Respiratory muscle decline in Duchenne muscular dystrophy.

Objectives: Duchenne muscular dystrophy (DMD) causes progressive respiratory muscle weakness. The aim of the study was to analyze the trend of a large number of respiratory parameters to gain further information on the course of the disease.

Study Design: Retrospective study.

Subject Selection: 48 boys with DMD, age range between 6 and 19 year old, who were followed in our multidisciplinary neuromuscular clinic between 2001 and 2011.

Methodology: Lung function, blood gases, respiratory mechanics, and muscle strength were measured during routine follow-up over a 10-year period. Only data from patients with at least two measurements were retained.

Results: The data of 28 patients were considered for analysis. Four parameters showed an important decline with age. Gastric pressure during cough (Pgas cough) was below normal in all patients with a mean decline of 5.7 ± 3.8 cmH2 O/year. Sniff nasal inspiratory pressure (SNIP) tended to increase first followed by a rapid decline (mean decrease 4.8 ± 4.9 cmH2 O; 5.2 ± 4.4% predicted/year). Absolute forced vital capacity (FVC) values peaked around the age of 13-14 years and remained mainly over 1 L but predicted values showed a mean 4.1 ± 4.4% decline/year. Diaphragmatic tension-time index (TTdi) increased above normal values after the age of 14 years with a mean increase of 0.04 ± 0.04 point/year.

Conclusions: This study confirms the previous findings that FVC and SNIP are among the most important parameters to monitor the evolution of DMD. Expiratory muscle strength, assessed by Pgas cough, and the endurance index, TTdi, which are reported for the first time in a large cohort, appeared to be informative too, even though measured through an invasive method.