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)