Abstract
It was not until 1962 that the Ad-Hoc Committee of the National Institute of Health first published a classification of headache syndromes by brief glossary definitions. The general disadvantage of such glossary definitions is that they require subjective interpretation. Therefore under the chairmanship of Prof. Jes Olesen, Copenhagen, the International Headache Society published in 1988 on the basis of empirical findings a first ever headache classification using operationalized criteria. The headache classification of the International Headache Society was immediately translated into the world's major languages and was adopted by all national headache societies represented in the International Headache Society, the World Health Organisation and the World Federation of Neurology. The new classification proved so successful and enjoyed such rapid international acceptance that no revision was undertaken until 1999. The second edition, again under the chairmanship of Prof. Jes Olesen, will probably be completed in 2002. The classification produced such a high degree of inspiration and motivation of pathophysiological and epidemiological research work that knowledge in the field of headache has displayed growth unparalleled in any other field of neurological research. This development was made possible by the determined work of the Chairman of the Headache Classification Committee, Prof. Jes Olesen. He succeeded in bringing together international researchers, motivating them and jointly turning the current fund of knowledge into a evidence-based classification. Prof. Jes Olesen thus performed the decisive pioneering work for all those who have to do with headaches-patients, doctors and scientists. The IHS classification is the most frequently cited text and one of the most important milestones in the history of the scientific study of headaches.
History of headache classification
The characteristic feature of modern diagnostic classification systems is precise, operationalized diagnostic criteria. Operationalized criteria may be inclusion criteria. Such inclusion criteria indicate whether a particular manifestation is to be used for the diagnosis of a disease. Exclusion criteria, on the other hand, indicate what manifestations must not be present in a given disease. It is only through diagnostic criteria in classification systems that scientific and clinical communication is possible at all. The characteristic feature of the precursors of modern classification systems was that they did not make use of operationalized classification criteria. Accordingly they sought either to include aetiological concepts or to string together a large number of different classification criteria without any operationalized approach.
Until 1960 there was no international basis for classifying headaches that was accepted and used consistently world-wide. It was not until 1962 that an Ad-Hoc Committee of the National Institute of Health first published a classification of headache syndromes. This was a first step towards a consensus-orientated terminology in the classification and diagnosis of headaches. The basis for this headache classification was the assumed aetiology of the headache syndrome. It attempted to assign delimitable headache causes to the various diagnostic entities. The headache syndromes were characterized by brief glossary definitions. The general disadvantage of such glossary definitions is that they require subjective interpretation. As a result of these limitations the ad-hoc classification was very controversial from the start (1 –4).
International Headache Society classification
The International Headache Society was founded in 1982. At international conferences it very soon became evident that headache researchers and headache experts in the various countries were unable to communicate clearly with each other because they had no clear operationalized headache classification at their disposal. A committee was therefore set up without delay in 1985 to draw up an international headache classification capable of consensus. This headache classification committee was made up of renowned headache experts under the chairmanship of Professor Jes Olesen, Copenhagen. Subcommittees drew up operationalized criteria on the basis of empirical findings and, if these were not available, on the basis of consensus among the experts. After 3 years of intensive work they were finally able to publish in 1988 a first ever headache classification on the basis of clear operationalized criteria. The original classification manual has 96 pages and describes a total of 165 different diagnoses. Before the final publication of this classification, several public discussions were held, and the drafts of the new classification were available to all for discussion.
The members of the Headache Classification Committee of the International Headache Society were aware at the time of publication that this new classification was a first step. A crucial problem here was the fact that, owing to the lack of empirical findings, a large part of the work was based on expert opinions and consensus. For this reason it was planned to publish a second edition in 1993. However, the new classification proved so successful and enjoyed such rapid international acceptance that no revision was undertaken in 1993. Instead it was decided not to publish the second edition before 1998. In view of its world-wide acceptance, however, a revision was not initiated until 1999. The second edition, again under the chairmanship of Professor Jes Olesen, will probably be completed in 2002. The headache classification of the International Headache Society was immediately translated into the world's major languages and was adopted by all national headache societies represented in the International Headache Society and the World Federation of Neurology.
The structure of the IHS classification
Under the classification of the International Headache Society (IHS), headache syndromes are divided into 13 main groups. A distinction is made between two important sub-groups of headaches: primary headache syndromes and secondary headache syndromes.
Primary headaches encompass headaches that do not exhibit any pathological findings using the usual clinical and technical investigation methods. Primary headaches are independent diseases, not a secondary symptom.
This is true of migraine, tension-type headache, cluster headache and chronic paroxysmal hemicrania, and of various headache types not accompanied by structural lesions (IHS Groups 1 –4).
Groups 5–11 list the secondary headache syndromes. In the case of secondary headaches it is possible, with clinical investigation methods, to detect pathological findings that can be associated with the headache syndromes. Thus secondary headaches are symptoms of an identifiable disorder.
The IHS codes allow differentiated specification to a depth of four digits. This has the advantage that although doctors can use the classifications without difficulty and with sufficient accuracy in routine clinical work, it is also possible for experimental and clinical research to undertake a highly differentiated subdivision.
Headache classification of the World Health Organization (WHO)
In everyday clinical work, diseases have to be coded and classified using the international classification ICD-10 of the World Health Organization (WHO). WHO has adopted the headache classification of the International Headache Society and taken it into account in the ICD-10 and the ICD-10 NA (NA: neurological application). In addition, the World Health Organization publishes Fascicles or Guides for particularly important groups of diseases. These Fascicles provide an exact operational breakdown of these disease fields with regard to their classification. The reason for the additional publication of the Fascicles or Guides is that owing to the great scope of the diagnosis it is not possible to specify the operationalized diagnostic criteria in the ‘parent text’ of the ICD-10. The Fascicles set out these diagnostic criteria in detail. The ‘ICD-10 Guide for Headache’ is the first Fascicle that has been prepared by the World Health Organization in the field of neurological disorders on the initiative of Professor Olesen.
Thus headache classification is currently playing a pioneering role with regard to precise operationalized classification of neurological disorders. The ICD-10 NA is now also able to delimit headache syndromes precisely by means of intra- and interindividually reproducible operationalized criteria. Although the ICD-10 NA was developed in close co-operation with the classification committee of the International Headache Society, it was not possible to adopt the IHS classification approach directly because of requirements of form for coding the diagnoses in accordance with the ICD code. Furthermore, the smaller scale of administrative problems had permitted faster development of the IHS headache classification to the publication stage. By contrast, the preparation of the ICD-10 NA had to pass through international consultation processes and was therefore not published until 1997.
The structure of the ICD-10
The tenth revision of the International Classification of Diseases (ICD-10) was published in 1993. The work on the tenth edition started as early as September 1983. The ICD is used primarily for preparing national and international statistics on mortality and morbidity. Since the ICD is used world-wide, it is necessary to secure agreement within the member states, the specialist societies, the specialists and the users - and hence achieve international acceptance - before it is published. The ICD determines international and national communication and thus professional thinking about diseases. The coding of the ICD code is based primarily on aetiological information.
If this is not available, the symptomatology of the disease is taken as a basis for classification. The ICD initially codes diseases on an alphanumeric basis. The letter G corresponds to diseases of the nervous system. This is followed by a dot and up to three further digits, which take the form of numbers. The second digit embraces important groups of diseases that are grouped on the basis of their localization or pathophysiology. The third digit in the code subdivides these main groups into further subgroups depending on their disease conditions, symptomatology, anatomical location or other pathological conditions and finally, a fourth digit permits a further subdivision of the disease entity in question.
Links between ICD-10 and the IHS headache classification
The ICD-10 NA contains a classification system for headache and facial pain which displays a large measure of substantive agreement with the IHS headache classification. The classification of the primary headache forms, i.e. migraine, tension-type headache, cluster headache and headache without structural lesions, is completely identical. The possibility of multiple coding in the ICD-10 NA means that symptomatic headache types can be coded the same in both systems. For some headache types the ICD-10 NA offers a more detailed classification, e.g. for headaches in the context of infections that do not relate to the head. Here the ICD-10 NA permits an exact specification of the trigger cause.
On the other hand the IHS classification offers more far-reaching possibilities for describing headaches when it comes to adding additional phenomenological headache types to the individual headache diagnoses. The ICD-10 NA headache classification is indispensable for administrative purposes in practice.
Importance of headache classification
The IHS classification has been translated into 12 major world languages. Modern and effective headache therapy is not possible without it, as the development of new therapeutic methods is based on the identification of headache syndromes in accordance with the IHS or ICD-10 classification. The IHS classification produced such a high degree of inspiration and motivation of pathophysiological and epidemiological research work that knowledge in the field of headache syndromes has displayed growth unparalleled in any other field of neurological research. This development was made possible by the determined work of the Chairman of the Headache Classification Committee, Professor Jes Olesen. He succeeded in bringing together international researchers, motivating them and jointly turning the current fund of knowledge into a evidence-based classification. Professor Jes Olesen thus performed the decisive pioneering work for all those who are involved with headaches, patients, doctors and scientists. The IHS classification initiated by Professor Jes Olesen and compiled under his chairmanship is the most frequently cited text and one of the most important milestones in the history of the scientific study of headaches.