We generally assume that anyone who itches probably has dry skin, an insect bite, an allergy or a skin disorder such as eczema. But itching is often far more complex than that and can also result from surprising causes, such as regular use of painkilling medication… kidney or liver disease… or even cancer.
To learn more about when an itch is more than just an itch, Bottom Line/Health spoke with Zhou-Feng Chen, PhD, a leading medical authority on itching (also known as pruritus).
Why do humans itch? No one knows for sure, but it’s probably a defense mechanism that alerts us to potential threats.
Suppose that an insect lands on your arm. It will depress the skin surface, move the tiny hairs or even slightly damage the outer layer of skin. Nerve receptors in the skin detect these changes. The receptors then send signals to the brain that say, in essence, “Get this thing off me.” We feel these signals as an itch. Our natural impulse is to rub or scratch the area. This relieves the itch — and provides a “reward” for eliminating the irritant.
Itches also can originate in deeper levels of skin. Anything that disrupts the body’s normal environment can trigger the release of inflammatory substances that cause nerve irritation and itch.
What are the main causes of itching? The most common causes of mild, temporary itching are, of course, environmental irritants, such as bug bites, poison ivy or chemicals in household products.
Some people feel itchy when they wear latex gloves. This so-called latex allergy is quite common. Scabs also can itch. And we all know that skin diseases, such as eczema and psoriasis, cause itching as do lice and scabies, which are transmitted by close contact with a person who is infected with lice or the scabies mite.
Few people realize, however, that melanoma, the deadliest form of skin cancer, causes severe itching in most patients (initially in the area around the malignancy). If you discover a skin growth or discoloration that won’t stop itching, see a doctor right away.
Certain types of lymphoma and — to a lesser extent — malignancies of the stomach, lung, breast, brain and pancreas may also cause itching on any part of the body.
In addition, people can experience chronic itch from iron-deficiency anemia and celiac disease (malabsorption of gluten). Anything that affects the nervous system, such as diabetes or multiple sclerosis, can cause itching. So can opioid painkillers such as morphine.
Haven’t scientists traditionally thought that itching is a mild form of pain? That has been a common belief, but new research is now suggesting a different cause.
For example, researchers at the Washington University School of Medicine conducted an experiment in which normal laboratory mice were exposed to an itch-causing substance. As you would expect, the animals itched and scratched vigorously.
Then we used the same substance on mice whose itch-sensing neurons had been killed by a toxin in the spinal cord. The mice didn’t itch or scratch at all.
However, the same mice still felt pain from other causes. What this told the researchers is that the sensations of itch and pain involve entirely different nerve cells and nerve pathways.
Why is this finding important? Now that we’ve identified “itch cells,” it may be possible to develop medications or other treatments for severe and/or chronic itching (lasting at least six weeks) — a condition that, according to one scientific paper, could affect up to 15% of adults at some time in their lives.
For most of us, an itch is just an annoyance. But for those with chronic itch, the discomfort can be almost unbearable. Some people who have been plagued by itching have even committed suicide. New and better treatments for itch could make a big difference for these patients. Also, the fact that itch and pain don’t share the same pathways could change current treatments.
How does scratching help? Actually, scratching may hurt more than it helps. It’s enjoyable for temporary relief. We don’t know why it feels so good. It may activate the reward parts of the brain and trigger the release of the “feel-good” neurotransmitter dopamine, but this is just speculation.
People who scratch all the time can damage the skin, sometimes severely. This does not stop the itching. Rather, it increases the severity.
What’s the best way to treat itch? Over-the-counter topical treatments, such as hydrocortisone cream and calamine lotion, are mildly helpful for skin-related itching. No one product is superior for every case — people must experiment to find out what works for them.
Those with itch that is caused by histamine may get relief by taking diphenhydramine (Benadryl). It blocks the effects of histamine, a chemical produced in response to allergies and some types of skin damage. Benadryl causes drowsiness, so it’s best to take it at bedtime. The nonsedating antihistamines, such as loratadine (Claritin), cause less drowsiness but aren’t as effective for itch.
Also helpful: Combine an antihistamine with aspirin or ibuprofen (Advil) — nonsteroidal anti-inflammatory drugs reduce the inflammation that contributes to itch. Use all these drugs according to label instructions.
If you prefer natural approaches, you can try cool, wet compresses… a cool bath with baking soda or Aveeno Oatmeal Bath Treatment… or menthol or camphor ointment. Ultraviolet light therapy is useful for psoriasis and other skin conditions — it may help stimulate vitamin D synthesis.
Is it true that itching, like yawning, is contagious? Yes. We know that itching has psychological and emotional components — that’s why the more you think about an itch, the more it itches.
In an interesting study recently published in British Journal of Dermatology, volunteers with eczema who watched videos of people scratching scratched more frequently themselves. They also reported a greater itch intensity than when they watched videos that showed people merely relaxing.
It’s not known what parts of the brain are involved in so-called visually induced itching. In the future, it may be possible to use MRI scans or other imaging techniques to better understand the underlying mechanisms and develop better treatments for all types of itch.
