[Autonomic Disorders in Trigeminal Autonomic Cephalalgias: An Update]

The term trigeminal autonomic cephalalgias (TACs) was introduced in the 2^nd edition of the International Classification of Headache Disorders, and has been retained in the 3^rd edition. TACs include cluster headache, paroxysmal hemicrania, short-lasting unilateral neuralgiform headache attacks, hemicrania continua, and probable trigeminal autonomic cephalalgia. Headaches classified as TACs share clinical features with unilateral headache and usually present with prominent lateralized and ipsilateral cranial parasympathetic autonomic features, including the following: (1) conjunctival injection and/or lacrimation, (2) nasal congestion and/or rhinorrhea, (3) eyelid edema, (4) forehead and facial sweating, (5) miosis and/or ptosis. "Clinical Practice Guideline for Headache 2021 (Japanese version)" will serve as a useful aid for diagnosis and treatment of TACs. Currently, hypothalamic activation, activation of the trigeminal-autonomic nerve reflex, internal carotid artery dilatation, and the action of some neuropeptides are implicated as pathophysiological mechanisms underlying TACs; however, it is not unequivocal. Further studies are warranted to gain deeper insight into several unclear aspects associated with TACs.