Introduction
Neuromuscular disorders (NMD) are rare in the general population with an estimated prevalence of approximately 1 in 3000.(1) These conditions predominantly have a genetic basis and often present during childhood. Respiratory morbidity leading to respiratory failure is the common pattern seen in children and adults with neuromuscular weakness.(2) The degree of respiratory involvement in these disorders is variable and can present at differing ages in children.(2) This predominantly depends on the underlying NMD with other factors including lower respiratory illness, scoliosis and pulmonary aspiration having an effect on the age of presentation of respiratory failure.(2-4) Appropriate screening for respiratory failure and subsequent intervention has been shown to reduce unplanned hospital admissions and improve life expectancy.(2, 5-7)
Involvement of respiratory muscles causes significant clinical sequelae, with recurrent respiratory illness and consequently routine respiratory monitoring is recommended in children with NMD.(8) Detailed evaluation relies on additional testing, which includes both invasive tests and non-invasive tests. Amongst the invasive tests the most reliable is the measurement of the oesophageal (Poes ) and gastric pressures (Pgas ). Amongst the non-invasive tests, vital capacity (VC), maximal inspiratory/expiratory pressures (MIP/MEP), sniff nasal inspiratory pressure (SNIP), the peak expiratory flow (PEF) and cough peak flow (CPF) have been studied extensively and are in clinical use in a number of centres.(9) These tests are limited by the need for patient cooperation to achieve the required technical quality standards.(8, 10) Patients with NMD are typically at risk for sleep disordered breathing (SDB) and hypoventilation. SDB is often the first signs of progressive respiratory decline which can manifest as REM- associated hypoventilation and then into continuous nocturnal hypoventilation.(10)
Spirometry measured in the supine position has been studied in adult patients with NMD. (11, 12) In the seated position it is estimated that the diaphragm contributes to nearly 70% of tidal breathing and the intercostal muscles approximately 30%.(1, 13) However, in the supine position, the diaphragm contributes nearly 90% of breathing done by a normally functioning diaphragm when upright.(14, 15) Studies in adults with amyotrophic lateral sclerosis demonstrated that supine spirometry has a sensitivity of 79% and specificity of 90% to detect diaphragmatic weakness.(14, 15) Supine spirometry has been suggested as a screening test to detect diaphragmatic weakness in children with NMD based on a small number of studies that predominantly involve adults.(11, 16) This screening test has been included in paediatric management guidelines despite being based on data extrapolated from adult studies.(17, 18){, 2012 #449;Wang, 2012 #489} There is a paucity of data on supine spirometry in healthy children, let alone children with neuromuscular disease. The aims of our study were to (1) test the feasibility of supine spirometry in normal children and children with neuromuscular diseases and (2) to correlate the degree of respiratory dysfunction measured by supine spirometry in children with neuromuscular disease with polysomnography derived parameters of the effectiveness of gas exchange.(19-23)