Understanding Migraines

6 frequently asked questions about this infamous type of headache.

Although migraines are notorious for causing pain, they've created a lot of confusion, too. We hope the answers to the six questions below will clear up a few misunderstandings and provide some useful information along the way. If you have a question about migraines, please write us, and we'll answer as many as possible on our Web site at www.health.harvard.edu/healthextra.

1. What exactly is a migraine?

Some people use the term migraine to describe any severe headache, but not all severe headaches are migraines nor are all migraines severe, although many do live up to their reputation for excruciating pain.

The "classic" migraine is preceded by aura, which typically consists of strange visual disturbances — zigzagging lines, flashing lights, and, occasionally, temporary vision loss. Numbness and tingling affecting one side of the lips, tongue, face, and the hand on the same side may also occur. But only about a third of migraine sufferers experience aura, and fewer still with every attack.

The migraine headache, with or without aura, tends to produce pain that usually begins (and sometimes stays) on one side of the head. The word migraine comes from the Greek hemi-, for half, and kranion, for skull. A migraine headache often has a pulsating quality to it. Many people also experience nausea, extreme sensitivity to light or sound, or both. But, as with aura, one has to be careful about generalizing: some studies have found that about 40% of migraineurs have headache pain on both sides of the head, not just one, and children with migraines usually have pain on both sides.

It's also possible to confuse other sorts of headaches with migraines. Migraines can cause nasal congestion and a runny nose, so they're sometimes mistaken for sinus headaches. And the regular headache that most of us have experienced can have some of the features of a migrainous one, such as unilateral pain and nausea.

At this point, there are no blood tests for migraines. Migraines don't cause brain abnormalities that a CT scan or an MRI can detect, although these tests are sometimes ordered to diagnose other problems that cause severe headaches such as bleeding in the brain.

The epidemiology of migraine can be helpful in raising, or allaying, suspicions. It's well documented that women are three times more likely to have migraines than men, that the tendency to have migraines runs in families, and that they occur less often as people age. But, obviously, these are guideposts, not diagnostic criteria.

In short, arriving at a definition and diagnosis for migraine is complicated. Yet a simple headache diary — keeping track of headaches and factors that might have provoked them — can be very helpful in making a diagnosis.

Three theories about migraines

2. What causes a migraine?

For years, doctors believed that migraine headaches originated with the blood vessels, particularly those that supply the meninges, the thin membranes wrapped around the brain inside the skull (see illustration). It was thought that when those blood vessels widened — dilated is the medical term — they impinged on pain receptors on the lacy network of trigeminal nerves that service the meninges and other parts of the head. According to this vascular theory, aura was caused by low blood supply from the narrowing of those blood vessels before they rebounded and widened, causing pain. The vascular explanation had considerable intuitive appeal because of the pulsating quality of migraine headaches. Some doctors elaborated on the theory, assigning different sorts of pain to different blood vessels.

But now there is near-total agreement that migraines originate in the brain, not with the blood vessels that surround it. One prevailing theory is that migraines are caused by rapid waves of brain cell activity crossing the cortex, the thin outer layer of brain tissue, followed by periods of no activity. The unwieldy (and potentially confusing) name for this phenomenon is cortical spreading depression. A Brazilian researcher, Aristides Leão, first observed it in rat brains in 1944, but many studies since have confirmed it occurs in the human brain as well.