When a patient has headache for the first time, or a new headache type, and at the same time develops a brain tumour, it is straightforward to conclude that headache is secondary to the tumour. Such patients shall be given only one headache diagnosis – 7.4 Headache attributed to intracranial neoplasia (or one of its subtypes) – even when the headache phenomenologically appears to be migraine, tension-type headache or cluster headache. In other words, a de novo headache occurring with another disorder recognized to be capable of causing it is always diagnosed as secondary.
The situation is different when the patient has previously had a type of primary headache that becomes worse in close temporal relation to the occurrence of another disorder. Three possible explanations for this worsening exist: that it is coincidental; that it is an aggravation of the primary headache, causally related to the other disorder; that it represents a new headache, again causally related to the other disorder. The general rules for attribution developed in ICHD-II allowed one or two diagnoses in such circumstances, but relied on judgement. They were modified in ICHD-3 (beta) to be less open to interpretation, and these modifications are retained.
- When a new headache occurs for the first time in close temporal relation to another disorder that is 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 or 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.
ICHD-II standardized the format of the diagnostic criteria for secondary headaches, but this was not without problems. A revision was adopted in ICHD-3 (beta), and this too is retained:
General diagnostic criteria for secondary headaches:
- Any headache fulfilling criterion C
- Another disorder scientifically documented to be able to cause headache has been diagnosed1Evidence of causation demonstrated by at least two of the following2:
- headache has developed in temporal relation to the onset of the presumed causative disorder
- either or both of the following:
- – headache has significantly worsened in parallel with worsening of the presumed causative disorder
- – headache has significantly improved in parallel with improvement of the presumed causative disorder
- headache has characteristics typical for the causative disorder3
- other evidence exists of causation4
- Not better accounted for by another ICHD-3 diagnosis.
- Since headache is extremely prevalent, it can occur simultaneously with another disorder by chance and without a causal relation. Therefore, a secondary headache can be definitely diagnosed only when solid evidence exists from published scientific studies that the disorder specified in criterion B is capable of causing headache. Scientific evidence can come from large clinical studies observing close temporal relationships between the disorder and headache outcomes after treatment of the disorder, or from smaller studies using advanced scanning methods, blood tests or other paraclinical tests, even if these are not readily available to the diagnosing physician who will use these criteria. In other words, study methods that are not useful in routine use of the diagnostic criteria may nonetheless be useful for establishing general causal relationships as the basis of criterion B. Throughout ICHD-3, however, diagnostic criteria restrict themselves to information reasonably available to the diagnosing physician in a typical clinical situation.
- The general criteria require at least two separate evidential features to be present, and allow up to four types of evidence, as set out. Not all of these four types are appropriate for all disorders, and not all four need form part of the specific criteria for a particular secondary headache when this is so. There are a few secondary headaches for which evidence of causation depends very heavily upon onset in temporal relation to the presumed cause. Examples are the subtypes of 7.2 Headache attributed to low cerebrospinal fluid pressure, which are usually orthostatic but not invariably, so that this characteristic cannot be relied upon as a diagnostic criterion. In such cases, criterion D is of particular importance.
- An example is very sudden (thunderclap) onset of headache in 6.2.2 Acute headache attributed to non-traumatic subarachnoid haemorrhage. The characteristics (if any) must be specified for each secondary headache.
- This is to be specified (if appropriate) for each secondary headache. One example of this kind of evidence is accordance between the site of the headache and the location of a presumed causative disorder. Another is variations in parallel between headache features (such as intensity) and markers of activity of the presumed causative disorder (eg, changes on neuroimaging, or in other laboratory measures [such as erythrocyte sedimentation rate in 6.4.1 Headache attributed to giant cell arteritis]).