The cause of hemicrania continua is unknown. As with Migraine disease and many other headache disorders, there is no definitive diagnostic test for hemicrania continua. Tests such as an MRI may be performed to rule out other causes for the headache. When a patient has the symptoms of hemicrania continua, it's considered "diagnostic" if they respond completely to indomethacin.

Resources:

International Headache Society. International Classification of Headache Disorders, 2nd Edition.

Tepper, Stewart J., M.D. "Understanding Migraine and Other Headaches." University Press of Mississippi. 2004.

Young, William B., M.D.; Silberstein, Stephen D., M.D. "Migraine and Other Headaches." American Academy of Neurology Press. 2004.

Evans, Randolph W.; Mathew, Ninan T. "Handbook of Headache," Second Edition. Lippincott Williams & Wilkins. 2005.