Excessive daytime sleepiness (EDS) is the primary concern for many patients with sleep disorders and is characterized by the inability to stay awake, alert, and optimally functional throughout the day. EDS has been estimated to affect between 2.5% and 26.1% of the general population, varying depending on the definition applied and method of assessment.
The most common causes of EDS are insufficient sleep and poor sleep quality. Sleep continuity is a crucial factor in determining sleep quality, with sleep breathing disorders such as sleep apnoea representing a common cause of sleep disruption. Many neurological and psychiatric disorders are associated with EDS.[3, 4]
In narcolepsy, a chronic disorder characterized by the dysregulation of the sleep/wake cycle, EDS is the cardinal symptom[5, 6] and is generally present in all patients.[5–8] Narcolepsy is characterized by recurrent, uncontrollable brief episodes of sleep and lapses in consciousness; other symptoms of narcolepsy include hypnagogic and hypnopompic hallucinations, cataplexy, sleep paralysis, and disrupted nocturnal sleep. The prevalence of narcolepsy in the general population is relatively low, estimated to be around 0.04%.
EDS is also a prominent symptom in obstructive sleep apnoea (OSA), which is characterized by frequent partial arousals that occur throughout sleep as a result of recurrent episodes of partial or complete collapse of the upper airway. OSA has a high prevalence in the general population; a 2019 study estimates OSA affects 936 million adults aged 30 to 69 years globally, or approximately 12% of the world’s population. This study extrapolated country-specific prevalence rates, including five European countries (France, Germany, Italy, Spain, United Kingdom [UK]), with prevalence of moderate to severe OSA estimated at 21.4%, ranging from 4.8% in the UK to 36.3% in France. While many studies examining the prevalence of OSA in Europe have shown varying rates between countries, one consistent finding is that the prevalence of OSA is higher in men than women.[11–16] The prevalence of OSA associated with EDS is approximately 3 to 7% for men and 2 to 5% for women in the general population, and it has been found that residual EDS remains in up to 13% of patients even after continuous positive airway pressure (CPAP) treatment.
EDS is a significant public health problem with serious economic consequences. EDS is associated with poor health-related quality of life (HRQoL), poor performance in the workplace and increased risk of accidents. Narcolepsy is associated with significant impact on HRQoL, with EDS emerging as a strong predictor of poor HRQoL in patients with narcolepsy.[20–23] Furthermore, treatment with stimulants or wakefulness-promoting agents at dosages high enough to alleviate EDS in patients with narcolepsy often leads to adverse effects.
There is also increasing evidence that higher levels of EDS in patients with OSA account for an increased burden of disease. This includes an increased risk of cardiovascular disease, depression, and diabetes, greater deficits in work productivity, increased health-care utilization, and worse HRQoL.[29, 30]
EDS may be assessed by the Epworth Sleepiness Scale (ESS), an 8-item patient-reported outcome scale that scores respondents on how likely they are to doze off during certain daily activities (e.g., sitting and reading, watching TV, etc.). Given the impact of EDS, the ability to predict health-related utility from ESS scores is valuable for examining the cost effectiveness of treatments for EDS. Indeed, while developing the technology appraisal guidance for CPAP machines, the National Institute for Health and Clinical Excellence (NICE; the UK’s health technology assessment agency) assessment group identified three studies that examined ESS score and SF-36 and/or EQ-5D data, and using this data, used a linear regression model to predict utility from ESS scores. This led to the development of a mapping algorithm to convert ESS scores into utility scores in the 2008 NICE CPAP appraisal; the cost effectiveness of CPAP machines was then examined by applying the mapping algorithm to data on mean difference in ESS scores between individuals treated with CPAP machines compared to those treated with placebo or compared to those treated with dental devices. However, the mapping algorithm based on EQ-5D was generated based on data from only 94 patients with OSA.
The current study aimed to expand on the previous research undertaken by NICE by including data from a larger number of patients, including patients with OSA and/or narcolepsy, and examining alternative models to simple linear regression to explain the association between EDS and EQ5D utility scores. Therefore, the objective of the current study was to examine the association between EDS and HRQoL in patients with OSA and/or narcolepsy to predict EQ-5D utility scores from ESS scores.