Can migraine be defined?

The commentary by Schulte and May (1) states in essence that the definition and classification of an entity such as ‘migraine’ – or indeed any other form of primary headache – is based entirely on a consensus regarding its diagnostic criteria. However, with regard to the ICHD-3 beta, this consensus is largely confined to the ‘experts’ who establish these ‘diagnostic criteria’ – which according to Shevel and Shevel (2) lack empirical proof – and then declare that ‘for research, the classification is indispensable’ and demand that ‘every patient entered into a research project, be it a drug trial or a study of pathophysiology or biochemistry, must fulfil … ’these criteria (3). We suspect that it would now be impossible to publish a paper on headache without referencing the ICHD-3 beta or reiterating the ICHD-3 beta criteria for the headache entity under consideration.

As Schulte and May infer, the strictures imposed by the ICHD-3 beta definition of migraine are fine if one wishes to simply ascertain patients fulfilling these particular criteria; but it would not be true to conclude that these criteria therefore ‘define migraine’. They do not.

Primary headaches are symptomatic entities without any independent biological markers. The term ‘migraine’ is a French word derived from the Galenic ‘hemicrania’ – a one-sided headache. We could then simply call any one-sided headache ‘migraine’. However, from the days of Hippocrates, it was clear that most such headaches were accompanied by a variable number of additional symptoms that we now recognise as comprising the syndrome we call ‘migraine’. However, we also now know that these same symptoms can rarely be caused by intracranial pathology (‘symptomatic migraine’) and that ‘migraine’ can sometimes occur without any headache at all! The ‘object’ of classification, to use Schulte and May’s terminology, has entirely changed its characteristics in such cases but continues to be called ‘migraine’.

Like the Shevels (2), we are concerned that the ICHD-3 beta criteria have assumed a status that is not justified by evidence and that the ICHD-3 beta consensus criteria might actually hinder headache research. In real life, we often diagnose ‘migraine’ in patients who do not fulfil the ICHD-3 beta criteria for ‘migraine’; the fact that the classification includes the category ‘probable migraine’ for such patients is useful but ducks the issue!

We have proposed that all primary headaches are part of a continuum bounded by parameters of headache intensity, duration or inverse of frequency and the presence or absence of trigeminal autonomic symptoms, driven by a common mechanism with different semiologies depending on the neural pathways engaged (4). Some patients change their headache characteristics several times during a lifetime and the symptoms of their habitual attacks can also vary and can overlap the artificial boundaries specified for different primary headaches by the ICHD-3 beta consensus criteria (4,5).

It might be argued from this that ‘all (primary) headache is migraine’. If we had an intervention that worked sufficiently far upstream in the mechanism it could treat all headaches. We do not yet have such an intervention and the main benefit of a primary headache classification is that it provides a basis for the recognition that some treatments work better for some forms of primary headache than for others. A headache classification is indeed essential but to pretend that complex symptom-based syndromes can be constrained by the strict diagnostic criteria of the ICHD-3 beta is unhelpful.