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I have an occasional “phantom” itch in the middle of my back in a place I can’t reach. I use a long-handled comb to give it a good scratch. There’s no obvious cause — no rash, no irritation or redness, no diagnosed skin disorder. It’s annoying, but it doesn’t disrupt my life.

Unfortunately, that’s not the case for everyone who itches. About 15 percent of the population suffers from chronic itch, according to Brian Kim, co-director of the Center for the Study of Itch at the Washington University School of Medicine. “It’s a very big problem,” Kim says. “Studies have shown that its impact on quality of life is equivalent to chronic pain. Many of my patients who have had both prefer pain over itch. Itch tends to be more maddening.”

Rockville, Md., dermatologist Thomas M. Keahey says itching is the chief complaint of about 20 percent of his new patients. Also, his older patients frequently raise the issue during their annual skin cancer screenings. Most of the time their problems are minor, but “sometimes, it’s a serious request for help,” he says.

There are hundreds of reasons people itch. These range from dry skin and such skin disorders as psoriasis, to “contact” dermatitis from rough clothing, pet dander, soaps, laundry detergents and perfume — collectively known as eczema — as well as more painfully familiar conditions such as bug bites or poison ivy.

Some people will break into hives after exposure to some external stimulus, such as cold air or the sun. “Can you fathom breaking out with itchy hives by walking outdoors into the cold or sunlight, or following a ‘healthy’ workout?” Keahey says.

There also are unexpected causes, some of them serious. These include diabetes, kidney disease and some cancers.

“One thing that may surprise people is that having a bad neck or back can cause itching due to damage to the nerves that come from your spinal cord,” Kim says. “Another thing people may not know is that in rare cases, cancer — particularly lymphomas and leukemias — can present with itch.”

All-over itching caused by blockage of the bile ducts can be a sign of pancreatic cancer, for example. In cutaneous T-cell lymphoma, a type of non-Hodgkin lymphoma, the cancer begins in the white blood cells and attacks the skin, causing a chronic, itchy rash “often confused with benign forms of eczema,” Keahey says. Also, about a third of patients suffering from end-stage kidney disease experience itching “due to a buildup of toxins, not well defined,” Keahey says.

Researchers are studying the itch-scratch cycle, trying to unravel the mysteries of what makes people itch, then scratch — and keep scratching. Scratching causes damage to the skin, which causes inflammation, Kim says. “This increased inflammation, like with many rashes, causes more itch in a feed-forward manner,” he says. “Thus it’s a vicious ‘itch-scratch’ cycle.”

Kim and others believe the body’s immune system is a player. “We may think our immune responses end in our immune system,” Kim says. “But the itch-scratch cycle engages the immune system with the whole body, interacting with behavior and the environment as well.”

Recent research in mice suggests there is a link between itching and food allergies, which also are an immune response. In the animals, scratching the skin prompted an increase in the number of activated mast cells — immune cells involved in allergic reactions — in the small intestine, indicating a possible relationship between food allergies and atopic dermatitis, a type of eczema, according to a study by scientists at Boston Children’s Hospital.

The brain also may be involved. In another mouse study, researchers at the Chinese Academy of Sciences showed that tinkering with a small subset of neurons in a brain region that processes sensory information, including pain, could prompt or halt scratching in mice, suggesting that these neurons are connected to the itch-scratch cycle.

Experts believe the cycle evolved over time among animals as a protective behavior.

“Itch sensation plays a key role in detecting harmful substances, especially those that have attached to the skin,” one of the Chinese researchers Yan-Gang Sun says. “As itch leads to scratching behavior, this allows the animal to get rid of the harmful substances.”

If an itch lasts more than a month, it’s probably time to see a doctor. Most people are reluctant to do so for a minor itch, and resort to over-the-counter remedies, which are too weak to have an effect, Keahey says.

“When the itch begins to affect quality of life — such as sleep — or is associated with a disfiguring rash, people will start to make their way into the dermatologist’s office,” he says.

Kim says there are numerous therapies, but the best ones depend on the nature of the itch: “Dry skin is best helped with moisturizers, whereas if you have eczema, certain anti-inflammatory drugs have better anti-itch properties than others.”

As for my “phantom” itch, both Keahey (who is my dermatologist) and Kim believe I probably have a fairly common ailment called notalgia paresthetica, which shows up as an itch but really involves the nerves.

“We think the nerves that relay sensation from your back become damaged or dysfunctional, causing you to itch,” Kim says. “You’re right, it is a bit of a phantom itch because there’s no primary stimulation in the skin. Rather, the nerve itself is misfiring. It’s precisely what the classic ‘back scratcher’ was invented for.”