It happens to all of us - beginning, perhaps, as a little tickle, hardly noticeable.
Maybe you're in an important meeting and you don't want to fidget. Or maybe your hands are full. So you try to ignore it, but the sensation grows - an irritating feeling that gradually occupies more and more of your attention. Finally, you can't take it any longer.
You have to scratch the itch.
Itching is as fundamental a sensation as pain and hunger, one we share with other creatures: "Every two-legged and four-legged animal itches and scratches," says Dr. Gil Yosipovitch, a dermatologist at Wake Forest University Baptist Medical Center in Winston-Salem, N.C. Yet for such a seemingly simple sensation, it's also surprisingly complicated.
Scientists have long wondered why pain - for example, from scratching - relieves an itch. They've searched for the nerves that send the itchy signal to the brain, and they've wondered what switches those nerves on and off.
And doctors and patients alike have wondered why the sensation can be so hard to expunge in those unfortunates who experience the extreme end of itching - an itch that just won't go away.
Today, a small group of dermatologists and neuroscientists are starting to look at the biological mechanisms that lie behind itching. As they do so, they are finding curious overlaps between itching and that different-seeming sensation, pain. Though sometimes pain is itching's opposite, the latest findings are showing more and more similarities between the two.
And as they begin to understand the sensation's biology, dermatologists - including ones at the world's only clinic dedicated entirely to the treatment of itch - are developing new therapies for people who suffer the torment of chronically itchy hides.
For most people, itching is only a mild annoyance, relieved by a quick scratch or maybe some skin cream. For others, the itch stays, stays - and stays.
"It starts like any other itch, like you've been bitten or something," said David Hayes, a Los Angeles computer technician who has psoriasis, a noncontagious disease that causes skin inflammation, probably from an overactive immune system. "But then it keeps on going. You've got to scratch it, and you've got to keep scratching until you're almost bleeding before it stops."
For people such as Hayes with psoriasis, or for others with the skin allergy eczema, the sensation can be unbearable. "Itching is the worst thing," says Susan Lipworth of Bloomfield Hills, Mich., who has had eczema for 13 years. "It never stops - it never stops - it wears you down."
Lipworth, who is also a board member of the National Eczema Association, a patient advocacy group, says that scratching becomes so automatic, she even does it while sleeping. "I'm waking my husband up with my scratching," she says.
An estimated 4.5 million adult Americans have psoriasis and 9 million have eczema, according to the American Academy of Dermatology, most tolerating long-term itching that doesn't go away with scratching.
Burn patients, and people with certain kinds of nerve damage, also often have severe itching - as can people with liver and kidney diseases and some who are infected with HIV, because of the infection itself and the medicines they must take.
Such itches can erode a person's mental health, experts say.
Many studies have found that people with severe itching from psoriasis, eczema and kidney dialysis are more likely to be depressed than others. In a 1998 study published in the British Journal of Dermatology, for example, researchers from the University of Western Ontario in Canada reported that almost 10 percent of 217 patients with psoriasis had had suicidal thoughts.
For many people, the itchiness also prevents them from sleeping. In a study in 2002, Yosipovitch found that 84 percent of 102 eczema patients had trouble falling asleep because of itching.
Lack of sleep is a particular problem in children with eczema, which affects more children than adults. Rebecca Litke, a professor at California State University, Northridge, has a 12-year-old daughter with eczema. "My daughter, for the first eight years, rarely slept through the night," she says. That meant that Litke rarely did, either. "It has been awful," she said. "For years you're functioning on three or four hours a night."
Lack of sleep from itching can affect people's physical health, Yosipovitch says. "It affects even mortality," he says. Last year, a group of scientists lead by Ronald L. Pisoni at the Arbor Research Collaborative for Health in Ann Arbor, Mich., completed a study of almost 19,000 people over eight years who had itching because of kidney dialysis. They found that moderate to severe itching resulted in a 17 percent higher chance of dying, mostly linked to lack of sleep.
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.
Seesaw of itch, scratch
Studying this interplay between itch and scratch has led 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.
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.
Therapies on 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 because of 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."
Eric D. Tytell wrote this article for the Los Angeles Times.