Essentially, chronic Migraine (CM) is Migraines or tension-type headaches 15 or more days a month, which means that people with chronic Migraine have a Migraine or headache more often than not.
It should be stated here that a diagnosis of "chronic Migraine" does not replace the more standard diagnosis of which form of Migraine we have - Migraine without aura, Migraine with aura, basilar-type Migraine, etc. However, as research is conducted, medications are submitted to the FDA for approval for use in the treatment of Migraine, and other issues arise, we're seeing that chronic forms of Migraine often respond differently to treatment than episodic Migraine, patient needs are different, and that there are many reasons to standardize criteria for classifying Migraine as chronic.
As in other areas of medicine, when diagnosing and classifying types of Migraine and other headache disorders, there is a "gold standard," a set of recognized criteria and classifications that are generally followed. This is helpful to patients as well as health care providers because the standardization keeps us all on the same page and, hopefully, understanding which form of Migraine or headache we're discussing. Today, in the field of "headache medicine," the International Headache Society's (IHS) International Classification of Headache Disorders, 2nd Edition (ICHD-II), is considered the gold standard.
Chronic Migraine was not addressed in the first edition of the ICHD, which was released in 1988. In 2004, chronic Migraine appeared in the ICHD-II as a "complication of Migraine." The description and criteria were brief. It was described as "Migraine headache occurring on 15 or more days per month for more than three months in the absence of medication overuse." The criteria were simple. Chronic Migraine had to have headache fulfilling the criteria for Migraine without aura on 15 or more days per month for three months and could not be attributed to another disorder.1
Between ICHD-II and ICHD-II, Dr. Stephen Silberstein and Dr. Richard Lipton established criteria for what they called transformed Migraine (TM). The described TM as "Daily or near-daily headache with migraine that begins with episodic migraines and as the headaches grow more frequent over months to years the associated symptoms become less severe and less frequent."4 (see Transformed Migraine - The Basics for more details.)
Finally, in 2006, the IHS revised the ICHD-II with this criteria for chronic Migraine:
Appendix 1.5.1 Chronic migraine2
A. Headache (tension-type and/or migraine) on 15 or more days per month for at least 3 months
B. Occurring in a patient who has had at least five attacks fulfilling criteria for 1.1 Migraine without aura
C. On 8 or more days per month for at least 3 months headache has fulfilled C1 and/or C2 below, that is, has fulfilled criteria for pain and associated symptoms of migraine without aura
1. Has at least two of a–d
(a) unilateral location
(b) pulsating quality
(c) moderate or severe pain intensity
(d) aggravation by or causing avoidance of routine physical activity (e.g. walking or climbing stairs)
and at least one of a or b
(a) nausea and/or vomiting
(b) photophobia and phonophobia
2. Treated and relieved by triptan(s) or ergot before the expected development of C1 above
D. No medication overuse and not attributed to another causative disorder
Although it's not part of the ICHD-II, most of the current working definitions of chronic Migraine also include that each headache or Migraine last at least four hours if not treated. This has carried over into the definition of CM that the FDA uses in evaluating new drug applications for medications submitted to the FDA for the treatment of chronic Migraine. At this time, the only medication that has FDA approval for the treatment of CM is onabotulinumtoxinA (Botox.)