That's why scratching relieves itch, they thought -- the same nerves couldn't transmit both itch and pain at the same time.
But in 1997, Schmelz discovered distinct nerve cells in the skin that respond to itchy stimuli, and itch researchers revised their theories. They now believe that the sensations of itch and pain travel along separate pathways.
In cases of chronic itching, though, things go wrong. Some of the nerves that would normally transmit pain start to send itch signals instead. A 2004 study led by Schmelz, for example, found that people with eczema perceived pinpricks and electric shocks as itchy, not painful.
"The nerves are acting wacky," Yosipovitch says.
In such cases, scratching doesn't make the itch go away -- in fact, over the long run it can make people even more sensitive to itchy sensations. Repeated damage to the skin makes it grow thicker and sprout even more itch nerves.
Yet it's hard to avoid scratching in response to an itch -- even when you know you're doing yourself no favors. "I consider it almost an addiction," said Diana Cordio, a human resources assistant in San Mateo, who has psoriasis. "Scratching the itch -- you know that it's not helping you in any way. But there were some times I would have to break down, and get [finger]nails to skin, or whatever was handy, like a hairbrush."
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Therapies on the horizon
It has been only about a decade since dermatologists started studying itch specifically. As they've come to understand some of its biological mechanisms, this has opened the door on new therapies.
Opioids have turned out to play an important role. Although some of them, such as morphine, are itch-promoting, scientists now know that others are itch-reducing. One now-common treatment for people who itch because of liver disease is to prevent the body's own itch-promoting opioids from working, using drugs such as naloxone or naltrexone (also used to treat morphine overdose and alcoholism). Patients experience less itch -- the downside is they also experience more pain. They often find this preferable.
"I tell you, if I scratch till it hurts, it's actually relief when it hurts," says Rita Kirshner, a retiree in Chicago who has itched due to a liver condition for more than eight years. She started taking naltrexone this year and says it has helped her somewhat. "Right now I feel fine, so it's hard to even imagine what [the itch] feels like."
