This extensive document is not intended to be learned by heart. Even members of the Headache Classification Subcommittee 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 subtypes 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 and every patient entered into a research project, be it a drug trial or a study of pathophysiology or biochemistry, must fulfil a set of diagnostic criteria.
This classification is hierarchical and you must decide how detailed you want to make your diagnosis. This can range from the first-digit level to the fourth. First one gets a rough idea about which group the patient belongs to. Is it for example 1. Migraine or 2. Tension-type headache or 3. Cluster headache and other 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 whilst in specialist practice and headache centres a diagnosis at the third- or fourth-digit levels is appropriate.
Patients receive a diagnosis according to the headache phenotypes that they currently present or that they have presented within the last year. For genetic and some other uses, occurrence during the whole lifetime is used.
Each distinct type of headache that the patient has must be separately diagnosed and coded. Thus, a severely affected patient in a headache centre may receive three diagnoses and codes: 1.1 Migraine without aura, 2.2 Frequent episodic tension-type headache and 8.2 Medication-overuse headache.
When a patient receives more than one diagnosis these should be listed in the order of importance to the patient.
If one type of headache in a particular patient fulfils two different sets of explicit diagnostic criteria, then 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 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. Cluster headache and other trigeminal autonomic cephalalgias, or any of their 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 1.6 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 whilst other attacks meet another set. In such cases, two diagnoses exist and both should be coded.
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 explicit 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.
The full set of explicit diagnostic criteria is provided for some headache disorders only at the first- and second-digit levels. Diagnostic criteria at the third- and fourth-digit levels then demand, as criterion A, fulfilment of the criteria for levels one and/or two and, in criterion B and onwards, specify the further specific criteria to be fulfilled.
The frequency of primary headache disorders varies from attacks every 1-2 years to attacks daily. The severity of attacks also varies. The International Classification of Headache Disorders, 2nd edition, does not generally provide a possibility to code for frequency or severity, but recommends that frequency and severity be specified in free text.
Primary or secondary headache or both: If a new headache occurs for the first time in close temporal relation to another disorder that is a known cause of headache, this headache is coded according to the causative disorder as a secondary headache. This remains true even when the headache has the characteristics of migraine, tension-type headache, cluster headache or one of the other trigeminal autonomic cephalalgias.
When a pre-existing primary headache is made worse in close temporal relation to another disorder that is a known cause of headache, there are two possibilities and judgment is required. The patient can either be given only the diagnosis of the pre-existing primary headache or be given both the primary headache diagnosis and a secondary headache diagnosis according to the other disorder. Factors that support adding the secondary headache diagnosis are: a very close temporal relation to the causative disorder, a marked worsening of the primary headache, very good evidence that the causative disorder can aggravate the primary headache in the manner observed and, finally, improvement or disappearance of the headache after relief from the presumed causative disorder.
Many patients with headache attacks fulfilling one set of explicit diagnostic criteria also have attacks that, whilst 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.
When a patient is suspected of having more than one headache type 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.
In each chapter on the secondary headaches the most well-known and well-established causes are mentioned and criteria for these are given. However, in many chapters, for example 9. Headache attributed to infection, there are an almost endless number of possible 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 infection. The same system is used in the other chapters on secondary headaches.
The last criterion for most of the secondary headaches requires that the headache greatly improves or resolves within a specified period after relief from the causative disorder (through treatment or spontaneous remission). In such cases, fulfilment of this criterion is an essential part of the evidence for a causal relationship. Very often, there is a need to code patients before this disorder is treated or before the result of treatment is known. In such cases the diagnosis should be Headache probably attributed to [the disorder]. Once the treatment results are known, the diagnosis becomes Headache attributed to [the disorder], or is changed if the criterion is not fulfilled.
In a few cases, post-traumatic headache being a good example, chronic headache subforms are recognised to occur. In such cases, the initially acute headache may persist, and causation is neither proved nor disproved by the duration of the headache in relation to onset of or relief from the causative disorder. The last criterion instead distinguishes between acute and chronic subforms, specifying resolution of headache within (for the acute subform) or persistence of headache beyond (for the chronic subform) a period of 3 months after occurrence, remission or cure of the causative disorder. In the course of the disorder, the diagnosis may therefore change after 3 months to Chronic headache attributed to [the disorder]. In the example, 5.1 Acute post-traumatic headache changes to 5.2 Chronic post-traumatic 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.