Primary or secondary headache or both?
When a headache occurs for the first time in close temporal relation to a disorder of homoeostasis, it is coded as a secondary headache attributed to that disorder. This remains true when the new headache has the characteristics of any of the primary headache disorders classified in Part One of ICHD-3 (beta). When a pre-existing headache with the characteristics of a primary headache disorder 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 a disorder of homoeostasis, both the initial headache diagnosis and a diagnosis of 10. Headache attributed to a disorder of homoeostasis (or one of its subtypes) should be given, provided that there is good evidence that that disorder can cause headache.
The mechanisms behind causation of the various subtypes of 10. Headache attributed to disorder of homoeostasis are various. Nevertheless, it is possible to set out general diagnostic criteria, applicable in most cases, as follows:
A. Headache fulfilling criterion C
B. A disorder of homoeostasis known to be able to cause headache has been diagnosed
C. Evidence of causation demonstrated by at least two of the following:
1. headache has developed in temporal relation to the onset of the disorder of homoeostasis
2. either or both of the following:
a) headache has significantly worsened in parallel with worsening of the disorder of homoeostasis
b) headache has significantly improved after resolution of the disorder of homoeostasis
3. headache has characteristics typical for the disorder of homoeostasis
D. Not better accounted for by another ICHD-3 diagnosis.