Abstract
The International Classification of Headache Disorders (ICHD), through its three substantive editions (1–3), has provided the medical community with an invaluable tool that informs clinical practice and frames research efforts. We again (4) acknowledge the very considerable debt to the chair of the first three editions, Professor Jes Olesen, who, with vision and great capability, established the foundation on which we build.
ICHD has proven useful in many dimensions. Initially, by necessity, boot-strapped from expert opinion, which will remain a key element of its review and construction, the classification has proved robust in many ways. The ICHD case definitions have provided a foundation for epidemiologic and biological research, which in turn has established headache disorders as very common and disabling (5). To those who argue its biological basis, albeit mirrored in phenotypes (6), one can point to functional imaging data that has identified robust, reproducible, core findings in migraine (7,8) and cluster headache (9). Moreover, ICHD has provided the diagnostic foundation for the development of multiple classes of new migraine treatments, notably serotonin 5-HT1B/1D receptor agonists, triptans; 5-HT1F receptor agonists, ditans, drugs targeting the calcitonin gene-related peptide (CGRP) pathway, monoclonal antibodies, and receptor antagonists, gepants, neuromodulation devices and most recently, pituitary adenylate cyclase activating polypeptide PACAP blockers (10), all of which are based on pathophysiological knowledge achieved through using iterations of ICHD. While there are areas worthy of improvement in ICHD, the existing editions have facilitated substantial improvements in the lives of people with headache disorders all over the globe.
ICHD’s success and widespread use has led to changes, such as those around the definition of chronic migraine (11), which is still a subject of debate. The ICHD-4 Committee met in Seoul at the 21st International Headache Congress. Our minutes will be available to members on the International Headache Society website. We undertook to: minute our meetings and provide them to members of the International Headache Society after review by the Board; be prepared to publish beta versions of changes to individual areas should we find a pressing issue with sufficient evidence; invite comments about the classification, and any issues that have arisen, with data where possible, understanding that we cannot directly respond to all queries. We agreed the work to produce ICHD-4 is likely to take five to six years to complete. Additional experts will be invited to join the committee and certainly sub-committees will be mindful of inclusivity and diversity.
Some areas in need of consideration were tabled, without viewing the list as exhaustive. Matters such as the division between episodic and chronic migraine, the classification status of menstrual migraine, classification of attacks versus disease, the role of biomarkers and genetics, and how classification impacts treatment were considered. The complex question of accounting for similar phenotypes across primary and secondary headaches needs careful thought. Tension-type headache, new daily persistent headache and medication overuse headache need only be mentioned and most readers will initially have a view. Another important aspect is how we account for regional variations in phenotype, and the modifications seen with age and sex.
We understand ICHD is a resource to be respected and curated with care. Headache medicine is an evolving field and headache classification is a dynamic pursuit that we embark upon with a mixture of caution and excitement.
Classification Committee of the International Headache Society, 4th Edition
Peter J. Goadsby, Chair
Stefan Evers, Secretary
Amy Gelfand
Richard B. Lipton
Arne May
Patricia Pozo-Rosich
Jean Schoenen
Todd Schwedt
Cristina Tassorelli
Gisela Terwindt
Shuu-Jiun Wang
