PROXIMO: A Cohort Study to Investigate the Incidence of Obstructive Sleep Apnoea Using Prolonged Overnight Oximetry in an Unselected Population Referred to Secondary Care Reporting Excessive Daytime Sleepiness

Obstructive sleep apnoea (OSA) is a common condition, effecting an estimated 1.5 million people in the UK and it is believed up to 80% of them may be undiagnosed [1]. During sleep the muscles supporting the upper airway tend to relax and the throat narrows. In patients with OSA the narrowing leads to collapse and pauses in breathing. These pauses in breathing are accompanied by a decrease in arterial oxygen saturation and to overcome the airway collapse the body responds by producing a surge in the sympathetic nervous system. This has the effect of stimulating the pharyngeal muscles to reopen the throat but also causes an increase in heart rate, blood pressure and disrupts the sleep cycle waking the patient up. These repeated disruptions to sleep leads to the patient feeling unrefreshed by sleep, excessively sleepy during the day and at increased risk of a sleepiness-related accident. The repetitive surges in the sympathetic nervous system also contributes to the risk of long term cardiovascular disease. The frequency with which these events occur determines the severity of obstructive sleep apnoea, less than 5 events per hour is considered normal, 5-15 mild, 15-30 moderate and more than 30 severe disease. However the disease severity does not correlate with the degree of symptoms [2] meaning it is important to identify patients even if they have mild and moderate disease.

OSA is effectively treated with continuous positive airway pressure (CPAP) therapy. These machines blow air into the throat through a nose or face mask. This helps to splint the airway open and reduce the disruption to sleep. CPAP is an effective therapy improving sleepiness, snoring, mood and reduces the long term risk of cardiovascular disease seen in untreated patients with OSA.

Full inpatient polysomnography testing is the gold standard investigation for diagnosis and exclusion of OSA [3]. However because polysomnography generally requires a hospital bed, significant technical input to set up and is time intensive to report it is impractical for the large numbers of patients referred to sleep services to undergo this test. Overnight oximetry recording has been established as an effective alternative for screening patients and identifying OSA in many sleep centres [4-6].

For a single test to be effective it needs to be highly sensitive and the condition being investigated should be stable. However significant variation in the severity of obstructive sleep apnoea has been demonstrated from night to night in individual patients [7, 8]. Therefore using oximetry or even polysomnography recording for a single night may not truly reflect the overall condition. Patients with mild and moderate disease show the most significant variations on a night to night basis, as the oxygen desaturation index (ODI, number of times the oxygen saturations dip per hour) rises, i.e. more severe OSA, less night to night variability is observed [9]. When patients with previously diagnosed OSA are withdrawn from CPAP treatment and overnight oximetry performed for two weeks only 22% of participants consistently remain within one severity category. The majority of these patients were in the severe category. Other studies have shown 40-50% of patients changing severity category in two to four nights recordings [7, 10] and that 16-20% of patients would not have received CPAP treatment based on a single night study but did so after repeating between one to three studies [10, 11].

The most clinically relevant distinction to make is between no OSA and mild and moderate OSA as this determines whether a trial of CPAP therapy would be offered. These previous trials suggest it is within this milder spectrum of the disease where a single night's oximetry recording is least reliable and more than one night's recording may be required.