Comorbidities and Clinical Overlap of Cluster Headache with Other Primary Headache Disorders: A Narrative Review.

Cluster headache (CH) is recognized as one of the most severe primary headache disorders and belongs to the group of trigeminal autonomic cephalalgias (TACs). Traditionally regarded as a distinct clinical entity under the International Classification of Headache Disorders, 3rd edition (ICHD-3), CH has been increasingly documented over the past two decades to frequently co-occur or overlap with other primary headache disorders, challenging this conventional view. This narrative review synthesizes available evidence from the PubMed and ScienceDirect databases on the comorbidity or clinical overlap between CH and other primary headache disorders. It focuses primarily on the co-occurrence of CH with migraine, tension-type headache, other TACs, primary stabbing headache, and medication-overuse headache. In addition, it explores potential clinical overlaps between CH and several other primary headaches, including cough headache, headache associated with sexual activity, thunderclap headache, and external-pressure headache. This review highlights that comorbidity of CH with other primary headache disorders is clinically significant yet frequently underrecognized, underscoring an urgent need for systematic prospective investigations to improve diagnostic accuracy and treatment outcomes.