Short communication: Persistent socio-economic inequality in frequent headache among Danish adolescents from 1991 to 2014.

<bold>Background: </bold>The association between socio-economic status (SES) and headache among adolescents is an understudied issue, and no study has examined whether such an association changes over time. The aim was to examine trends in socio-economic inequality in frequent headache among 11- to 15-year-olds in Denmark from 1991 to 2014, using occupational social class (OSC) as indicator of SES.<bold>Methods: </bold>The study applies data from the Danish part of the international Health Behaviour in School-aged Children (HBSC) study. HBSC includes nationally representative samples of 11-, 13- and 15-year-olds. This study combines data from seven data survey years from 1991 to 2014, participation rate 88.6%, n = 31,102. We report absolute inequality as per cent difference in frequent headache between high and low OSC and relative inequality as odds ratio for frequent headache by OSC.<bold>Results: </bold>In the entire study population, 10.4% reported frequent headache. There was a significant increase in frequent headache from 8.0% in 1991 to 12.9% in 2014, test for trend, p < 0.0001. This increasing trend was significant in all OSCs. The prevalence of frequent headache was significantly higher in low than high OSC, OR = 1.50 (95% CI: 1.34-1.67). This socio-economic inequality in frequent headache was persistent from 1991 to 2014.<bold>Conclusion: </bold>There was a significant and persistent socio-economic inequality, i.e. increasing prevalence of frequent headache with decreasing OSC. The association between socio-economic position and headache did not significantly change over time, i.e. the statistical interaction between OSC and survey year was insignificant.<bold>Significance: </bold>The prevalence of frequent headache among adolescents increases with decreasing SES. This socio-economic inequality has been persistent among adolescents in Denmark from 1991 to 2014. Clinicians should be aware of this social inequality. [ABSTRACT FROM AUTHOR]