Educational Aims
The reader will come to appreciate:
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Supine spirometry is feasible in most children with DMD.
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Supine spirometry did not predict sleep hypoventilation in children with DMD complicated by mild restrictive lung disease.
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There are limited data regarding normative values for supine spirometry.
Abstract
Aim
To explore the relationship between postural changes in lung function and polysomnography (PSG) in children with Duchenne muscular dystrophy (DMD).
Methods
In this prospective cross-sectional study, children with DMD performed spirometry in sitting and supine positions. A control group of age and gender matched healthy children also underwent postural lung function testing. PSG was performed within six months of spirometry.
Results
Seventeen children with DMD, aged 12.3 ± 3 years performed sitting spirometry. 14 (84%) performed acceptable spirometry in the supine position. Mean FEV 1 sit and FVC sit were 77% (SD ± 22) and 74% (SD ± 20.4) respectively, with mean% ΔFVC ( sit–sup ) 9% (SD ± 11) (range 2% to 20%), and was significantly greater than healthy controls 4% (n = 30, SD ± 3, P < 0.001). PSG data on the 14 DMD children with acceptable supine spirometry showed total AHI 6.9 ± 5.9/hour (0.3 to 29), obstructive AHI 5.2 ± 4.0/hour (0.2 to 10), and REM AHI 14.1 ± -5.3/hour (0.1 to 34.7). ΔFVC(sit–sup) had poor correlation with hypoventilation on polysomnography.
Conclusion
Children with DMD and mild restrictive lung disease showed greater postural changes in spirometry than healthy controls but lower supine spirometry was not predictive of sleep hypoventilation.
Introduction
Neuromuscular disorders (NMD) are rare in the general population with an estimated prevalence of approximately 1 in 3000. Duchenne muscular dystrophy (DMD) is the most prominent neuromuscular disorders affecting about 1 in 5000 male live births and caused by gross deletions or duplications (70% of cases) and point mutations (30% of cases) in X-linked DMD gene. Boys with DMD typically present in early childhood with proximal lower limb weakness and markedly elevated creatine kinase (CK) levels secondary to ongoing breakdown of muscle fibres. Given the multiorgan expression of dystrophin, respiratory insufficiency, dilated cardiomyopathy and cognitive impairment are also common features. The degree of respiratory involvement in DMD is variable and can present at differing ages in children. This depends predominantly on factors including lower respiratory illnesses, scoliosis and pulmonary aspiration having an effect on the age of presentation of respiratory failure. Appropriate screening for respiratory failure and subsequent intervention has been shown to reduce unplanned hospital admissions and improve life expectancy. .
Involvement of respiratory muscles causes significant clinical sequelae, with recurrent respiratory illnesses and, consequently, routine respiratory monitoring is recommended in children with DMD. Detailed evaluation relies on additional testing, including both invasive and non-invasive tests. Amongst the invasive tests the most reliable is the measurement of oesophageal (P oes ) and gastric pressures (P gas ). The non-invasive tests, vital capacity (VC), maximal inspiratory/expiratory pressures (MIP/MEP), sniff nasal inspiratory pressure (SNIP), peak expiratory flow (PEF) and cough peak flow (CPF) have been studied extensively and are in clinical use in a number of centres. These tests are limited by the need for patient cooperation to achieve the required technical quality standards. Patients with DMD are typically at risk for sleep disordered breathing (SDB) and hypoventilation. SDB is often the first sign of progressive respiratory decline which can manifest as REM-associated hypoventilation and then progress to continuous nocturnal hypoventilation. .
Spirometry measured in the supine position has been studied in adult patients with neuromuscular disorders(NMD). In the seated position it is estimated that the diaphragm contributes to nearly 70% of tidal breathing and the intercostal muscles approximately 30%. However, in the supine position, the diaphragm contributes nearly 90% of breathing done by a normally functioning diaphragm when upright. Studies in adults with amyotrophic lateral sclerosis (ALS) demonstrated that supine spirometry has a sensitivity of 79% and specificity of 90% to detect diaphragmatic weakness. 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. This screening test has been included in paediatric management guidelines despite being based on data extrapolated from adult studies. {, 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 test the feasibility of supine spirometry in otherwise well children and children with DMD and to correlate the degree of respiratory dysfunction measured by supine spirometry in children with DMD with PSG- derived parameters of the effectiveness of gas exchange. .
Methods
This was a cross-sectional, prospective study approved by the Human Research Ethics Committee at the Children’s Hospital at Westmead (LNR/12/SCHN/280). Patients diagnosed with DMD managed through the Neurogenetics clinic at the Children’s Hospital at Westmead were invited to participate in the study. Controls were siblings or friends of neuromuscular patients or children of staff members who had no previous significant respiratory health issues. The inclusion criteria were a proven diagnosis of DMD (via genetics, muscle biopsy or nerve conduction studies), aged 8 to 18 years, anticipated cognitive capacity to perform seated and supine spirometry and willingness to have an overnight PSG. Written informed consent was provided by the parent or primary caregiver and children gave their verbal consent to perform the test. The recruitment of patients and healthy controls ran for 24 months,
Data collection was performed during two visits. During the first visit, demographic data were recorded and pulmonary function testing was performed. At the second visit overnight PSG parameters were recorded. Demographic information was gathered on the following: (1) diagnosis, (2) age at diagnosis, (3) height, (4) weight, (5) body mass index (BMI), (6) gender, (7) use of wheelchair or walking aids and (8) other medical conditions (e.g. asthma). Testing was offered during their routine clinic visits (scheduled at six monthly intervals). Forced vital capacity (FVC) and forced expiratory volume in one second (FEV 1 ) were measured with a Lilly type pneumograph (Viasys Healthcare, California, USA) according to ATS/European Respiratory Society (ERS) standards. Children were tested in a conventional upright seated position followed by a supine position while wearing a nose clip. As per ATS/ERS guidelines, the best effort, determined as the measurement with the highest sum of FVC and FEV 1 , was recorded for the study. Values were expressed as a percentage of predicted normal values (based on healthy children of the same age, gender, and height). Reference values were derived from published data. As there are no published reference standards for supine FVC, percent predicted supine FVC was calculated using predicted values for upright FVC. Children who were unable to produce acceptable and repeatable spirometry in sitting positions according to standardised ATS/ERS criteria were excluded. Children with acceptable and repeatable sitting lung function were asked to perform spirometry in the supine position during the same clinic visit.
Overnight PSG was only performed in the children with DMD, and all were undertaken at the David Read Sleep Unit, in The Children’s Hospital at Westmead, NSW, Australia, within six months of performing pulmonary function tests. Children established on NIV previously had their most recent PSG parameters of the diagnostic component of sleep study accessed for this study. Corresponding spirometry performed on the day of the PSG was used for the analysis. PSG was performed in accordance with the 1997 ATS guidelines using the Sandman Elite® Version 9.2 system (Embla Systems, Broomfield, CO, USA). Data were collected according to standardised recommendations, commenced between 19:30 and 21:00, and ended at 06:00 the following morning. Data analyses were performed in accordance with the 2007 AASM guidelines. The modified Epworth Sleepiness Scale (mESS) questionnaire was completed by the child or the parent at the time of the PSG.
Respiratory events were scored if they were at least two respiratory cycles long, and significant oxygen desaturations were defined as ≥3% desaturation from baseline. Children were classified as having SDB if polysomnography results showed an AHI > 1.0 events/h. The severity of SDB was further classified. Mild SDB was defined as an AHI of 1.0 to 4.9 events/h, moderate SDB as 5 events/h to 9.9 events/h and severe SDB as an AHI > 10 events/h. Nocturnal hypoventilation was defined according to AASM as > 25 % of sleep time with a TcCO 2 > 50 mmHg or a PCO 2 rise of >10 mmHg from baseline .
Pearson correlations and linear regression models were used to examine associations between the measures of respiratory function and PSG. Sensitivity and specificity results were calculated. Logistic regression was used to examine the ability of change in ΔFVC ( sit–sup ) to predict a binary outcome of an AHI ≥ 5 events/hour. All analyses were conducted in SAS version 9.3 (SAS Institute Inc., Cary, NC, USA). There was no adjustment made for multiple statistical comparisons. A P- value of <0.05 was considered statistically significant.
Results
Out of seventeen patients who were enrolled in the study, fourteen (82%) were able to perform spirometry in both sitting and supine positions. The mean age at testing was 11.5 years (SD ± 3.0). Thirty controls without respiratory or sleep pathology were recruited. Characteristics of the two study groups are shown in Table 1 . Children with DMD were further categorised as spontaneously breathing (SB) (N = 10) or established on nocturnal NIV (N = 4).
| Parameter | DMD(n = 14) | Controls(n = 30) |
|---|---|---|
| Age mean (±SD) years | 11.5 (±3) | 12 (±2) |
| Gender (M/F) | 14/0 | 18/12 |
| Weight median (range) (kg) | 39 (19.8 to 60) | 43 (24 to 54) |
| Height median (range) (cm) | 140.3 (114 to 166) | 143 (123 to 166) |
| BMI median (range) (kg/m 2 ) | 20.8 (10 to 28) | 17.6 (15.1 to 19) |
Pulmonary function testing
An average of 15 min was taken to perform reliable supine spirometry after performing sitting spirometry. In the DMD group, the mean sitting FEV 1 % and sitting FVC% were 78% (SD ± 21.9) and 75% (SD ± 20.5), respectively, as shown in Table 2 . The percentage difference in mean% ΔFVC ( sit–sup ) between sitting and supine in these DMD children was 9% (±11).
| NMD (n = 14) | Controls (n = 30) | P -Value* | |||
|---|---|---|---|---|---|
| All (n = 14) | SB (n = 10) | NIV (n = 4) | |||
| Age mean year (±SD) | 12.3 (±3.0) | 11.5 (±2.8) | 14 (±2.1) | 12 (±2) | NS |
| Gender (M/F) | 14/0 | 10/0 | 4/0 | 17/13 | NS |
| BMI kg/m 2 median (range) | 20.8 (10 to 28) | 19.9 (11 to 28) | 21.7 (10 to 25) | 17.6 (15.1 to 19) | 0.024 |
| FEV 1 (sit) mean % (±SD) | 78 (±21.9) | 83 (±21.9) | 72 (±17) | 95 (±5.2) | <0.001 |
| FVC (sit) mean % (±SD) | 75 (±20.5) | 78 (±20.5) | 70 (±24) | 98 (±4.8) | <0.0001 |
| FEV 1 (sup)% | 72 (±22.1) | 83 (±24.1) | 53 (±22.2) | 94 (±6) | <0.0001 |
| FVC (sup)% mean (SD) | 66 (±21) | 70 (±22) | 58 (±18) | 94(±4) | <0.0001 |
| FEV 1 /FVC% mean (SD) | 86 (±7) | 86 (±7) | 85 (±7) | 95 | 0.003 |
| FVC % (sit–sup) mean (SD) | 9 (±11) | 7 (±8) | 12 (±6) | 4 (±3) | 0.03 |
| % ΔFVC(sit–sup) | 12 | 8.9 | 17 | 4 | 0.03 |
| PSQ-22 median (range) | 0.30 (0.12 to 0.5) | 0.28 (0.12 to 0.5) | 0.31 (0.2 to 0.5) | 0.25 | NS |
| mESS median (range) | 7 (0 to 6) | 5 (0 to 6) | 9 (0 to 10) | 5 | NS |
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