The article by Guest et al. (1) addresses a hotly debated topic. While many studies have examined the impact of obstructive sleep apnea (OSA) on glucose metabolism and cardiovascular disease in patients without diabetes, little is known about the impact of OSA on clinical outcomes in patients with type 2 diabetes—hence the importance of the findings by Guest et al. The importance of this area is further emphasized by the high prevalence of OSA in patients with type 2 diabetes (2,3).
The impact of continuous positive airway pressure (CPAP) on glycemic measures is unclear. Although uncontrolled studies have shown that CPAP improved glycemic control in patients with type 2 diabetes, the only randomized controlled trial has not shown such benefit (2,4). This could be due to a true lack of effect or to factors related to the study populations and duration and compliance with CPAP (2,5).
The current case-control observational study showed an average HbA1c difference of 3.9% between treated and untreated groups by the end of the 5-year follow-up. This resulted from a greater deterioration in glycemic control in the control group compared with the CPAP-treated group. Although these results are intriguing, the deterioration in glycemic control occurred very early in the study in both groups; the mean HbA1c increased by 1.3% and 5.2% in the CPAP-treated and control groups, respectively, by year 2 of follow-up. Taking into account that the glycemic control did not deteriorate any further (in fact it improved slightly) in the control group despite not having CPAP, these differences observed as early as year 2 are difficult to attribute to CPAP treatment. Although deterioration in glycemic control in patients with type 2 diabetes due to the continuous decline in β-cell function is to be expected, an HbA1c worsening of 5.2% over 1 year in the control group is highly unusual in real-life or research settings. This is supported by the fact that the HbA1c only deteriorated by 0.5% between the two HbA1c measurements in the control group during year 1. Such significant worsening in HbA1c in the untreated group might reflect factors that were not measured in this observational study.
Furthermore, despite the deterioration in glycemic control in the control group over the 5-year period, there appears to have been no significant intensification of insulin treatment. In fact, the prescription of insulin was twice as much in the CPAP-treated versus control group, although this was not statistically significant. In addition, the CPAP-treated group showed a very high level of compliance with CPAP—much more than that reported in research studies (as highlighted by the authors)—and such a group of motivated patients are likely to be adherent to other treatments, including glucose-lowering agents, although objective data about CPAP usage was not available for the majority of the study population. All these factors make it difficult to draw firm conclusions from this study regarding the impact of CPAP treatment on glycemic control.
The improvements in blood pressure observed in the study are more compelling, although the observational nature of the study will still limit interpretation of the findings.
Well-designed clinical trials of adequate treatment duration are needed to answer the question of the impact of CPAP on clinical outcomes in patients with type 2 diabetes, although early studies suggest a favorable impact (6,7).
Funding. A.A.T. is a clinician scientist supported by the National Institute for Health Research in the U.K.
Duality of Interest. No potential conflicts of interest relevant to this article were reported.
The views expressed in this publication are those of the author(s) and not necessarily those of the National Health Service, the National Institute for Health Research, or the Department of Health.