To the best of our knowledge, this study is the first to compare the sensitivity of the ICHD-3 beta criteria for “sleep apnoea headache” in OSAS patients with morning headaches. In our study, we assessed headaches upon awakening in the morning but not headaches upon awakening during sleep or after a nap because the ICHD-3 beta criteria clearly state that a sleep apnoea headache is a morning headache caused by sleep apnoea.
This study demonstrated that 60.4 % and 81.3 % of patients with morning headache met the criteria for “sleep apnoea headache” established by ICHD-2 and ICHD-3 beta, respectively (Tables 1 and 2). The increased frequency of individuals who qualified for diagnosis was likely attributable to the extension of headache duration from 30 min to 4 h. This modification had clinical significance, as demonstrated in this study. Out of all morning headaches (n = 48), 81.3 % (n = 39) responded to CPAP therapy, and 19.7 % (n = 10) of the CPAP-treatable morning headaches did not fulfil the ICHD-2 criteria, although they all fulfilled the ICHD-3 beta criteria for “sleep apnoea headache”. Previous studies have demonstrated that 80-90 % of morning headaches improved after appropriate treatment (CPAP or uvulopalatopharyngoplasty) for OSAS [1, 13]. Among habitual snorers, 32 % of all morning headaches lasted less than 30 min ; in our study of OSAS patients, 31.3 % of all morning headaches lasted less than 30 min. In other studies, the proportions of morning headaches lasting less than 1 h were 26.3 %  and 26.7 %  in OSAS patients. In our study, the proportion of morning headaches resolving within 4 h, that is, the duration specified by the ICHD-3 beta criteria, increased to 52.1 %.
Kristiansen et al.  recently reported an increased frequency of morning headache in participants with OSAS (11.8 %) compared with those without OSAS (4.16 %) in the general population. The frequency of morning headache (20.4 %) in our study was lower than previously reported in patients with OSAS (18-60 %) [1–3, 14, 16–18]. The proportion of total OSAS patients in this study who had sleep apnoea headache was 12.3 % and 16.6 % based on ICHD-2 and ICHD-3 beta, respectively; similarly, the latest review reports a prevalence of sleep apnoea headache of 12-18 % in the middle-aged population . Göder et al.  compared PSG recordings from the nights before morning headaches with those from nights that were not followed by morning headaches in patients with OSAS, and they showed that the occurrence of a morning headache was associated with decreases in total sleep time, sleep efficiency and the amount of rapid eye movement sleep as well as with an increase in awake time during the preceding night. Oxygen desaturation and AHI did not differ when nights with and without subsequent morning headaches were compared. In our previous study including 36 OSAS patients, PaCO2 values upon waking did not significantly differ between patients with and without morning headaches . In contrast, Aldrich and Chauncey  evaluated the frequency and characteristics of morning headaches in various sleep disorders, including OSAS, but they failed to find a significant association between morning headaches and OSAS. The possible mechanisms of sleep apnoea headache include repeated respiratory events, nocturnal hypoxia, hypercapnia-induced vasodilation and increased intracranial pressure [3, 13, 22]. Our study did not support a role of nocturnal hypoxia in the development of sleep apnoea headaches or morning headaches in OSAS patients because the PSG parameters, including AHI and nocturnal oxygen desaturation, did not differ depending on whether the patient had sleep apnoea headaches or morning headaches.
Another important finding in our study is that OSAS patients with morning headache frequently exhibited characteristics of migraine and tension-type headache. Furthermore, a significant number of morning headache patients having characteristics of migraine and tension-type headaches were diagnosed as having sleep apnoea headache. This finding suggests that for patients with OSAS, CPAP therapy can effectively treat not only morning headaches with tension-type features but also morning headaches with migraine-like features. By contrast, Chen et al.  found that morning headaches were more strongly associated with migraines (adjusted odds ratio, 6.3) than with OSAS (adjusted odds ratio, 2.6) in habitual snorers. The authors suggested that for habitual snorers, managing migraines and insomnia may be more helpful for morning headaches than managing OSAS. In the study including 4759 patients with OSAS from the Taiwan Longitudinal Health Insurance Database, OSAS patients had a higher likelihood of developing tension-type headache than did patients without OSAS . However, the authors did not assess the presence of morning headache and whether tension-type headache was associated with morning headache. Johnson et al.  reported that of 82 chronic headache patients with migraine or tension-type headache or both, 63 % had concomitant OSAS.
The limitations of our study included the cross-sectional study design and the lack of a healthy control group. In this study, all of the OSAS patients were under CPAP treatment; therefore, the headache frequencies reported by the patients might have been underestimated due to the efficacy of CPAP therapy. However, the strength of this study is that sleep apnoea headaches were diagnosed after a confirmation of the effectiveness of CPAP on headaches, which could eliminate other headaches that mimicked sleep apnoea headaches; in the previous studies, morning headaches were evaluated before confirming the effectiveness of CPAP on headaches , or the CPAP effects on sleep apnoea headaches were evaluated in a limited number of OSAS patients [1, 3, 13]. In our study, most of the patients had an AHI of 20 or greater, likely because we selected stable OSAS patients receiving CPAP therapy who visited our outpatient clinic every month, and in Japan, CPAP use is generally restricted to OSAS patients with AHI ≥ 20. Further investigations are needed to evaluate sleep apnoea headache in OSAS patients with AHI < 20. A further prospective study is required to assess the associations between morning headaches and OSAS.