A 32-year-old woman presents to you for evaluation of headache. The headaches began at age 18, were initially unilateral and worse around the time of her menses. Initially the use of triptans two or three times a month would provide complete relief. Over the past several years, however, the headaches have become more frequent and severe. Triptans provide only partial relief; the patient requires a combination of acetaminophen, caffeine, and butalbital to achieve some improvement. Prophylactic medications including beta-blockers, tricyclics, and topiramate have been unsuccessful in preventing the headaches, and she has been to the emergency room three times over the past 2 weeks for a "pain shot." The general physical examination is unremarkable. Her funduscopic examination shows no evidence of papilledema, and a careful neurological examination is likewise normal. What is the most likely explanation for her headache syndrome?