Migraine and scotoma. Illustration of the silhouette of the head of a person experiencing the pain (lightning flashes) of a migraine, along with the associated visual disturbance known as a scotoma (right). Migraines are recurrent throbbing headaches that characteristically affect one side of the head. In some sufferers a visual disturbance called a scotoma occurs just before the attack. This typically appears as a slowly expanding area of blindness surrounded by an irregular, sparkling edge that occupies up to half the visual field. The scotoma clears up after about 20 minutes and is followed by a severe, one-sided headache accompanied by nausea and vomiting. The best therapy is prevention by avoiding any known triggers. A migraine starts when blood vessels to the head dilate after a period of constriction.