How many Danes have migraine?

Some of the world's most thorough headache studies were done in Copenhagen. The numbers are bigger than most people walk around believing.

A Danish population survey found that 25% of the women in the sample had, at some point in their lives, had migraine. One in four. Not headache in general — migraine, the specific diagnosis. Among men the figure was 8%. These are numbers that are easy to miss in everyday life, because migraine is an illness people keep to themselves.

Two measurements, twelve years apart

Some of the most cited figures in the field come from two studies of 25 to 36-year-olds in Copenhagen. In 1989, 11.3% of participants had migraine. In 2001, the figure was 15.5%. That looks like a rise of four percentage points over twelve years, and it's tempting to read it as a clear sign that more and more people are getting migraine.

That isn't what the researchers themselves concluded. The samples were under 300 participants each, and with that uncertainty the rise wasn't statistically significant. The prevalence didn't change significantly within the 11-15% range between the two years. That's an important distinction: the figures went up, but you can't say migraine became more widespread.

New cases, every single year

It's worth dwelling on just how thoroughly this field has actually been studied in Denmark. There isn't just one figure, but several independent surveys — lifetime prevalence, point prevalence at two points in time, and incidence — each measuring something slightly different, and yet agreeing with each other in the bigger picture.

Beyond how many have ever had migraine, there are figures for how many get it for the first time. A Danish follow-up study found an incidence of 8.1 new cases per 1,000 person-years. The ratio between men and women was 1:6 — for every man who got migraine for the first time, there were six women who did the same.

Put those two figures together, and the picture becomes clear without needing to calculate anything the studies didn't themselves measure: migraine is neither rare nor evenly distributed. It affects a broad range of people, and it affects women markedly more often than men. Eight new cases per 1,000 person-years doesn't sound like much on its own — but it's an ongoing intake, year after year, on top of the many who already have the diagnosis.

It's worth holding onto what the studies behind these figures actually asked. The two Copenhagen studies cover a single, narrow age group — 25 to 36 years. The lifetime prevalence figures of 8% and 25% come from a broader Danish population sample. It isn't the same figure measured twice; it's different slices of the population, asked by different studies at different times. That they point in the same direction — widespread, and markedly skewed between the sexes — is interesting in itself.

The most common complaint of all

That picture recurs elsewhere. In a German population survey of weather sensitivity, headache and migraine were by far the most common complaint among those who experienced themselves as weather-sensitive — mentioned by 61%, far ahead of everything else on the list. Wherever you ask, headache and migraine come out on top.

None of these figures say how many Danes react to air pressure. That's a completely different question, and it isn't answered by a population survey — it can only be answered in the individual, over time. A population survey can tell you how many have the diagnosis. It can't tell you what triggers a particular attack in a particular person, and that isn't what it's designed to answer at all.