Modular headache theory: a new approach.

Many people experience headaches that do not fulfill the International Headache Society's criteria for a specific headache disorder, yet behave biologically like that disorder. Others fulfill criteria for one headache disorder but have features of another. To explain these observations, we propose that groups of neurons, called modules, become activated to produce each symptom of a primary headache disorder, and that each module is linked to other modules that together produce an individual's headache. Headaches develop phenotypic stability through a process referred to as learned stereotypy. This theory has implications for the classification, research, and treatment of primary and secondary headache patients.