Chronic daily headache: nosology and pathophysiology.

Systematic scientific classification of primary headaches is inexact, relying on clinical features because the disorders lack diagnostic markers, although the International Headache Society classification has been successful in providing relatively homogenous clinical groups for pathophysiological and therapeutic studies. One area in which there have been particular difficulties and uncertainty is in classifying patients with frequent headache, particularly chronic daily headache. Clinical research on the topic is limited, and imprecise because of uncertainties of definition. Rigorous basic or applied clinical research is a rarity, attested to by a paucity of new publications in the past year. Accordingly, the scientific basis of chronic daily headaches remains to be determined. There is agreement on one issue: for headache specialists and neurologists this is an important clinical problem. We take the position that chronic daily headache is what it says--frequent headache. As hematologists make a diagnosis of anemia, which invites further investigation and sub-classification, neurologists might diagnose chronic daily headache not to imply that all its causes are the same but simply to begin the clinical process.