Chronic migraine best characterizes those patients with a history of migraine who experience headache more than half the time. This condition was previously called Transformed Migraine. However, transformation requires that the sufferer recall precisely how their headaches have changed over time. This new description will provide a usable definition for medical research studies of new treatment options.

Some patients will describe a daily, chronic tension-type headache as well as a recurrent hard or "sick" migraine-like headache. This combination of tension-type and migraine headaches, previously known as the mixed headache syndrome, is now identified as coexisting migraine and tension-type headaches. Other terms used for it include: transformed migraine, chronic migraine and chronic daily headache.

Transformed migraine (TM) is a migraine condition that initially began as episodic migraine attacks, which then increase in frequency over a period of month to years. Transformed migraines occur daily or almost daily and appear to be a mixture of tension-type headaches and migraine attacks. The pain associated with transformed migraines often drops to a mild to moderate level of severity. Patients with TM often have a history of episodic migraines originating in the early teens or twenties. The process of transformation yields attacks that are frequently accompanied by phonophobia, photophobia, and nausea, but these symptoms tend to lessen in severity and frequency over time. Many other symptoms of migraine remain, such as unilateral pain, gastrointestinal symptoms, as well as aggravation by other triggers. Many of the patients with TM are women, 90% of whom have a history of migraine without aura.