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How to use the classification

This extensive document is not intended to be learned by heart. Even members of the Classification Committee are unable to remember all of it. It is a document that should be consulted time and time again. In this way you will soon get to know the diagnostic criteria for 1.1 Migraine without aura, 1.2 Migraine with aura, the major types of 2. Tension-type headache, 3.1 Cluster headache and a few others. The rest will remain something to look up. In clinical practice you do not need the classification for the obvious case of migraine or tension-type headache, but it is useful when the diagnosis is uncertain. For research, the classification is indispensable: every patient entered into a research project, be it a drug trial or a study of pathophysiology or biochemistry, must fulfil an agreed set of diagnostic criteria.

  1. This classification is hierarchical, and you must decide how detailed you wish to make your diagnosis: from the first-digit level only to the fifth. First one forms an impression as to which group the patient belongs to. Is it, for example, 1. Migraine or 2. Tension-type headache or 3. Trigeminal autonomic cephalalgias? Then one obtains information allowing a more detailed diagnosis. The desired detail depends on the purpose. In general practice only the first- or second-digit diagnoses are usually applied, while in specialist practice and headache centres a diagnosis at fourth- or fifth-digit levels is appropriate.
  2. For most purposes, patients receive a diagnosis according to the headache phenotypes currently present, or that have presented within the last year. For genetic and some other uses, occurrence during the whole lifetime is used.
  3. Each distinct type, subtype or subform of headache that the patient has must be separately diagnosed and coded. For example, a severely affected patient in a headache centre may receive three diagnoses and codes: 1.1 Migraine without aura, 1.2 Migraine with aura and 8.2 Medication-overuse headache.
  4. When a patient receives more than one diagnosis, these should be listed in the order of importance to the patient.
  5. When one type of headache in a particular patient fulfils two different sets of diagnostic criteria, all other available information should be used to decide which of the alternatives is the correct or more likely diagnosis. This could include the longitudinal headache history (how and when did the headache start?), the family history, the effect of drugs, menstrual relationship, age, gender and a range of other features. Fulfilment of the diagnostic criteria for 1. Migraine, 2. Tension-type headache or 3. Trigeminal autonomic cephalalgias, or any of their types or subtypes, always trumps fulfilment of criteria for the probable diagnostic categories of each, which are last-described in the respective groups. In other words, a patient whose headache fulfils criteria for both 1.5 Probable migraine and 2.1 Infrequent episodic tension-type headache should be coded to the latter. Nevertheless, consideration should always be given to the possibility that some headache attacks meet one set of criteria while other attacks meet another set. In such cases, two diagnoses exist and both should be given and coded.
  6. To receive a particular headache diagnosis the patient must, in many cases, experience a minimum number of attacks of (or days with) that headache. This number is specified in the diagnostic criteria for the headache type, subtype or subform. Further, the headache must fulfil a number of other requirements described within the criteria under separate letter headings: A, B, C etc. Some letter headings are monothetic: that is, they express a single requirement. Other letter headings are polythetic, requiring for example any two out of four listed characteristics.
  7. The full set of diagnostic criteria is provided for some headache disorders only at the first- and second-digit levels. Diagnostic criteria at the third-, fourth- and occasionally fifth-digit levels then demand, as criterion A, fulfilment of the criteria for levels one and/or two and, in criterion B and onwards, state the further specific criteria to be fulfilled.
  8. The frequency of primary headache disorders varies widely, from attacks every 1-2 years to attacks daily. The severity of attacks also varies. ICHD-3 does not generally provide a possibility to code for frequency or severity, but recommends that frequency and severity be specified in free text.
  9. Primary or secondary headache or both: When a new headache occurs for the first time in close temporal relation to another disorder known to cause headache, or fulfils other criteria for causation by that disorder, the new headache is coded as a secondary headache attributed to the causative disorder. This remains true even when the headache has the characteristics of a primary headache (migraine, tension-type headache, cluster headache or one of the other trigeminal autonomic cephalalgias). When a pre-existing primary headache becomes chronic in close temporal relation to such a causative disorder, both the primary and the secondary diagnoses should be given. When a pre-existing primary headache is made significantly worse (usually meaning a two-fold or greater increase in frequency and/or severity) in close temporal relation to such a causative disorder, both the primary and the secondary headache diagnoses should be given, provided that there is good evidence that the disorder can cause headache.
  10. The last criterion for almost every headache disorder is “Not better accounted for by another ICHD-3 diagnosis”. Consideration of other possible diagnoses (the differential diagnosis) is a routine part of the clinical diagnostic process. When a headache appears to fulfil the criteria for a particular headache disorder, this last criterion is a reminder always to consider other diagnoses that might better explain the headache. In particular this applies to assessing whether headache is secondary or primary. It may also apply to alternative causative disorders: for example, headache occurring in close temporal relation to acute ischaemic stroke may be a consequence not of the stroke but of the cause of the stroke (eg, dissection).
  11. Many patients with headache attacks fulfilling one set of explicit diagnostic criteria also have attacks that, while similar, do not quite satisfy the criteria. This can be due to treatment, inability to recall symptoms exactly or other factors. Ask the patient to describe a typical untreated or unsuccessfully-treated attack and ascertain that there have been enough of these to establish the diagnosis. Then include the less-typical attacks when describing attack frequency.
  12. When a patient is suspected of having more than one headache type or subtype, it is highly recommended that he or she fill out a diagnostic headache diary in which, for each headache episode, the important characteristics are recorded. It has been shown that such a headache diary improves diagnostic accuracy as well as allowing a more precise judgement of medication consumption. The diary helps in judging the quantity of two or more different headache types or subtypes. Finally, it teaches the patient how to distinguish between different headaches: for example between migraine without aura and episodic tension-type headache.
  13. In each chapter on the secondary headaches, the most well-known and well-established causes are mentioned and criteria for the consequent headaches are given. However, in many chapters, for example 9. Headache attributed to infection, there are an almost endless number of possible infective causes. In order to avoid a very long list, only the most important are mentioned. In the example, rarer causes are assigned to 9.2.3 Headache attributed to other systemic infection. The same system is used in the other chapters on secondary headaches.
  14. The diagnostic criteria for the secondary headaches no longer require remission or substantial improvement of the underlying causative disorder before the headache diagnosis can be made. The diagnostic citeria of ICHD-3 can be applied already on presentation or as soon after as the underlying disorder is confirmed. Criterion A is presence of the headache; criterion B is presence of the causative disorder; criterion C is the evidence of causation. In acute conditions, a close temporal relation between onset of headache and onset of the presumed causative disorder is often sufficient to establish causation, while less acute conditions usually require more evidence of causation. In all cases, the last criterion must be applied as a check: “Not better accounted for by another ICHD-3 diagnosis”.
  15. In a few secondary headaches, 5.2 Persistent headache attributed to traumatic injury to the head being a good example, persistent headache types or subtypes are recognized to occur; that is, headache that was caused initially by another disorder fails to remit after that disorder has resolved. In such cases, the diagnosis changes from the acute type (eg, 5.1 Acute headache attributed to traumatic injury to the head) to the persistent type (5.2 Persistent headache attributed to traumatic injury to the head) after a specified time interval (3 months in this example). Evidence of causation depends upon earlier fulfilment of the criteria for diagnosis of the acute type, and persistence of the same headache. Most such diagnoses are in the Appendix because of insufficient evidence of their existence. They will not usually be applied, but are there to stimulate research into better criteria for causation.
  16. The Appendix is for research. It helps clinical scientists study orphan entities for later inclusion in (or, in some cases, exclusion from) the main body of the classification. Most diagnoses and diagnostic criteria in the Appendix are either new or alternatives to criteria in the main body. Some are old entities not yet sufficiently validated; these are expected to be deleted in the next revision of ICHD if evidence is not produced.