Indomethacin-responsive trigeminal autonomic cephalgias: a review of key characteristics and pathophysiology.

Trigeminal autonomic cephalgias (TACs) are a well-defined subset of uncommon primary headaches that share comparable onset, pathophysiology and symptom patterns. TACs are characterised by the presentation of one-sided and high-intensity trigeminal pain together with unilateral cranial autonomic signs, which can include lacrimation, rhinorrhea, and miosis. The International Classification of Headache Disorders 3rd Edition recognises four different headache entities in this group, with cluster headache as the most recognised among them. Hemicrania continua (HC) and paroxysmal hemicrania (PH) are both distinctive cephalgias of which the diagnostic criteria include an absolute response to indomethacin. Consequently, for this reason they are often referred to as 'indomethacin-responsive' TACs. The main focus of this review was to discuss the state of knowledge regarding the pathophysiology and key characteristics of PH and HC. Given the limited understanding of these conditions, and their exceptionally uncommon prevalence, a correct diagnosis can pose a clinical challenge and the search for an effective treatment may be prolonged, which frequently has a serious impact upon patients' quality of life. The information provided in this review is meant to help physicians to differentiate indomethacin-sensitive cephalgias from other distinct headache disorders with a relatively similar clinical presentation, such as cluster headache, trigeminal neuralgia, and various migraine conditions.