Participants and Data

The study was approved and conducted in compliance with the regulations set by the Institutional Review Board. We retrospectively screened all adult patients who had a full-night PSG at two different tertiary university hospital sleep centers in South Korea between January 2013 and November 2015. Based on the PSG and clinical findings, 594 patients received a diagnosis of OSA^16,17; no patients had prior history of treatment for OSA. Inclusion criteria were ages 20–75 years and an AHI of ≥ 5 events/h. In our sleep laboratories, we use AHI to define the severity of OSA instead of respiratory effort-related arousals and the respiratory disturbance index. We excluded OSA with comorbid sleep disorders other than insomnia (ie, restless legs syndrome, narcolepsy, parasomnia, and epilepsy) (Figure 1). A total of 476 adult patients with OSA were enrolled in this study.

AHI = apnea-hypopnea index, ISI-K = Insomnia Severity Index, Korean version, OSA = obstructive sleep apnea, PSG = polysomnography, RLS = restless legs syndrome.

The PSG tests were performed using 32-channel Grass-Telefactor Comet digital recording polysomnographic system (Natus Medical Incorporated, Pleasanton, California, United States) and were manually scored.¹⁸ Scoring of sleep stages, respiratory events including apneas, hypopneas, arousal, and periodic limb movement were performed according to the American Academy of Sleep Medicine guidelines.¹⁹ Apnea was defined as a drop in the peak signal excursion by greater than or equal to 90% of the preevent baseline using an oronasal thermal sensor lasting for 10 seconds or longer. Hypopnea was defined as a nasal pressure signal drop to more than 30% of baseline that lasted for 10 seconds or longer and was associated with either more than 3% oxygen desaturation or an arousal.²⁰ Sleep onset latency, wake after sleep onset, and sleep efficiency were recorded. Arousal index was recorded both as total, as well as three subcategories of apnea-related, periodic limb movement-related, and spontaneous.

Health conditions were evaluated using a questionnaire inquiring about heart disease, hypertension (HTN), diabetes mellitus (DM), hyperlipidemia (HL), and current smoking habits. Smokers were classified by their use of tobacco, regardless of the amount. Heart disease included ischemic heart disease, arrhythmia, and valvular disease. Subjects with any cardiac history or current medications for heart disease were classified as having heart disease. There were no subjects with congenital heart disease. All subjects with HTN, DM, and HL were under the care of a physician. All subjects completed sleep-related and psychiatric questionnaires including the Korean versions of the Insomnia Severity Index (ISI-K),² the Pittsburgh Sleep Quality Index (PSQI-K),²¹ the Epworth Sleepiness Scale (ESS-K),²² the Beck Depression Inventory (BDI-2-K),²³ and the Short-Form 36-Item Health Survey (SF-36-K).²⁴ The results from these questionnaires were compared between male and female patients. As the ISI-K measures symptoms other than sleepiness (eg, worry, distress, and daytime interference, which are shared with OSA), we analyzed the results using the subtotal of ISI-K score in the first three sleep items.²⁵

We considered insomnia as an independent entity as defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5)²⁶ without stratifying it into different subtypes. A validated questionnaire, ISI-K, was used to measure the patients' perception of insomnia.² ISI-K is a seven-item self-reported questionnaire measuring daytime effect as well as impairments caused by insomnia. An ISI score of 15 or greater is considered clinically significant.² The patients were divided into two groups based on their ISI-K score. Those with an ISIK score ≥ 15 were categorized as OSA with insomnia (OSA+I), and those with an ISI-K score < 15 were categorized as OSA without insomnia (OSA−I). Given the mismatch between abnormal PSG findings and clinical symptoms in some elderly patients, prior studies have used sleepiness as an added screening tool.²⁷ Sleep propensity was defined as ESS-K score ≥ 11.²² In what follows, we refer to OSA+I and OSA−I groups as “the two groups” for brevity.

Demographic and clinical characteristics were compared between the two groups, as well as the subgroups of OSA−I based on ESS-K (Table 1). We also screened 77 patients who had started treatment with continuous positive airway pressure (CPAP), and telephone interviews were conducted in order to investigate differences in CPAP adherence between the two groups. A questionnaire was used for the telephone interviews that included current use of CPAP, average hours used per night, the number of nights per week, duration of use, and the reason for discontinuation. We defined adherence as using CPAP for more than 4 hours per night and more than 5 nights per week.²⁸ All subjects were contacted in order to assess their adherence by using a questionnaire that was designed for this purpose (Figure 2).

Demographic and clinical characteristics.

This algorithm was used to determine CPAP adherence in patients with OSA. CPAP = continuous positive airway pressure, OSA = obstructive sleep apnea.