Introduction: Obstructive sleep apnea (OSA) is a common chronic disorder consisting of episodes with impaired breathing due to obstruction of the upper airways. Treatment with Continuous Positive Airway Pressure (CPAP) is a potentially effective treatment, but adherence is low. Several potential factors affecting adherence, e.g., subjective sleepiness and personality, are only quantifiable through questionnaires. Better knowledge about psychometric properties of such questionnaires might improve future research on CPAP adherence and thus lead to better treatment options.

Aim: Study I: To describe the devlopment and initial testing of the Side Effects of CPAP treatment Inventory (SECI) questionnaire. Study II: To describe the prevalence of Type D personality in OSAS patients with CPAP treatment longer than 6 months and the association with self-reported side effects and adherence. Study III: To study whether any of the items in the Epworth Sleepiness Scale (ESS) exhibit differential item functioning and, if so, to which degree. Study IV: To examine the evolution of CPAP side effects over time; and prospectively assess correlations between early CPAP side effects and treatment adherence.

Patients and Methods: In study I, SECI items were based on a literature review, an expert panel and interviews with patients. It was then mailed to 329 CPAP-treated OSAS patients. Based on this, a principal component analysis was performed, and SECI results were compared between adherent and non-adherent patients. In study II, the population consisted of 247 OSAS patients with ongoing CPAP treatment. The DS14 was used to assess the prevalence of type D personality, and SECI and adherence data from medical records were used to correlate Type D personality to side effects and adherence. In study III, the population consisted of pooled data from 1,167 subjects who had completed the ESS in five other studies. Ordinal regression and Rasch analysis were used to assess the existence of differential item functioning for age and gender. The cutoff for age was 65 years in the Rasch analysis. In study IV, SECI was sent to 186 subjects with newly diagnosed OSAS three times during the first year on CPAP. SECI results were followed over time within subjects, and were correlated to treatment dropout during the first year and machine usage time after 6 months.

Results: SECI provides a valid and reliable instrument to measure side effects, and non-adherent patients have higher scores (i.e., were more bothered by side effects) than adherent patients (study I). Type D personality was prevalent in approximately 30 % of CPAP treated OSAS patients, and was associated to poorer objective and subjective adherence as well as more side effects (study II). Differential item functioning was present in items 3, 4 and 8 for age in both DIF analyses, and to gender in item 8 the Rasch analysis (study III). Dry mouth and increased number of awakenings were consistently associated to poorer adherence in CPAP treated patients. Side effects both emerged and resolved over time (study IV).

Conclusions: Differences in previous research regarding side effects and CPAP adherence might be explained by differences in how side effects and adherence are defined. While some side effects are related to adherence, others are not. Side effects are furthermore not stable over time, and might be related to personality. ESS scores are also related to CPAP adherence according to previous research, but might be affected by other factors than sleepiness, such as age and possibly gender.