The comorbidity of headache with other pain syndromes.

In this article, we consider the possible reasons and supporting evidence for the comorbidity of chronic pain conditions. To simplify the discussion, we primarily focus on the epidemiology of headache with other pain conditions, dividing studies into those based on children or adolescents and those based on adults. We consider exogenous and endogenous factors, and methodological challenges, in understanding whether and how 2 pain conditions may be related.