Utility of Supine Spirometry in Children with Neuromuscular Disorders

Abstract

Background Children with neuromuscular disorders (NMD) have progressive loss of muscle strength and function which can lead to respiratory failure. Postural change in spirometry has been previously used in adult patients to detect diaphragmatic dysfunction, sleep disordered breathing and early respiratory failure. Postural spirometry changes in children with NMD have been advocated in guidelines for the assessment of NMD but not extensively studied. Aim To explore the relationship between postural changes in forced vital capacity [FVC] and polysomnography (PSG) in the assessment of early sleep disordered breathing in children with NMD. Method In this prospective cross sectional study, children with NMD from neurogenetic and respiratory clinics performed spirometry in sitting (si) and supine (su) position. Control group consisting of age and gender matched healthy children also underwent lung function testing. PSG was performed within 6 months of spirometry. Spirometry was acceptable according to ATS standards and PSGs scored according to AASM guidelines. Results Of 40 children with NMD who performed spirometry, 30 were able to perform acceptable supine spirometry. Underlying diagnoses were heterogeneous, with majority having Duchenne Muscular dystrophy (n=17). Mean age was 12.3 years (range 8 to 17yrs,SD ±3) with 21 males and 9 females. Mean FEV1sit and FVCsit were 78% (SD ±22) and 75%(SD±20.4) respectively. SpO2 mean 96% [92 to 98%] and SpO2 nadir was 87% (84 to 96%). Mean% ΔFVC (sit – sup) in these children was 9% (range +6 to -28%,SD±11). When compared with control group, there was a significant difference in supine spirometry between the two groups (p<0.001). PSG data was available on 30 children with NMD. Mean total AHI 6.9/hr (0.3 to 29,±5.9), obstructive AHI 5.2/hr (0.2 to 10), REM AHI 14.3/hr (0.1 to 34.7). On comparing supine spirometry with Total AHI, there was a moderate correlation with with presence of sleep disordered breathing (r=0.62, p=0.001) in those breathing spontaneously. Children established on non-invasive ventilation (NIV) showed a poor correlation with supine spirometry. In order to demonstrate the relationship of supine spirometry with evidence of hypoventilation, correlation was performed between change in postural spirometry and rise in CO2 from non-Rem to REM sleep. The mean (SD) rise was 6mmHg (±1.9) with a range of 4 mmHg to 11 mmHg. Coefficient of correlation was 0.04 (p=0.8) and therefore was not significant. When the NMD group was split between those spontaneously breathing and those established on NIV, there was a poor correlation between supine spirometry and rise in CO2 in both the groups spontaneously breathing (r=0.02,p=0.9) NIV (r=0.13,p=0.74) Conclusion In this cohort study, children with NMD with mild restrictive lung disease have postural changes in spirometry which are significantly greater than healthy controls. The changes in supine spirometry however do not correlate with nocturnal hypoventilation on polysomnography in patients with mild neuromuscular disease