphragm. Grossly irregular patterns of breathing were seen in several patients. A defect of medullary respiratory control has been proposed in myotonic dystrophy but there is no direct evidence for this and the possible influence of disordered afferent information from the diseased muscles remains to be explored.