Because, in healthy subjects, vital capacity (VC) increases with growth during the first decade of life, early respiratory muscle dysfunction may be masked until VC does not reach its plateau. Accordingly, in DMD patients, VC progressively decreases by about 5–6 % per year, usually starting around 12 years of age. However, the rate of this decline may be variable: in particular, a sudden reduction in VC can be occasionally precipitated by intercurrent events such as a worsening of scoliosis or infective complications leading to the development of hypoventilation. A forced vital capacity (FVC) ≤1 l is a predictor of poor outcome, with a survival rate at 5 years of only 8 % if assisted ventilation is not provided [2].

Reductions in maximum inspiratory pressure occur early in the clinical course of DMD and may precede the reduction in VC. However, despite significant weakness of the respiratory muscles, pulmonary symptoms are often minimal in the early stages of the disease. Particularly, hypercapnia is uncommon in patients with DMD in the absence of pulmonary infections. This is believed to be due to a relative preservation of diaphragm function until the later stages of the disease. Anyhow, once hypercapnia develops, the course is rapidly progressive [1].