The lesson of chronic migraine.

The hypothesis that central sensitization/allodynia is the common final mechanism responsible for the progression of migraine pain is supported by the possibility of tracing back to allodynic mechanisms the action of the main risk factors for chronic migraine validated by the recent literature. The comorbidity between migraine and idiopathic intracranial hypertension without papilledema is emerging as a new, commonly overlooked risk factor for migraine progression whose putative mechanism might also converge on the sensitization of central pain pathways. If headache progression always occurs at the end of a pathogenetic sequence typical of an individual susceptibility to allodynia, then the primary character of chronic migraine might be debated. Allodynia is not specific to migraine but is implied in the progressive amplification of pain after repeated stimuli, a universal adaptive phenomenon. Being largely conditioned by the individual comorbidity profile, allodynia may only in part be defined as primary in itself. Many migraine comorbid conditions, including a hidden idiopathic intracranial hypertension without papilledema, may emphasize susceptibility to allodynia and promote chronic migraine. These factors and comorbid conditions require to be individually assessed and adequately treated to optimize the therapeutic response.