In this study, we have investigated the prevalence of OSA and other measurements reflecting sleeping patterns assessed by a home sleep-monitoring device in a group of well-characterised patients with AS in comparison with matched controls. In addition, we have searched for AS-disease-related and non-disease-related factors associated with OSA in patients with AS. We found no difference in the prevalence of OSA between patients with AS and controls. About half of the AS patients and controls fulfilled the criteria of OSA. Furthermore, the most important determinants for OSA in patients with AS were higher age and BMI and lesser chest expansion, reflecting more severe AS disease.
A link between AS and OSA has been suggested, but knowledge has been limited. Prior small studies have reported a higher prevalence of OSA in patients with AS, in indirect comparisons with the prevalence in the general population [5, 6]. Furthermore, an elevated prevalence of sleep apnoea was reported in AS patients compared with controls in a large US administrative claims database . In contrast, we did not find a higher prevalence of OSA in AS patients in the current controlled study. The discrepancy between our and previous findings might be explained by the disparate methods used in these studies, such as different ways of recruiting the patients, diverse devices to assess OSA, and indirect comparisons with controls versus direct comparison in our study.
Concerning the non-AS-disease-related factors, we found, in the adjusted logistic regression analyses, that OSA was related to higher BMI and age, in line with findings in the general population [21, 22]. Regarding the AS-related characteristics, various factors were related to the presence of OSA in the non-adjusted logistic regression analyses, while most of the associations, except lesser chest expansion, disappeared when adjusting for age and sex. Not surprisingly, we showed a significantly higher ESS-score among the AS patients with OSA compared to those without OSA. This suggests that ESS may be used as a screening tool for OSA in patients with AS.
The latest guidelines from the American Academy of Sleep Medicine (AASM) recommend treatment for sleep apnoea in patients with AHI ≥ 15 events/h and/or AHI ≥ 5 events/h with symptoms . Thus, according to the AASM guidelines, 13/63 (20.6%) of the investigated patients with AS qualified for treatment. For biological DMARDs, the literature shows conflicting results about the association between TNF-inhibitors and OSA, one study found a lower frequency of OSA among TNF-inhibitor-treated patients, while two other studies did not, in line with our findings [23,24,25].
There are some limitations to be acknowledged. Only 63 (40.6%) of 155 patients with AS were completely assessed with the home sleep-monitoring device, which means that one may not draw too strong conclusions in sub-group analyses. The golden standard, polysomnography for diagnosing sleeping disorders, was not performed, but a simplified home sleep-monitoring device was used. The strengths of this study are the use of a matched control group, the systematic use of the same validated device  in both the patients and the controls, and the very well-characterised patients with AS. Although only 40.6% of the AS patients completed the assessment, the 63 examined patients did not differ in any clinical characteristics compared to the 92 patients not examined with the device. Furthermore, the 46 patients with AS who were compared with the matched controls from SCAPIS did not differ in AS-related characteristics from the 109 patients with AS that were not compared with the SCAPIS controls. Thus, we believe that the studied patients with AS quite well represent patients with AS followed at a university hospital in northern Sweden.
In conclusion, the prevalence of OSA in patients with AS was not higher than in matched controls. The AS patients with OSA had more daytime sleepiness, had a higher BMI, were older, and, importantly, had also lesser chest expansion, which reflects a more severe AS disease, compared with patients without OSA. These are the most important factors to consider when trying to identify OSA in patients with AS.