Earlier decline in sniff nasal inspiratory pressure than peak expiratory flow in children with Duchenne muscular dystrophy.

In Duchenne muscular dystrophy (DMD), progressive weakness of the respiratory muscles leads to a restrictive ventilatory defect contributing to early morbidity and mortality. We have recently shown by a longitudinal assessment of vital capacity and sniff nasal inspiratory pressure (SNIP) in young DMD children that SNIP was an earlier marker of decline in respiratory muscle strength than vital capacity [1]. However, early involvement of the expiratory muscles is a characteristic feature of DMD, as shown by the earlier decrease in expiratory (PEmax) than inspiratory maximal pressure (PImax) [2]. In the absence of bronchial obstruction, peak expiratory flow (PEF) reflects maximal expiratory muscle strength [3]. In patients with respiratory weakness, PEF is reduced and correlated with PEmax [4]. Recently, it was shown that DMD patients on a 1-year idebenone treatment improved in expiratory muscle strength (evaluated by PEF) while patients on placebo deteriorated [5]. No difference between treatments groups were observed for measures such as vital capacity [6], suggesting that PEF could be used as an outcome parameter to assess the effect of early therapeutic interventions on respiratory muscle strength in DMD. As candidate drugs for the treatment of DMD enter clinical trials, it is important to determine the natural evolution of pulmonary function parameters that could be used as outcome measures for efficacy studies in DMD children. However, to our knowledge, there are no data on the natural evolution of PEF in young DMD children and there is no information on the age of PEF decline compared with the age of SNIP decline. Therefore, we aimed to assess the PEF and SNIP change with age in glucocorticoid-naı̈ve DMD children in order: 1) to document the natural evolution of PEF measurement; and 2) to identify the ages of decline in PEF and SNIP in DMD children.