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 neoplasm – even if the headache phenomenologically is migraine, tension-type headache or cluster headache. In other words, a de novo headache occurring with another disorder recognised 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. In the first edition of The International Classification of Headache Disorders we concluded after many discussions that only a new headache could be regarded as secondary. During the work with the second edition it has become obvious, however, that this results in some unacceptable situations. What about a patient who throughout her life has had ten migraine attacks but who, after a head trauma, begins to have migraine attacks twice a week and becomes disabled by these headaches? According to the system of the first edition this patient could only receive the diagnosis of migraine. Another example is a patient who has had tension-type headache which becomes worse, whilst retaining the same characteristics, in association with a brain tumour. The diagnosis of 7.4 Headache attributed to intracranial neoplasm could not previously be given. Finally, nothing in the past could be diagnosed as medication-overuse headache because this is always an aggravation of a primary headache, usually migraine, which would remain the only diagnosis.
For these reasons, we introduce a new way of diagnosing and coding primary headaches that are made significantly worse in close temporal relation to another disorder known from good scientific studies to be able to cause headache. Such patients can now receive two diagnoses: the primary headache diagnosis and the secondary headache diagnosis. In theory the new system is more open to interpretation than the old but, in fact, the old system has never been followed when it led to unreasonable diagnoses. The problem with the new system is to decide, in patients whose primary headache worsens in relation to another disorder, whether to use only the primary diagnosis or whether to add a secondary headache diagnosis also. The following factors support the use of two diagnoses: a very close temporal relation, marked worsening of the primary headache, the existence of other evidence that the other disorder can aggravate primary headache in the manner observed, and remission of the headache after cure or remission of the other disorder.
In the first edition of The International Classification of Headache Disorders the diagnostic criteria for secondary headaches varied a great deal and were often uninformative about headache characteristics. For this second edition it has been decided to standardise the format and give more headache characteristics whenever possible. The diagnostic criteria therefore have the following disposition:
Diagnostic criteria for secondary headaches:
A. Headache with one (or more) of the following [listed] characteristics1;2 and fulfilling criteria C and D
B. Another disorder known to be able to cause headache has been demonstrated
C. Headache occurs in close temporal relation to the other disorder and/or there is other evidence of a causal relationship
D. Headache is greatly reduced or resolves within 3 months (this may be shorter for some disorders) after successful treatment or spontaneous remission of the causative disorder3
1. For most secondary headaches the characteristics of the headache itself are poorly described in the scientific literature. Even for those where it is well described, there are usually few diagnostically important features. Therefore, diagnostic criterion A in the standard set of criteria is usually not very contributory to establishing causation. However, criteria B, C and D usually effectively establish causation. This makes it possible to use criterion A not only as a defining feature but also to tell as much about the headache as possible or to show how little we know of it. This is why the formulation of criterion A now allows mention of a number of features. Hopefully, this will stimulate more research into the characteristics of secondary headaches so that, eventually, criterion A for most of these headaches can become much more clearly defined.
2. If nothing is known about the headache, it is stated “no typical characteristics known”.
3. Criterion D cannot always be ascertained and some presumed causative disorders cannot be treated or do not remit. In such cases criterion D may be replaced by: “Other causes ruled out by appropriate investigations”.
In many cases sufficient follow-up is not available or a diagnosis has to be made before the expected time needed for remission. In most such cases the headache should be coded as Headache probably attributed to [the disorder]: a definite relationship can only be established with full confidence once criterion D is fulfilled. This is especially so in situations where a pre-existing primary headache has been made worse by another disorder. For example, the great majority of patients otherwise fulfilling the criteria for 1.5.1 Chronic migraine are overusing medication and will improve after this overuse ceases. The default rule in this case, pending withdrawal of the overused medication, is to code according to the antecedent migraine subtype (usually 1.1 Migraine without aura) plus 1.6.5 Probable chronic migraine plus 22.214.171.124.8 Probable medication-overuse headache. Following withdrawal, criterion D for 8.2 Medication-overuse headache is not fulfilled if a patient does not improve within 2 months and this diagnosis must then be discarded in favour of 1.5.1 Chronic migraine. A similar rule applies to patients overusing medication but otherwise fulfilling the criteria for 2.3 Chronic tension-type headache.
In most cases criterion D has a time-limit for improvement of the headache after cure or spontaneous remission or removal of the presumed cause. Usually this is 3 months but it is shorter for some secondary headaches. If headache persists after 3 months (or a shorter limit) it should be questioned whether it was actually secondary to the presumed cause. Secondary headaches persisting after 3 months have often been observed but most have not been of scientifically-proven aetiology. Such cases have been included in the appendix as Chronic headache attributed to [a specified disorder].