Objectives To determine: (a) prevalence of clinically unsuspected nocturnal hypoventilation (NH) in a clinic population of children with progressive neuromuscular disease; (b) whether NH can be predicted from clinical/laboratory parameters; and (c) change over 1 year in pulmonary function decline, quality of life and attention in children with NH treated with non-invasive positive pressure ventilation (NPPV) compared with children without NH.

Design Prospective cohort study.

Setting Two tertiary-care paediatric neuromuscular clinics.

Patients 46 children (6–17 years) with progressive neuromuscular disease without neurocognitive impairment or dystrophinopathy.

Interventions Polysomnography, pulmonary function, manual muscle strength, quality of life (CHQ-PF50) and Conners questionnaires.

Outcome measures (a) Prevalence of NH; (b) predictive value of surrogate clinical measures for NH; and (c) differences in change over 1 year in pulmonary function, muscle strength, quality of life and attention between children with and without NH.

Results Prevalence of NH was 14.8%, 95% CI 8.0% to 25.7%. Maximal sensitivity and specificity for NH were achieved with thresholds of forced vital capacity <70% and forced expiratory volume in 1 s <65% predicted (sensitivities: 71.4, 71.4; specificities: 64.1, 79.5). Scoliosis also predicted NH (sensitivity 88.9; specificity 80.4). Over 1 year, those with NH had a greater increase in residual volume/total lung capacity (0.075 (−0.003 to 0.168) vs −0.03 (−0.065 to 0.028)), decline in muscle strength (−0.67 (−0.90 to 0.10) vs 0.53 (−0.05 to 0.90)) and worsened perception of health status.

Conclusions 15% of subjects had clinically unsuspected NH, predicted by moderate pulmonary function test impairment and scoliosis. Over 1 year those with NH had increased gas trapping, decline of muscle strength and worse perception of health status, despite NPPV.