Dose-dependent effect of individualized respiratory muscle training in children with Duchenne muscular dystrophy
Introduction
In patients with Duchenne muscular dystrophy (DMD), progressive respiratory muscle weakness is responsible for the deterioration in respiratory function and could result in inadequate ventilation in the face of increased mechanical loads and high ventilatory requirements. This may lead to respiratory failure [1], which generally occurs late in the disease and is the cause of mortality [2]. Most investigators have therefore supported the specific training of respiratory muscles in patients with neuromuscular disease as an important component among the treatment modalities to improve respiratory function [3], [4], [5], [6]. Training is best begun in the early stages of DMD in children with only moderate impairment, as determined from lung function tests [3], [7], [8].
The training results are, however, contradictory. Some authors have found that respiratory muscle training led to no improvement [9] or small, non-significant [7] improvement in strength or endurance. Most [3], [4], [5], [8] have shown that specific training induced improvement, essentially in muscle endurance.
The contradictory results could be due to many factors. First, the methods of evaluating endurance differ from study to study. These methods include the maximal time the subject is able to sustain a predetermined percentage of the maximal minute ventilation [8] or the transdiaphragmatic pressure [3], the maximal ventilation produced during a predetermined period [7], or the maximal pressure sustained by the respiratory muscles for 10 min [9]. Second, as we suggested in a recent study [10], the contradictory results may be in part explained by a lack of similarity in the methods used for training and evaluation [8], [9], in addition to a lack of a standardized and reproducible method of evaluating endurance. Indeed, with the exception of a study by Wanke et al. [3], patients in previous studies were able to modify their tidal volume (Vt) and inspiratory and expiratory time (Ti, Te), even though a meta-analysis [11] concluded that a constant breathing pattern should be imposed during the loaded inspiratory period to make the method reproducible. In consequence, we developed a standardized method using controlled breathing pattern [10] that allows us to have reproducible endurance measurement in DMD children without learning disability.
The aim of this double-blind, placebo-controlled study was thus to evaluate the effects of short term and low intensity respiratory muscle training at home, using a reproducible method of evaluating respiratory muscle endurance, and the same method both to evaluate and to train children.
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Subjects
Sixteen male patients with DMD followed by the Neuropediatrics Department of St Eloi University Hospital Center in Montpellier, France, were studied for 2 years. All children who participated in our study completed it. Before beginning the study, the patients were randomized to receive either placebo training (eight subjects) or specific inspiratory muscle training (eight subjects). This study was double-blinded in that neither the physician who performed the tests nor the children were aware
Results
All children who were included in the study finished it. Mean age, anthropometric characteristics and spirometric values of the training and control groups are reported in Table 1. None of the differences between groups were statistically significant. The values of spirometric parameters after the period of training in the training group and control group (VC: 1.78±0.31 vs. 1.76±0.79 l; FRC: 1.87±0.62 vs. 1.47±0.41 l; and TLC: 3.24±0.48 vs. 2.79±0.88 l) were not significantly different from those
Discussion
The main finding of the present study is that 6 weeks of low intensity inspiratory resistive training at home, using the same method with control of breathing pattern to evaluate and train the children, significantly improved inspiratory muscle endurance without improving strength. We also found a positive correlation between endurance improvement and total time of respiratory muscle training. Thus this significant effect on endurance seems to be dose-dependent. This result highlights the
Acknowledgements
We thank Dr Humberclaude and the Institute of Palavas for their collaboration. This investigation was supported by the ‘Association Française contre les Myopathies’ (AFM), (grant no 5395).
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