Trigeminal autonomic cephalalgias: from pathophysiology to clinical aspects.

The strictly unilateral headaches, more commonly known as trigeminal autonomic cephalalgias (TACs), are characterised by severe, strictly unilateral pain in the territory of the distribution of the trigeminal nerve, associated with autonomic manifestations. The recent International Headache Society classification lists the strictly unilateral headaches as cluster headache (CH), episodic and chronic paroxysmal hemicrania, and short-lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing. CH is the most common and best-defined of the TACs, whose pathophysiologies have not been adequately defined. Convincing proposals for pathophysiologic mechanisms must explain the trigeminal distribution of the pain, the homolateral autonomic manifestations; and, for CH, the usually periodic recurrence of the crises and clusters. With regard to CH, (i) the pain is always located periorbitally-frontally, implicating nociceptive mechanisms involving the trigeminal nerve; (ii) the autonomic manifestations homolateral to the pain seem to be both parasympathetic (lacrimation and rhinorrhoea) and sympathetic (ptosis and miosis); and (iii) the periodicity of the attacks and seasonal recurrence of the cluster periods suggest involvement of a biological clock within hypothalamus.