Tension-type headache and sleep apnea in the general population

This is a cross-sectional population-based study. An age and gender stratified random sample of 40,000 persons aged 20–80 years old were drawn by the National Population Register. Each of the ages 30, 35, 40, 45, 50, 55 and 60 years included 2,000 persons of each gender, while the remaining ages included 1,000 persons of each gender. The participants were residing in Akershus, Hedmark or Oppland County, Norway. The Counties have both rural and urban areas, and Akershus County is situated in close proximity to Oslo. Data from Statistics Norway has shown that the sampling area were representative for the total Norwegian population regarding age, gender, marital status and level of education [19]. The employment rate was equal, but employment in trade, hotel/restaurant and transport were overrepresented while industry, oil and gas and financial services were underrepresented in the sampling area as compared to the total Norwegian population. As shown in Fig. 1, the sample size was reduced to 38,871 because of error in the address list (n = 1,024), multihandicap (n = 4), dementia (n = 23), insufficient Norwegian language skills (n = 3) and deceased (n = 75). All participants received a mailed standard letter containing information about the project and a short questionnaire including the Berlin Questionnaire. The Berlin Questionnaire was used to classify respondents to be of either high or low risk of obstructive sleep apnea [20]. This instrument was developed in 1996 and is a ten item self-report questionnaire designed to predict the risk of obstructive sleep apnea. It contains questions in three categories addressing snoring, daytime sleepiness and the presence of hypertension and/or obesity. A positive score in at least two of the three categories are required to classify a person as high risk of obstructive sleep apnea. All others are classified as low risk group of obstructive sleep apnea. Detailed definitions of each category have previously been published [21]. If the questionnaire evoked no response, a second mail was issued. The replies could either be on paper or electronically. The overall response rate was 54.5% (21,177/38,871), and it was significantly higher among women than men (n = 11,120 vs. n = 10,057; p < 0.001). A total of 1,442 questionnaires were not eligible. This was due to late response (n = 41), not containing a telephone number necessary for re-contact (n = 729) and incomplete filled in questionnaires that could not be classified as high or low risk of obstructive sleep apnea (n = 672). An age and gender stratified sample of the respondents aged 30–65 years were then invited by mail to a clinical evaluation and contacted by telephone. The clinical evaluation was conducted over a period of 2 years. If they could not be reached within three attempts, no further attempts were made (n = 202). Other exclusion criteria were: Use of Continuous Positive Airway Pressure (n = 10), pregnancy (n = 9), lack of Norwegian language skills (n = 5) and severe physical impairment (n = 4). A total of 378 persons with high risk and 157 persons with low risk of sleep apnea were included for further investigations. In case of technical failure in the polysomnography (PSG) recordings, the participants were asked to return for a second recording (n = 6). Two persons refrained from such a second PSG recording. Finally, all the participants diagnosed with migraine (n = 102) were excluded, in order to have a pure material without migraine as a confounder. The final study sample in the present study comprised of 431 (297 high risk and 134 low risk) persons. 585 persons refrained from participating. Participants and non-participants were not significantly different regarding self reported headache, depression, gender or age, while simple snoring was overrepresented in the low risk group, as compared to all low risk respondents of the questionnaire.

The participants were all admitted to Akershus University Hospital (Stensby Hospital), Norway and underwent an extensive clinical interview including a semi-structured headache interview and a physical and a neurological examination by one of three physicians. The physicians were blinded regarding the participants replies on the questionnaire. ICHD II was applied [14]. Since the participants with no headache and infrequent tension-type headache did not differ in any of the variables, they were grouped together in the analyses, as <12 days of headache per year. The Hospital Anxiety and Depression Scale (HADS) was used to screen for depression [22]. The replies were dichotomized and depression was defined by a score of ≥8 on the subscale of depression (HADS-D) [23]. Excessive daytime sleepiness was assessed by the Epworth Sleepiness Scale [24]. The results were dichotomized into scores ≤10 and >10, the latter is considered to represent clinically significant excessive daytime sleepiness [25]. Body mass index (kg/m²) was calculated from measured weight and height. All participants then underwent in-hospital PSG performed on standard, multichannel, Embla^TM A10, PSG devices (ResMed Corp Poway, CA, USA). The recordings included a two-channel electroencephalograph (C4/A1, C3/A2 according to the 10–20 international electrode placement system), a two-channel electrooculogram, a one-channel submental electromyogram, leg EMG (tibialis), SaO₂, breathing movements (Respitrace; Ambulatory Monitoring, Ardsley, NY, USA), air flow measured by a nasal air pressure transducer (Pro-Tech, Woodinville, WA, USA) and an oro-nasal thermistor, and body position monitoring. All electrophysiological signals were pre-amplified, stored and subsequently scored (30-s epochs using Somnologica 3.2 software package, Flaga-Medcare, Buffalo, NY, USA) according to the Rechtshaffen and Kales scoring manual by two US board certified PSG technicians who were blinded to the result of the Berlin Questionnaire [26]. Arousals were documented and classified [27]. Obstructive apneas were scored when a 90% decrease of flow occurred for more than 10 s. Hypopneas were defined as a 30% decrease in flow for more than 10 s with subsequent oxygen desaturation of at least 4%. The apnea hypopnoea index (AHI) was calculated as the average of total number of apneas and hypopneas per hour of sleep. In this study, the participants with AHI ≥5 were classified with obstructive sleep apnea.

The statistical analyses were performed using SPSS Base System for Windows 16.0. Power analysis determined that a total sample size of 398 participants was required to detect a difference of proportions of headache of 10%, with a power of 80 and a 95% confidence level. Chi-square tests and logistic regression modelling with 5% level of significance were used. Fisher’s exact test was used when appropriate. The Student’s t test and Mann–Whitney U test were used in comparing normally and non-normally distributed continuous variables. In our multivariate logistic regression model obstructive sleep apnea was used as the dependent variable, while tension-type headache, depression, gender, body mass index and age were independent variables.

The project was approved by The Regional Committees for Medical Research Ethics and the Norwegian Social Science Data Services.