How does Botox relieves migraine headaches?

Botox, or onabotulinumtoxinA was recently approved by the FDA for treatment of chronic migraine based on the results of two large studies. Botox is the only prophylactic therapy specifically approved for chronic migraine. Many patients and doctors alike wonder about the mechanism of action of Botox. We originally thought that Botox works by relaxing tight muscles around the scalp. Studies have shown that during a migraine attack, the muscles in the forehead, temples and the back of the head are in fact contracted. It is also typical for a person with a migraine to rub their temples or the neck, which provides some temporary relief. However, I have seen some patients who would report that injecting muscles around the head eliminated pain in the injected areas, but that they still had pain on the top of the head. There are no muscles on the top of the head and we usually do not inject Botox there, but in those patients who do have residual pain on the top, injecting Botox stops the pain. Recent research has shown that Botox in fact also exerts a direct analgesic (pain-relieving) effect. This is supported by my and other doctors’ observation that Botox also helps other types of pain, such as that of shingles or trigeminal neuralgia. These are so called anecdotal reports and cannot be relied on to make definitive conclusions – we need large trials can prove this. It appears that Botox helps by reducing pain messages sent to the brain from both muscles and peripheral sensory nerves. This explains why migraine, which is a brain disorder, can be helped by a procedure directed only at the peripheral nerves – with the reduction of the barrage of pain messages reaching the brain, the brain does not become more and more excitable, or “wound-up” and a migraine attack does not occur. Some patients tell me that after Botox treatment they sometimes feel that a migraine is about to start, but it does not.