If an itch is, at best, slightly annoying, and, at worst, causes a 17% higher chance of dying, it may make one wonder why our species evolved the bothersome sensation at all.
Itching exists for a reason, says Dr. Jeffrey Bernhard, a dermatologist at the University of Massachusetts Medical School in Worcester, Mass.: It helps one detect when an enemy is mounting an assault. The body's first line of defense is the skin, and itch is the alarm that tells us when something (for example, a tick or other insect) has penetrated that first line.
Itching, in turn, triggers a desire to scratch, so the tick or other unwelcome guest can be pulled out.
Of course, if a tick has burrowed deeply, one may have to gouge out a big chunk of skin to fully remove the parasite. Under normal circumstances, such gouging would be painful.
Conveniently, with an itch, the brain rewires itself so that the normally painful sensation of rough scratching feels pleasurable. We still feel the pain, says Dr. Martin Schmelz, a professor in the Clinic for Anesthesiology and Intensive Care at the University of Heidelberg in Germany. But somehow, when it's accompanied by itch, our brains don't interpret it the same way -- they let us perceive the damage to our skin as pleasurable.
The cycle of itching and scratching is generally useful, Schmelz says. The problem arises when there are no insects. Or when the itch isn't satisfied by scratching but is made even worse -- the case, unfortunately, for most people suffering from chronic itch.
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Seesaw of itch, scratch
Studying this interplay between itch and scratch has lead to some strange findings.
For healthy people, the relationship between pain and itch is like a well-balanced seesaw. Increasing pain (for example, by scratching) decreases itch. Decreasing pain (for example, by taking the painkiller morphine) increases itch.
But in chronically itchy people, the seesaw is off balance. In the best-understood cases -- some liver diseases such as cirrhosis -- the liver begins to produce chemicals called opioids, natural substances similar to morphine. Like those who take morphine, the sufferer begins to itch.
Scientists used to have a simple explanation for the seesaw relationship between itch and pain. Itch, they thought, was just a mild form of pain -- when pain nerves were slightly stimulated, it felt itchy, but when they were strongly stimulated, it felt painful.
