Dermatographia

How it works

Everyone’s skin reacts a little to a firm stroke. A pale line appears first, then a red flush around it, then a slight swelling along the line. This is called the triple response of Lewis, and it is normal³. In dermatographia the response is far stronger. The line becomes a raised, pale welt with a red border, like a hive, and it can itch¹.

The welt comes from mast cells. These sit in the skin and hold sacks of histamine. Pressure or friction makes them release it. Histamine makes small blood vessels widen and leak fluid into the skin, which causes the redness and the swelling². In dermatographia the mast cells release histamine much more easily than usual¹.

The welts show up within about 5 minutes and usually fade within 15 to 30 minutes¹. Tight clothes, scratching, rubbing a towel over the skin, and even clapping can bring them up. It is the most common form of physical urticaria, meaning hives caused by a physical trigger rather than an allergen¹.

About 2% to 5% of people have it¹. In most cases no cause is found. It sometimes begins after an infection, a course of medicine, or a stressful period, and it often fades on its own after months or years¹. It is not contagious and it does not scar.

Try it

1. Choose a patch of skin on the inside of your forearm.
2. Using the cap of a pen or a blunt fingernail, draw one firm line about 10 cm long. Press hard enough to feel it but not enough to hurt.
3. Wait 3 to 5 minutes.
4. Look at the line. A faint pink mark that fades quickly is the normal response.
5. A raised, pale welt with a red edge that lasts 15 minutes or more is dermatographia¹.

If you have it, you can write a word on your arm and watch it appear.