The present study was part of the MobilEe project, an epidemiologic investigation of possible effects of exposure to radiofrequency electromagnetic fields on wellbeing in children and adolescents conducted in Bavaria, Germany. It was funded by the German Mobile Telecommunication Research Programme (DMF). MobilEe is described elsewhere in more detail [33]. A population-based sample of adolescents was personally interviewed. In this survey, a broader scale of health conditions was investigated, including a screening question on prevalent headache during the last six months. Adolescents (13 to 17 years), who reported at least one episode of headache per month during the last six months, were invited to answer a questionnaire for a more detailed investigation of type of headache.

The research was approved by the Ethics Committee of the Medical Faculty of the Ludwig-Maximilians-University Munich (285/03). Written informed consent was obtained from the participants' parents and - if participants were older than 14 years - from the adolescents themselves as well.

Participants

Every resident in Germany is forced by law to register at the population registry of his place of residence; taken together these registries list all inhabitants of Germany. Participants in the MobilEe project were randomly drawn from the population registries in four Bavarian cities. Six thousand three hundred and eighty-six children and adolescents were invited to participate in a personal interview: 516 could not be approached and 2,848 refused to participate. From the 3,022 participants with completed interviews, 1,498 were children younger than 13 years and therefore not eligible for the present study. For 388 of the 1,524 remaining adolescents, inclusion in the headache study was not possible, since they had been recruited prior to the conception of the headache substudy, leaving 1,136 adolescents for the present study. Of these 1,136 subjects, 547 reported no headache episodes during the last six months, while 589 subjects reported headache and were therefore invited to complete the headache questionnaires. For 508 of these, the questionnaire was completed, but 8 had to be excluded because of inconclusive information. A further 11 subjects without headache and 11 subjects with valid headache questionnaires had to be excluded because of missing values in the potential socio-demographic confounding variables. This leaves a final study population of 1,025 subjects, of whom 489 had valid headache questionnaires ('any headache' group) and 536 were without any headache ('no headache' group; see Figure 1).

Figure 1 Flow diagram of subject participation. Subjects included in the headache study are marked shaded. Full size image

A non-responder analysis with respect to socio-demographic variables revealed that adolescents from families with higher levels of education were more willing to participate in the MobilEe study [33].

Data collection

Data were collected in public buildings, like city halls or public health departments. Computer-assisted personal interviews took between 15 and 25 minutes to complete. Participants who indicated that they had suffered from headache at least once per month during the last six months were asked to fill in a detailed questionnaire on type of headache.

Validated questions on socio-demographic variables from the German Health Interview and Examination Survey for Children and Adolescents (KiGGS) were used [34]. The definition of the adolescents' SES was based on their own level of education and on the job position of their parents. Level of education was assigned on a scale from 1 point (general secondary school) to 5 points (grammar school). In a first step, parental jobs were classified using the International Standard Classification of Occupations (ISCO-88). Afterwards, the jobs were assigned to the International Socio-Economic Index of Occupational Status [ISEI; [35, 36]]. Each ISCO code holds a specific ISEI value, where a high value stands for a job with a high prestige. ISEI values were then divided into quintiles. Level of education and assigned ISEI quintile were added together and revealed the following SES levels: low SES (2-4 points), middle SES (5-7 points), high SES (8-10 points) [see [7]]. For the present analyses, low/middle and high SES were dichotomized.

Earlier studies using MobilEe data [7, 37, 38] revealed that sex (male, female), age group ('14 years and younger', '15 years and older'), family condition ('complete' if living together with both parents, otherwise 'incomplete') and SES (low/middle, high) are associated with headache and/or with electronic media use and are therefore to be considered as socio-demographic confounding variables in the present study.

Use of mobile phones was assessed with the following item: 'On an average day, how long (in minutes) do you use your mobile phone for voice calls?' with the response categories 'not at all', 'less than 5 minutes', '6 to 15 minutes', '16 to 30 minutes' and 'more than 30 minutes'.

Questions investigating use of the other electronic media were taken from the KiGGS interview [3]: Average daily use of computer/internet, watching television/videos, playing with game consoles and listening to music was assessed using a 5-point response scale ('never', 'approximately 30 minutes', 'approximately 1 to 2 hours', 'approximately 3 to 4 hours', 'more than 4 hours'), separately for weekdays and weekend days. Average use of each electronic media was estimated as weighted means from weekdays and weekend days. An overall media index for use of electronic media was estimated by adding up time spent with computer/internet, television/videos and playing with game consoles [3].

Headache questionnaire

The headache screening question in the interview ('How often did you experience headache within the last six months?') was part of the symptom checklist developed for the survey of Health Behaviour in School-aged Children [39]. Subjects who indicated that they had suffered from headache at least once per month were given a detailed questionnaire on type of headache, based on the International Classification of Headache Disorders - 2nd edition (ICHD-II) [40].

Any type of migraine and any type of TTH were classified according to the ICHD-II criteria as primary headache disorders. The classifications of pure migraine included the subtypes migraine (with or without aura) and probable migraine according to the strict criteria for adults. Furthermore, probable migraine was classified according to a modified criterion for children with a shorter duration of headache (i.e., attacks between 30 minutes and 72 hours). This criterion was chosen for the sake of comparability with another German study on headache in adolescents [[20]; see also [37, 38]] although it does not quite match the conventional ICHD-II criterion for children, which requires attack rates of at least one and up to 72 hours. The classification of pure TTH included the subtypes infrequent episodic TTH, frequent episodic TTH, chronic TTH, probable infrequent episodic TTH, probable frequent episodic TTH and probable chronic TTH. The diagnostic criteria for probable migraine and any probable TTH require agreement with all but one of the respective diagnostic criteria for the migraine or TTH, respectively. Therefore, a double diagnosis of 'migraine plus TTH' could arise.

All other subjects with headache who did not match any of these diagnoses for primary headache were considered as miscellaneous headache (MH).

Statistical analysis

Differences in categorical variables (i.e. media use) were tested by using Cochrane-Armitage tests for underlying trends with ordered categories (i.e. different durations of media use), studying dose-response effects. Multivariate analyses were done using logistic regression models adjusted for age group, sex, family condition and SES as potential socio-demographic confounding variables. Separate regression models for each type of electronic media (mobile phone, computer/internet, television/videos, game consoles, music, overall media index) and each type of headache ('pure migraine', 'pure TTH', 'migraine+ TTH' and 'MH' against no headache) were calculated. Further regression models for each type of headache were calculated, considering all types of electronic media (mobile phone, computer/internet, television/videos, game consoles, music) and socio-demographic variables (age group, sex, family condition, SES). Odds ratios (OR) with 95% confidence interval (CI) for associations between no vs. different durations of media use and six-month prevalence of headache were reported. Calculations were performed with the SAS software package (version 9.1, SAS Institute Inc. Cary, NC, USA).