Primary headaches are a continuum driven by a common process

We propose that the primary headaches defined under the International Classification of Headaches (ICH3) lie within a semiological continuum which encompasses all primary headaches. It is likely that activation of a common ‘upstream’ brain mechanism engages divers ‘downstream’ neural networks and pathways to generate the symptoms. These pathways typically become entrained through recurrent activation so habitual headache patterns emerge, but semiologies can also change over time. Features which may accompany primary headaches, such as auras, cranial autonomic and vegetative symptoms, are also generated through neural networks activated by the mechanism but are functionally independent of the headache pathway. While they feature commonly in migraine, they can occur occasionally in other primary headaches, and also in the absence of headache. Several cranial nerves and the greater occipital nerves can be the source of head pains, but some also provide input to the migraine mechanism through the trigeminal cervical complex (TCC). Cranial neuralgias can also trigger the migraine mechanism directly and there is a close semiological and functional relationship between trigeminal neuralgia (TN), migraine, and the trigeminal autonomic cephalalgias (TACs), exemplified by TAC-tic and migraine-tic syndromes and the role of trigeminal neurovascular conflict in the pathogenesis of both TN and SUNHA (short-lasting unilateral neuralgiform headache attacks).