Deposition of iron in the periaqueductal grey matter (PAG) is a potential biomarker for chronic migraine (CM), say researchers. Domínguez and co-workers used 3T MRI to analyse the deposition of iron in the red nucleus, globus pallidus and PAG of 55 patients with CM, 57 patients with episodic migraine (EM) and 25 controls. Patients with CM had increased iron deposition in the red nucleus and PAG compared with patients with EM and controls. Patients with EM had increased iron deposition in the PAG compared with controls. No differences in iron deposition in the globus pallidus were observed between groups. Iron volume in the PAG correlated with serum level of biomarkers of endothelial dysfunction and blood–brain barrier disruption, and a threshold of 240 µl iron in the PAG showed 93% sensitivity and 97% specificity for diagnosis of CM.