Impact of Adherence to Continuous Positive Airway Pressure on Outcomes in Obstructive Sleep Apnea Chronic Obstructive Pulmonary Disease Overlap Syndrome

To the Editor:

Sterling and colleagues analyzed the impact of adherence to positive airway pressure (PAP) treatment in obstructive sleep apnea (OSA) and chronic obstructive pulmonary disease (COPD) overlap syndrome and concluded that adherence to PAP treatment was associated with reduced all-cause hospitalizations and emergency room visits, severe acute exacerbations, and healthcare costs (1). This study has a significant impact owing to its compelling strengths: this is a large real-life study, easy to measure and generalize to the population and the individual patient, and its results are compatible with the currently available data (2).

Nevertheless, this study has some limitations that make the results challenging to interpret.

First, a selection bias might have led to more hospitalizations and ER visits in the nonadherent group. This group included far more patients with psychotic disorders than the adherent group (98 vs. 57 patients, respectively, out of 712). This correlates not only with low adherence to PAP therapy but also to general medical nonadherence, less control of comorbidities, and increased risk of subsequent medical consequences (3). Moreover, the nonadherent group was less obese than the adherent group (34.7% vs. 40% morbidly obese and 34% vs. 30.2% nonobese, respectively). It is now known that a higher proportion of nonobese patients have a low respiratory arousal threshold, suggesting that nonanatomical causes may be particularly important for the pathogenesis of OSA in the absence of obesity (4). These patients are less likely to adhere to PAP therapy (4); in fact, the preferable treatment and the prognosis in these patients are still to be studied. Furthermore, 51.8% of the nonadherents were tobacco users, compared with 44.4% in the adherent group. Thus, the former group might be at a higher risk for adverse health events regardless of PAP therapy.

Second, previous COPD exacerbation, the single most important predictor of future exacerbations, which is among the leading causes of emergency room visits and hospitalizations (5), was not reported in the study. Although the overall emergency room visits and hospitalizations in the year before PAP therapy were reported, excluding previous COPD exacerbations in patients with COPD as inclusion criteria, may affect the reliability of the model.

Finally, the study does not distinguish between patients who are hypercapnic and nonhypercapnic, nor between continuous PAP and bilevel PAP. As noninvasive positive pressure ventilation reduces mortality in patients who are hypercapnic with COPD regardless of OSA status (6), the unpublished data regarding hypercapnia status makes results interpretation challenging.

Despite what preceded, we believe that OSA COPD overlap syndrome should be treated on the basis of the current guidelines for OSA and COPD, including PAP as a cornerstone of OSA treatment. We also believe that overlap syndrome has more health impact than either OSA or COPD on the basis of the cumulative data. However, the exact impact of PAP treatment and the causality for health improvement should be further studied.