Epidemiology and CV Impact of Obstructive Sleep Apnea (OSA)

Singapore is a Southeast Asian country with a population of 5.3 million people, comprising predominantly of Chinese, Malay and Indian ethnic groups. Obstructive sleep apnea (OSA), a sleep breathing disorder characterized by recurrent upper airway collapse, is estimated to be prevalent in up to 15% of the Singapore general population.¹ This estimate is significantly higher than the 1-5% prevalence estimates of OSA in Western populations.²

OSA is predominant in middle-aged, overweight Singaporean males, with the majority having moderate-to-severe OSA (AHI≥15). Similar to other Asian populations, it is observed that in Singapore, obesity is a relatively weaker risk factor for OSA compared to Caucasian populations.³ Taken together with the comparatively higher prevalence of OSA, it is hypothesized that local craniofacial characteristics, which may result in an anatomically narrow airway and a higher percentage of body fat, are important features in the increased predisposition of OSA in Asians.³

OSA has gained recognition as an important health concern in Singapore. The pathophysiological mechanisms of OSA, including oxidative stress, inflammation, hypercoagulability and endothelial dysfunction predispose patients with OSA to cardiovascular disease. It is established that untreated OSA is an independent predictor of future cardiovascular morbidities and mortality.^{4, 5} Of particular significance are the results from the Sleep heart health study, reporting OSA as a significant predictor of future major adverse cardiovascular events.⁶ The prevalence of OSA in patients with existing cardiovascular disease have been reported to be much higher than the general population. Existing studies report a 26% prevalence among coronary artery disease patients in Singapore who are at high risk of OSA.⁷ There is a need for a deeper understanding of the effects of OSA in cardiovascular disease patients for the diagnosis and management of OSA. In this article, we evaluate existing evidence relevant to this aspect in Singapore.

Prognosis of CVD Patients With OSA

There is an increasing focus on the implications of OSA in patients with coronary artery disease. In Singapore, it was observed that 65.7% of 120 patients presenting with acute myocardial infarction (AMI) had previously undiagnosed OSA (AHI > 15).⁸ The prevalence is consistent with clinical studies performed in the USA ⁹ and Portugal ¹⁰ that reported 66.4% and 43.1% of ACS patients had OSA (AHI > 10). However, a Japanese study reported a lower prevalence of 43% in a Japanese hospital when polysomnography was performed about two weeks after AMI onset.¹¹

After 18 months follow up of the patient cohort, it was reported that severe OSA was an independent predictor of adverse events.¹² Such patients with severe OSA have lower event-free survival rate and are 5.4 times more likely to suffer adverse events such as death, reinfarction, stroke, unplanned target vessel revascularization, and heart failure requiring hospitalization.

OSA is also associated with worse angiographic outcomes in patients with coronary artery disease. In a local study, the target vessel revascularization rate among patients with severe OSA was three times as high as those in the non-severe OSA group and that the late lumen loss of endothelial progenitor cell capturing stent was similar to that of bare metal stent.¹³

Advocating for More Effective OSA Screening

Earlier paragraphs have highlighted OSA as a growing health concern worldwide. However, some studies indicate that OSA remains highly underdiagnosed even in people with obesity or hypertension.¹⁴ According to a March 2011 ACC survey, 42% of cardiologists indicate a lack of satisfaction with the effectiveness of sleep apnea therapy as their primary barrier for referring patients to sleep centers and 29% believe the next barrier was the cost of a sleep study to the patient.

Perhaps one solution is to use a simple and validated screening tool such as the Berlin Questionnaire (BQ), Epworth Sleepiness Scale (ESS) and STOP-BANG questionnaire. The BQ contains 10 questions covering three categories including 1) snoring severity, 2) excessive daytime sleepiness, and 3) history of high blood pressure or obesity. The patient is instructed to answer questions to all three categories, and it is then scored by the physician or medical staff. The effectiveness of BQ is demonstrated by a number of studies which identified high risk OSA patients in different patient cohorts with prevalence ranging from 37.5% to 65%, including primary care setting, ¹⁵ acute coronary syndrome,¹⁶ recent myocardial infarction,¹⁷ cardiac rehabilitation,¹⁸ atrial fibrillation,¹⁹ and general cardiology service.⁷

Although screening questionnaires such as the BQ and ESS are relatively effective as OSA screening tools, the majority of these questions asked are non-specific symptoms which are highly present in CAD patients. As such, we set out to see if a clinical risk stratification tool (CHADS₂ score) would have a role in clinical identification of OSA CAD patients.²⁰ The CHADS₂ score is a clinical prediction tool that characterizes the risk of thromboembolic stroke²¹ in patients with non-valvular atrial fibrillation, and is widely used to aid in long-term anticoagulation decisions. Patients are scored by adding up the points that correspond to factors of CHADS₂. The factors include congestive heart failure, hypertension, age (at or above 75 years), diabetes mellitus and prior stroke. Each factor has been consistently reported to demonstrate an association with OSA in population-based epidemiological studies.^{22, 23} Using a different age cut-off of ≥65 years old because existing data shows a higher risk of OSA in people aged ≥65 ²⁴, the results showed that OSA patients had significantly higher CHA₆₅DS₂ scores than non-OSA patients (after adjusting for age, body mass index and clinical presentation). When compared to a patient with a CHA₆₅DS₂ score of 0, a patient with a CHA₆₅DS₂ score of one is 1.97 times more likely to have OSA. This risk increases to 2.85 times when CHA₆₅DS₂ score is at least two.

Singapore's Approach to OSA

Singapore is not spared from the insidious rising incidence of OSA. From an economic viewpoint, the majority of OSA patients comes from the economically-productive middle-aged segment of the local population. And from a medical viewpoint, OSA has several cardiovascular complications (e.g. arrhythmias, pulmonary hypertension, myocardial infarction, heart failure, systemic hypertension). We also highlighted the association of OSA is with a worse prognosis for patients who have co-morbid cardiovascular diseases.

One way to start is to address the under-diagnosis of OSA. Cardiologists in local practice can be encouraged to use screening questionnaires (such as the BQ or STOP-BANG) or a validated clinical prediction rule (CHA₆₅DS₂ score) to guide patient referrals for a sleep study. Such a focused approach would make more effective use of sleep studies. In the war against OSA, the goal is to achieve early diagnosis, start targeted treatment and improve long-term patient outcomes.

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Keywords: Ethnic Groups, Respiratory System, Singapore, Sleep Apnea, Obstructive, Sleep Apnea Syndromes

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