Limitations
Limitations to our study included involvement of only a single site where children were recruited from one neuromuscular clinic resulting in a small sample size even though all children with NMD who fulfilled inclusion criteria were approached to participate in the study. In addition, ccorrelations between spirometry and PSG parameters may have been apparent if PSG data had been obtained in the control group. Whilst historical cohorts looking at supine spirometry have included similar numbers of patients, we recognize that larger numbers of patients are needed. Children with severe restrictive lung disease (FVC<40%) who were already established on NIV were unable to perform reliable spirometry in both sitting and supine positions. Supine spirometry is therefore not useful in these children on the more severe end of the spectrum where it may be uncomfortable, impractical und unhelpful in predicting SDB. Lastly, the cross-sectional nature of the study did not allow examination of the relationship between supine spirometry and evolution of respiratory weakness over time.