Complexity of prevalence and incidence measures in headache research.

Defining disease status in population-based studies is often challenging, particularly when disease status changes between active and non-active stages. Migraine is a typical example of a fluctuating disease as its frequency and clinical features can vary considerably during one’s lifespan (1). In general, the data show that the vast majority of patients have an onset of their migraine in childhood or early adulthood and only 2% report a first migraine above the age of 50 (1). We are also accustomed to observing data which demonstrate that the ‘prevalence’ of migraine decreases after the age of 40. But what do we mean by prevalence? Prevalence is defined as the number of cases of a specific condition in a given population over a defined duration of time. Often, however, the duration of time is not well specified in population-based headache studies. If we think of lifetime history of migraine, then the substantial decrease in migraine prevalence could only be explained if patients with migraine would have a dramatically increased risk of death. While there is some evidence that migraine is associated with a modest increased risk of mortality (2, 3), particularly cardiovascular mortality, the effect would by far not explain the dramatic prevalence decrease. Thus, when we refer to a decline of migraine prevalence in midlife, we likely mean the prevalence of active migraine (such as a 1-year prevalence of migraine). However, such definition conflicts with the current International Classification of Headache Disorders, 2nd edition (ICHD-2) (4). A person is classified to have a migraine after at least five attacks with typical migraine characteristics. As there is no distinction between active and non-active phases in the ICHD-2, a person is a prevalent migraine case for the rest of their life after five typical migraine attacks. What about incidence? Incidence is the new occurrence of a specific disease in a given population of individuals without this disease in a defined follow-up time. Incidence can only be studied among individuals who are ‘at risk’ of getting the disease, which is referred to as the ‘at-risk population’. With regard to migraine, an incident event is a new onset of migraine in a person who never had a migraine before (based on the ICHD-2 criteria, this means no or less than five migraine-specific attacks). In this issue of Cephalalgia, Khil et al. (5) evaluate the role of headache incidence when different populations at risk are considered. Incidences were assessed in 1122 men and women who were randomly drawn from the general population in Germany and based on the ICHD-2 criteria using standardized headache questions. They defined three populations at risk: