Did you ever notice that during a migraine your senses seem more acute?
- Odors smell stronger.
- Lights look brighter.
- Sounds seem louder.
- Touch feels harsher.
This heightening of the senses is a sign that the brain and nervous system are excited and paying closer attention to changes in your environment. When a migraine is happening, the brain goes into a “high alert” state called central sensitization. Central sensitization makes the brain more efficient at detecting-and magnifying-even small changes in the body.
Central sensitization: a closer look
In the head, there are nerves outside the brain (peripheral nerves) and nerves within the brain tissue (central nerves). The first change that happens to nerve activity during a migraine is peripheral sensitization: the activation of nerves outside the brain. When these nerves are excited or sensitized, the head becomes more sensitive to minute changes in pressure around the brain and blood flow.
Normally, we can’t feel little pressure changes around the brain. But, during a migraine, the sufferer does experience pain with these small pressure changes because the nerves around the head are excited (peripheral sensitization). As a result, the person will experience an intense pounding, even though the pressure change is not that great. Migraineurs often say that they feel like they have to hold the sides of their head together during a migraine if they bend, cough, or sneeze because it feels like their head is going to explode.
Another symptom of peripheral nerve activation or sensitization is the throbbing pain that characterizes migraine-pain related to blood flow. When a person is having a migraine, the blood flow to the blood vessels around the brain increases slightly. Usually, we wouldn’t feel this tiny increase in blood vessel pulsing. When a migraine is taking place, however, the peripheral nerves are also more excited and so we become aware of the pulsing. A person may say that “it feels like my heart is beating in my head.” The blood vessels are actually always beating, but we can’t feel them unless the peripheral nerves are sensitized.
About one to two hours after the peripheral nerves are sensitized, the central nerves within the brain also become sensitized in a significant number of migraineurs. When the central nerves are activated, migraineurs experience allodynia: the sensation of pain when something non-painful touches the skin. A common complaint that can indicate allodynia is that the “hair hurts” during a migraine. Because the scalp develops allodynia, even gentle stroking of it or brushing the hair is felt as pain. Other symptoms of allodynia on the head are pain from wearing glasses or earrings and pain from shaving or washing the face. About half of migraineurs will also have allodynia in their arms or legs and may note that clothes feel unpleasantly tight during a migraine.
Why is central sensitization important?
Dr. Rami Burstein at Harvard University has done some important experiments testing the relationship of nerve sensitization and migraine therapy. His studies show that the expected benefit from acute migraine medication, such as a triptan, depends on which nerves are sensitized when the treatment is administered. If a triptan medication is taken at the beginning of a migraine when only the peripheral nerves are sensitized, migraine relief usually occurs. If, however, triptans are not taken until after central sensitization happens, then the migraine is usually not relieved. These studies show that there is a window of opportunity for treating a migraine to maximize the likelihood of good relief.
How can you use information about nerve sensitization to treat your own migraine?
People often wonder why a medication can work so well for one headache and not help another one. The new research on nerve sensitization provides an explanation for some of this difference in headache response. In many cases, an acute migraine medication didn’t work for one headache because it wasn’t used until after central sensitization had occurred. When the same medication is taken before central sensitization, it will likely become effective.
Fortunately, you can tell when you have developed both peripheral and central sensitization during a migraine. When you have a throbbing pain and your head pain is aggravated by bending, you have peripheral sensitization. This is the ideal time to treat your migraine because early treatment increases the effectiveness of acute migraine therapy.
Once you notice scalp sensitivity, pain with light touching of the head, or pain with wearing glasses or earrings, central sensitization has occurred and the acute migraine medication may be less likely to be effective.
It may be worthwhile to re-try therapy that didn’t work for you in the past if you previously waited more than one hour to start treatment.
What if you never develop allodynia?
Some people don’t develop allodynia and central sensitization. In Dr. Burstein’s studies, he found that those people were just as likely to respond well to a triptan if it was administered early in a migraine episode or delayed for a couple of hours. If you don’t typically develop symptoms of allodynia, then you will probably get similar migraine relief whether you take your medication early or later into a migraine episode.
Does central sensitization go away?
Allodynia generally occurs in migraineurs only during migraine episodes and then goes away in between migraine attacks. However, in addition to getting a temporary activation of the peripheral and central nerves during migraine episodes, migraineurs also have increased activity of the brain between migraine episodes. This may make migraineurs more sensitive to headache triggers and more likely to have frequent headaches.
The good news for migraineurs is that migraine preventive therapies, such as beta blockers (e.g., propranolol) and anti-epileptic therapies (e.g., valproate), have been shown to reverse changes of chronic brain excitation in migraineurs. The reversal may explain why some patients find that once their headaches become less frequent by using a preventive therapy, acute medications become more effective for the headaches they still have.
Take-home messages
- Nerves become excited and sensitized during migraine episodes.
- Throbbing pain and pain with bending are symptoms of peripheral sensitization.
- Sensitivity to touching the scalp or skin (“my hair hurts”) is a symptom of central sensitization.
- Acute migraine therapy is most effective when used during peripheral sensitization.
- Acute migraine therapy is less effective once central sensitization has occurred.
- Since central sensitization occurs within one to two hours of the onset of a migraine, acute therapy should usually be taken during the first hour of migraine symptoms.
–Dawn A. Marcus, MD. Multidisciplinary Headache Clinic, University of Pittsburgh. Pittsburgh, PA
From Headache, the Newsletter of ACHE, Fall 2004, vol. 15, no. 3.