Chronic migraine is a distinct and relatively recently defined sub-type of Chronic Daily Headache. The International Headache Society defines chronic migraine as more than fifteen headache days per month over a three month period of which more than eight are migrainous, in the absence of medication overuse. Episodic migraine is the other migraine sub-type, which is defined as less than 15 headache days per month.
Reference : Written by Dr Fayyaz Ahmed MD FRCP MBA, https://www.migrainetrust.org/about-migraine/types-of-migraine/chronic-migraine/ july 4th,2019