Can ‘migraine’ be defined? – Yes and we have to

We thank our esteemed colleagues Lane and Davies for their commentary on our letter on the importance of disease classifications in scientific research (1). Upon reading their correspondence, however, we encountered a fundamental conceptual difference to our understanding of the matter.

This difference is a logical one. Lane and Davies claim in essence that we need empirical evidence from e.g. epidemiological studies to accurately define migraine. However, as we have previously pointed out¹, the first step in any study, any scientific process, any dialogue and basically any cognitive process in a community is a definition of the object of research prior to investigating or even discussing it. Without this definition, basic understanding between members of a community and therefore scientific discourse and progress is not possible, as its lacking hinders comparability and reproducibility of data. The nature of a definition is to offer a short and concise explanation of the key features of an object (2) and thus to delineate it from other objects: determinatio negatio est (3). A definition offering criteria that do not clearly differentiate one category from another is indeed worthless, as it does not ‘de-fine’ in the original sense of the word. The definition of an object of research is the first step in a scientific process. Any study aiming at verification of this definition and at the same time relying on it for primary determination of the study object will necessarily fail as it will always only reproduce the definition. Studies using different primary definitions or criteria, however, do not investigate the same object and can thus also not verify or falsify the definition in question. Ergo: Empirical verification or falsification of an existing definition or classification is a logical impossibility (1).

Consequently, definitions are not only indispensable in a scientific community but also in a clinical context: only consensus about the key features of a disease among a clinical community makes communication among doctors possible. In clinical practice it is usually the syndrome not exactly matching diagnostic criteria that alerts us and lets us suspect other – possibly malignant – causes: clearly defined diagnostic criteria are thus of vast importance for diagnostic and eventually even therapeutic decisions. It is therefore with interest that we read that Drs Lane and Davies write that: ‘in real life, we often diagnose “migraine” in patients who don’t fulfill the IHC criteria for ‘migraine’. We would like to stress that the International Headache Society (IHS) classification was not written for clinicians but for scientists (see preface and introduction of the 1988 classification) (4). It has nevertheless proven to be so exact and useful that a diagnosis in daily clinical practice should not be done without it. This becomes evident by the fact that we can firmly determine that diagnostic imaging is not necessary in patients with headache, when the symptoms follow the IHS criteria for migraine and the neurological examination is normal (5). In that case, the rate of pathological findings is 0.2% and thus exactly the rate of chance finding in 1000 healthy volunteers (6). If the symptoms allow a diagnostic classification of the headache to any of the other IHS defined primary headache types, the rate of pathological findings in brain imaging increases to 2.4%, provided the neurological examination is normal (5). However, if the symptoms are somewhat ‘atypical’ or not classifiable by IHS criteria, the rate of pathological findings increases to 14%, even if the neurological examination is normal (7). It is indisputable that the hope of the authors of the first IHS classification that ‘ … it (the classification) will probably influence the way we diagnose patients in our daily work’ (4) has become true. It is also indisputable that the global scientific advances in headache research, be it bench or bedside, have enormously profited from the operational diagnostic criteria since 1988 and are indeed not thinkable without. The basis of this success is a low inter-rater variability. In this context, the most recent trend to make the classification (e.g. for chronic migraine) less rigid is to be seen with great caution or even suspicion. We simply will not learn enough about chronic migraine by broadening and consequently weakening the definition, something we should have learned from the definition of tension-type headache.

Regarding the question whether migraine can be defined: first and foremost ‘migraine’ is a word. Like all words, it has a meaning. This meaning is arrived at by definition and consensus. The ICHD takes this word (‘migraine’) and assigns a meaning to it (headache attacks with certain characteristics) based on an expert consensus (8). This is a definition. To define ‘migraine’, i.e. to arrive at a communal consensus about what we mean when we talk about migraine is thus not only possible, it is indeed crucial for any scientific investigation and also in a clinical context.