In the January issue of Sleep, there is an article that studied the effect of three months of CPAP use on daytime sleepiness, daytime mental functioning, and quality of life in patients diagnosed with moderate to severe obstructive sleep apnea (OSA).
The authors explain that excessive daytime sleepiness (EDS) is common in patients with and without OSA. Other causes of EDS are obesity, age, typical sleep duration, diabetes, and smoking. Also, studies have shown that EDS does not improve in every patient with OSA even if they are optimally treated with CPAP.
In this study, the authors performed a maintenance of wakefulness test (MWT) after three months of CPAP use, which measures a patient's ability to stay awake throughout the day. The Federal Aviation Administration uses the MWT results in pilots with OSA to determine if the CPAP is working such that they can fly safely. Interestingly, there was no improvement in the MWT results even in patients who were optimally compliant with CPAP.
The authors measured subjective sleepiness with a common question and answer scale called the ESS. Finally, the measured daytime mental functioning with subjective (question and answer tests) and objective computer testing.
The ESS scale showed improvement with more CPAP use. However, even if the patient used CPAP 7 hours per night, 19% of compliant OSA patients had an elevated ESS, indicating persistent sleepiness.
Daytime mental function levels varied considerably in OSA patients before treatment. There was a predictable improvement in some patients who used CPAP on some of the measures of daytime functioning, but not in everyone, including those who used the CPAP an optimal amount.
The results of this study are consistent with other studies' results and with my clinical experience. Not everyone gets all the way better with regular CPAP use. There are other reasons to use CPAP consistently, though, including cardioascular protection.
Another intersting point brought up by the authors was whether the MWT is the most appropriate measure of a pilot's ability to fly safely with regards to EDS. Perhaps these pilots should have a different daytime sleepiness measure such as the MSLT, which is used to diagnose narcolepsy.