Trigeminal autonomic cephalalgias: fancy term or constructive change to the IHS classification?

For the neurologist faced with the day to day grind of clinical work a change to terminology may seem like the academics “at it again”. I will try to set out this change and illustrate a physiology that may be attractive to understand, and hopefully one that enhances, clinical practice. Appreciating the physiology of the trigeminal-autonomic reflex can make patients presenting with varying degrees of cranial autonomic activation, such as lacrimation, conjunctival injection, nasal congestion or rhinorrhoea and the like, comprehensible at the bedside.1

The trigeminal autonomic cephalalgias (TACs) is a grouping of headache syndromes recognised in the second edition of the International Headache Society (IHS) classification.2 The term was coined to reflect a part of the pathophysiology of these conditions that is a common thread—that is, excessive cranial parasympathetic autonomic reflex activation to nociceptive input in the ophthalmic division of the trigeminal nerve.1 The TACs are classified in section III of the second edition of the classification,2 and include cluster headache,3 paroxysmal hemicrania, and short lasting unilateral neuralgiform headache attacks with conjunctival injection and tearing (SUNCT).4 In an early draft, hemicrania continua was included5 but this was finally classified in section IV. I will briefly review the underlying physiology of the trigeminal-autonomic reflex that underpins these conditions and set out their classification and differential diagnosis. I will point out some limitations and some directions for future research. Their therapy is beyond the scope of the present paper, but it has been recently reviewed.4

PATHOPHYSIOLOGY OF TACS

Any pathophysiological construct for TACs must account for the two major shared clinical features characteristic of the various conditions that comprise this group: trigeminal distribution pain and ipsilateral cranial autonomic features.1 …