WASHINGTON • An itch is annoying, but it usually does not disrupt a person's life.
Unfortunately, that is not the case for everyone who itches.
About 15 per cent of the American population suffers from chronic itch, according to Dr Brian Kim, co-director of the Centre for the Study of Itch at the Washington University School of Medicine.
"It's a very big problem," Dr 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."
American dermatologist Thomas M. Keahey says itching is the chief complaint of about 20 per cent 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 skin disorders such 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 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?" Dr 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," Dr Kim says.
"Another thing people may not know is that in rare cases, cancer - particularly lymphomas and leukaemias - can present themselves with itch."
All-over itching caused by the 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", Dr Keahey says.
Also, about a third of patients suffering from end-stage kidney disease experience itching "due to a build-up of toxins, not well defined", he says.
Researchers are studying the itch-scratch cycle, trying to unravel the mysteries of what makes people itch, then scratch - and keep scratching. Dr Kim says scratching causes damage to the skin, which causes inflammation.
"This increased inflammation, like with many rashes, causes more itch in a feed-forward manner," he says. "Thus, it's a vicious circle."
Dr Kim and others believe the body's immune system is a player.
"We may think our immune responses end in our immune system," Dr Kim says. "But the itch-scratch cycle engages the immune system with the whole body, interacting with behaviour 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 may also 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 behaviour.
"Itch sensation plays a key role in detecting harmful substances, especially those that have attached to the skin," Dr Sun Yangang, one of the Chinese researchers, says.
"As itch leads to scratching behaviour, this allows the animal to get rid of the harmful substances."
If an itch lasts more than a month, it is 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, Dr 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.
Dr Kim says there are numerous therapies, but the best ones depend on the nature of the itch: "Dry skin is best helped with moisturisers, whereas if you have eczema, certain anti-inflammatory drugs have better anti-itch properties than others."