Primary or secondary headache or both?
Three rules apply to headache with the characteristics of a trigeminal autonomic cephalalgia (TAC), according to circumstances.
- When a new headache with the characteristics of a TAC occurs for the first time in close temporal relation to another disorder 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 TAC becomes chronic in close temporal relation to such a causative disorder, both the initial TAC diagnosis and the secondary diagnosis should be given.
- When a pre-existing TAC 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 TAC diagnosis and the secondary headache diagnosis should be given, provided that there is good evidence that the disorder can cause headache.
The trigeminal-autonomic cephalalgias (TACs) share the clinical features of unilateral headache and, usually, prominent cranial parasympathetic autonomic features, which are lateralized and ipsilateral to the headache. Experimental and human functional imaging suggests these syndromes activate a normal human trigeminal-parasympathetic reflex, with the clinical signs of cranial sympathetic dysfunction being secondary.
Typical migraine aura can be seen, rarely, in association with TACs.