Background: Respiratory muscle weakness in Duchenne muscular dystrophy (DMD) leads to weak cough, impaired airway clearance, and ultimately hypoventilation. Poor expiratory function is often the first sign of respiratory impairment, yet the relationship with underlying expiratory muscle pathology is not well-described. Magnetic resonance imaging (MRI) can capture expiratory muscle fatty replacement, a biomarker of muscle health.
Objective: To assess disease progression in the expiratory muscles using MRI methods and to compare muscle progression to changes in clinical pulmonary function.
Approach: Participants with DMD (ages 7-18 yrs) were recruited to participate in this longitudinal, observational study. At each annual study visit, participants underwent chemical shift-encoded MRI of the abdomen to assess fatty infiltration of the expiratory muscles (rectus abdominis-RA; internal oblique-IO; external oblique-EO). After MRI scans, forced vital capacity (FVC), peak cough flow (PCF), and maximal expiratory pressure (MEP) were measured.
Results: Thirty-nine individuals with DMD enrolled in the study with 31 one-year and 24 two-year follow up visits. Fatty replacement of the expiratory muscles was evident at an early age, particularly in the IO. Longitudinal models of expiratory muscle fatty infiltration suggest that the IO reaches 50% fatty replacement at an average age of 12.7 yrs (EO=13.5 yrs, RA = 14.8 yrs). Absolute MEP values did not decrease significantly from baseline to 2 years, but increases in IO FF were related to decreases in percent predicted FCV (R2=.36) and MEP (R2=.49). Expiratory muscle fat fraction was significantly higher in participants using cough assist compared to those who did not (p<0.01), while there was no difference in PCF between the groups.
Conclusions: Imaging of the expiratory respiratory muscles may be a useful adjunct to traditional pulmonary function testing and may be helpful to determine who will benefit from cough assist therapy.