When can successful migraine prophylaxis be discontinued?

CASE HISTORY A 43-year-old woman has a 20-year history of migraine without aura. Two years ago, she was having about 8 migraines per month lasting 1–2 days with a variable response to triptans. She was placed on topiramate, 100 mg daily. Since then, there has had a dramatic reduction in the number of headaches to about once or twice monthly. She is typically pain free within 1 or 2 hours upon taking a triptan. She now asks if she can discontinue the topiramate. Questions.—Can she discontinue the topiramate? Do preventive medications alter the natural history of migraine? What are the general guidelines for discontinuing preventative medications?