Primary or secondary headache or both?
When a headache with the characteristics of any of the disorders classified here occurs for the first time in close temporal relation to another disorder that is a 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. When a pre-existing headache with the characteristics of any of the disorders classified here 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 initial headache diagnosis and the secondary headache diagnosis should be given, provided that there is good evidence that the disorder can cause headache.
This chapter includes a number of primary headache disorders that are clinically heterogeneous. Their pathogenesis is still poorly understood, and their treatments are suggested on the basis of anecdotal reports or uncontrolled trials. Headaches with similar characteristics to several of these disorders can be symptomatic of another disorder (ie, secondary headaches); when they first present, they demand careful evaluation by imaging and/or other appropriate tests.
The onset of some of these headaches, for example, 4.2 Primary exercise headache, 4.3 Primary headache associated with sexual activity and 4.4 Primary thunderclap headache, can be acute, and affected patients are sometimes assessed in emergency departments. Appropriate and full investigation (neuroimaging, in particular) is mandatory in these cases.
This chapter also includes some clinical entities, such as 4.7 Primary stabbing headache and 4.9 Hypnic headache, that are primary in most cases. In addition, more and more evidence indicates that 4.8 Nummular headache is a primary headache disorder, and therefore it has been moved from the Appendix of ICHD-II to this chapter of ICHD-3 (beta). Two headache disorders originally in chapter 13 of ICHD-II have also been moved to this chapter: 4.5 Cold-stimulus headache and 4.6 External-pressure headache. The latter includes 4.6.1 External-compression headache and the newly added entity, 4.6.2 External-traction headache, because these seem more likely to be primary headache disorders in that they are brought on by physiological (non-damaging) stimuli. In contrast, 3.4 Hemicrania continua, originally in this chapter in ICHD-II, is now moved to chapter 3 because evidence indicates that it rightly belongs to 3. Trigeminal autonomic cephalalgias.
The headache disorders in this chapter can be grouped into four categories: (1) headaches associated with physical exertion, including 4.1 Primary cough headache, 4.2 Primary exercise headache, 4.3 Primary headache attributed to sexual activity and 4.4 Primary thunderclap headache; (2) headaches attributed to direct physical stimuli, including 4.5 Cold-stimulus headache and 4.6 External-pressure headache; (3) epicranial headaches (ie, head pain over the scalp), including 4.7 Primary stabbing headache and 4.8 Nummular headache (as well as A4.11 Epicrania fugax in the Appendix); and (4) other miscellaneous primary headache disorders including 4.9 Hypnic headache and 4.10 New daily persistent headache. Therefore, the coding sequence is rearranged in ICHD-3 (beta) according to these groupings.