Effect of long-term steroids on cough efficiency and respiratory muscle strength in patients with Duchenne muscular dystrophy

OBJECTIVE. The objective of this study was to determine whether long-term steroid therapy is associated with increased peak cough flow in patients with Duchenne muscular dystrophy and to determine which pulmonary function test variable is most predictive of peak cough flow. METHODS. In this case-control study, the medical charts of patients who had Duchenne muscular dystrophy and had pulmonary function tests at our institution in the previous 2 years were examined. Steroid-treated patients were on therapy for at least 1 year. The measured pulmonary function tests included forced vital capacity, maximum expiratory pressure, maximum inspiratory pressure, maximum voluntary ventilation, and peak cough flow. Multiple linear regression analysis was used to determine which pulmonary function test measure was most predictive of peak cough flow and assess the influence of steroid treatment and patient age on peak cough flow. RESULTS. Ten steroid-treated and 25 untreated patients were analyzed. Peak cough flow and maximum expiratory pressure were significantly higher in the steroid-treated patients. Each of the pulmonary function test variables was significantly associated with peak cough flow. The linear model that had the highest adjusted r(2) value included only 2 variables: maximum voluntary ventilation and steroid treatment, demonstrating that steroid-treated patients had peak cough flow values that were 27 L/min higher than the untreated patients. The interaction between maximum voluntary ventilation and steroid was not statistically significant, suggesting that the steroid-associated increase in peak cough flow was approximately constant over the observed range of maximum voluntary ventilation values. The effects of maximum voluntary ventilation and treatment group on peak cough flow were not confounded with the patient age. CONCLUSIONS. Long-term steroid therapy is associated with improved peak cough flow and respiratory muscle strength in patients with Duchenne muscular dystrophy. Maximum voluntary ventilation may be a useful predictor of lung function in Duchenne muscular dystrophy.