Can we know the prevalence of MOH?

The Global Burden of Disease Survey 2010 (GBD2010) reaffirmed the importance of migraine as a cause of public ill-health and disability (1). It did this beyond doubt, and was welcomed (2). But whereas migraine is neither the most prevalent nor the most disabling of headache disorders, others that may be of similar importance remain largely overlooked. The earlier GBD2000 considered only migraine (3). GBD2010 at least included tension-type headache (TTH), finding it the second most common disease in the world (behind dental caries; migraine was third). But, with ictal disability estimated at just 4% (against 43.3% for migraine), TTH accounted for only 0.25% of all years of life lost to disability (YLDs) (against 3.1% for migraine) (1). Medication-overuse headache (MOH), heavily burdensome at the individual level (4) and very common in headache clinics, did not make the final analysis in GBD2010 (1). Nevertheless it was included in the worldwide consultation undertaken by GBD2010, comparing the various health states attributable to disease, and was allocated a disability weight of 22% (unpublished data). Assuming a global prevalence of 1.5% and headache present on most days (say 60%), a back-of-the-envelope estimate of total YLDs attributable toMOH comes in at about two-thirds of those caused by migraine – about 2%of all YLDs. I do not offer this as an accurate assessment, but do believe it demonstrates that MOH is a far from insignificant cause of public ill-health and disability. In health-policy terms, its importance is magnified not only because it consumes very substantial health-care resources (4) but also, sinceMOH is an avoidable condition, because this cost is unnecessary. MOH was omitted from the reports of GBD2010 (1), principally because prevalence data from around the world were inadequate to support regional estimates. This was true but is changing, and future iterations from GBD2013 onwards can be expected to include it. But do we know its prevalence? Population-based studies have not been easy because of two related problems: of case definition and case ascertainment. The definition of MOH has been unstable from the time of its recognition; diagnostic criteria have changed through the various editions of the International Classification of Headache Disorders (ICHD) (5–8), while alternative proposals (9) have led to an epidemiologically unhelpful conflation of MOH with chronic migraine. In this issue of Cephalalgia, Westergaard et al. directly confront one of these problems and offer proposals to overcome the other (10). They base their arguments on published studies of MOH prevalence, finding, from a systematic literature search, 27 reports of 24 datasets from 16 countries. Diagnostic criteria in these studies, they note, closely followed the consensus of their time. Perhaps surprisingly, therefore, estimates of adult prevalence clustered around 1–2%, although a few were much higher (up to 7.2%). What this may indicate is greater consistency in the criteria applied by most authors of these studies than existed in the definitions of the time, the result, probably, of pragmatic adaptations. Westergaard et al. want to make this approach explicit. ‘Diagnostic criteria that are useful in the clinic’, they observe, ‘are not always applicable in population-based research’. For example, demonstration of causation – a general requirement in ICHD for secondary headaches – and exclusion of other possible diagnoses demand careful evaluation beyond the means of the epidemiological researcher. The unspoken concern here is that, unless accepted diagnostic criteria are applicable in population-based research, prevalence can never be known. Westergaard et al. find the past experience of adapting criteria instructive as a guide for how future population-based studies can use ICHD-3 beta (8), and propose alternative criteria for MOH specifically for such studies. Essentially these omit criterion C. ‘Exclusion of other headache diagnoses cannot be easily implemented’, they argue, ‘and should not hinder efforts to estimate prevalence’.