Migraines, and all variations of it, are purely psychogenic. They are caused by suppressed aggression. Generally, people who get migraines are sweet natured people who wouldn't hurt a fly, but all humans have aggressive impulses. There is no way to eliminate them, but aggression always has to express itself somehow, and if an aggressive urge can't be gratified by attacking another person, it turns inward. It's like wishing to hit someone on the head, but since this is prohibited by ones own conscience, the only head a truly peaceable person can hit is his own. Thus the migraine. Any vertigo associated with the migraine is symbolic dizzyness from a symbolic punch.
The way Lexapro works for migraines is simply that it mellows out the afflicted's internal psyche. Marijuana generally works wonders on migraines for the same reason. Psychotherapy works in some instances, but in my humble opinion it is better to keep the migraines and stay sweet natured.