Also reduced was the mean standard deviation in nighttime glucose values for individual patients, from 20.0 to 13.0 mg/dL (P=0.005), indicating more stable glycemia levels during sleep.

"Our findings suggest that screening type 2 diabetics for obstructive sleep apnea and treating those with moderate to severe sleep-disordered breathing could improve the management of their hyperglycemia and might favorably influence their long-term prognosis," the researchers wrote.

But they noted that the study excluded patients with poor compliance with CPAP -- a perennial problem with the treatment -- and hence may not be generalizable to all patients with sleep apnea and type 2 diabetes. The study does add, though, to the evidence that treatment of sleep apnea can lead to improvement in conditions that frequently accompany it.

Besides type 2 diabetes, sleep apnea is often associated with obesity and cardiovascular disease.

Patients in the trial were on a stable diabetic treatment regimen and had overnight oximetry data suggesting moderate to severe sleep apnea. All were newly diagnosed with sleep apnea and had never used CPAP.

A total of 28 were initially enrolled, but three were subsequently excluded because they were unable to sleep at least four hours per night with CPAP. Five others withdrew or were excluded for other reasons, leaving 20 for analysis. Patients underwent continuous glucose monitoring during sleep using an interstitial fluid sampling system. It was calibrated with blood glucose from a fingerstick in each patient. Participants also had their sleep monitored with polysomnography.

These procedures were performed once at enrollment and again after at least four weeks of CPAP therapy. In addition to lower mean nighttime glucose levels, patients also showed improvements in various measures of sleep quality and nocturnal respiration.

Mean apnea-hypopnea index values declined from 63 per hour at baseline to 7.9 per hour at the follow-up exam (P<0.001).  Daytime sleepiness, as measured by the Epworth scale, fell to 6.1 from 11.2 at baseline.

Patients' weight increased by 1 kg (P=0.02) -- they were instructed not to try to lose weight -- and glycated hemoglobin levels remained stable. Dr. Dawson and colleagues said the latter was not surprising, because the study did not last long enough for altered daily glucose levels to be reflected in glycated hemoglobin.

Wake time after sleep onset decreased by 10 minutes from 71 minutes at baseline (P=0.02). Sleep arousals plummeted, primarily because of almost total elimination of respiratory arousals -- from a mean of 51 at baseline to 2 at follow-up (P<0.001).

The researchers found that daytime glucose levels were largely unchanged.

Dr. Dawson and colleagues said the finding of reduced nighttime variability in glucose levels was important because glycemic fluctuations have previously been shown to be an independent risk factor for death and microvascular complications of type 2 diabetes.

Practice Pearls:
Explain to interested patients that the study found that when patients with type 2 diabetes and sleep apnea used continuous positive airway pressure at night, their glycemic control during sleep improved

Point out that the investigators measured interstitial glucose during polysomnography and did not demonstrate changes in hemoglobin A1c

Journal of Clinical Sleep Medicine, Dec 15, 2008: Dawson A, et al "CPAP therapy of obstructive sleep apnea in type 2 diabetics improves glycemic control during sleep" JCSM 2008; 4: 538-43.

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