Itching (pruritus) and its partner scratching are universal, common to all people and most other mammals. Cows, horses, dogs, cats, monkeys, elephants, and rhinoceroses can be seen scratching themselves, though it’s not certain they experience itching exactly as humans do. Itching can drive you crazy, interfering with sleep and life, but most cases eventually go away, whatever you do.
Still, it’s astonishing how little we actually know about itching. Here’s a scratch at the surface.
What’s behind an itch
Many things cause itching. Itching can originate in the skin—the result of dryness, insect bites, poison ivy, or the healing of a small wound, for instance. (Dry skin is the most common cause of itching, particularly that all-over itch or the itch that covers a wide area such as the back or legs.) Itching is a feature of not only skin diseases such as eczema and psoriasis but also other conditions such as kidney and liver failure. Then there is athlete’s foot, as well as “jock itch,” generally brought on by fungal infections. Itching can be caused or worsened by emotional upset. It can be a reaction to certain foods or medications. Morphine and other opioids can cause itching. Often people itch for no discernible reason.
Why itches itch
An itchy skin irritant, such as an insect bite, provokes a complex response in your body. Various molecules, among them histamines (the same ones that cause allergic reactions) and prostaglandins (which can cause pain and are involved in many other processes in the body), congregate at the site. Though it’s been debated over the years whether or not itching and pain share the same nerve fibers (itches sometimes seem to hurt, after all), it’s now thought that they’re two distinct sensations. Itch impulses travel to the brain over their own system (though within the same bundle as pain fibers) and quickly activate the urge to scratch.
The trouble with scratching
It’s thought that scratching activates nearby nerves that transmit pain impulses, and this overrides the itching. This is the counter-irritation theory—rather like drowning out an unpleasant noise with a louder one. But relief is usually short-lived, and if you keep scratching, you risk damaging the skin. What was a minor problem (a mosquito bite or a dry patch) can become a big one—raw skin and possibly an infection. It is very hard not to scratch, though. If you itch in your sleep, you may scratch in your sleep. Some people scratch while fully awake without being aware of it.
Can you “catch” an itch?
Itching can indeed be contagious, research suggests. For instance, a small study in the British Journal of Dermatology showed that scratching could be visually induced in people with atopic dermatitis (a common type of eczema), and to a lesser extent in people without the skin condition, when they watched videos of other people scratching, whether their forearms had been treated with histamine (to stimulate itching) or a saline control. Moreover, they scratched all over their bodies, not just their arms.
As with contagious yawning, the mechanism is not clear, but it may similarly involve “mirror neurons” in the brain, which fire both when an action is performed and when the same action is observed by another. “Contagious itch may represent another expression of humans’ ability to emulate others and it could also be seen as a form of empathy,” the authors concluded. Or it could just be that humans are highly susceptible to visual and mental suggestions.
An odd shoulder itch
Some people experience a constant itch by the shoulder blade (scapula). This area of the back can of course be itchy due to the same things that cause itchy skin anywhere, notably dry skin or conditions like eczema or psoriasis. But there’s also a curious condition called notalgia paresthetica, involving chronic itching below the scapula, typically on one side, most often the left. There may also be tingling, pain, or numbness in that spot, and the itchy patch of skin may be hyperpigmented. It’s thought to result from impingement of nerve roots that emerge from specific vertebrae, and it may indicate degenerative changes in the spine.
If the itch continues, see your doctor. Though it’s hard to cure, possible remedies include capsaicin cream (from hot chili peppers), transcutaneous electrical nerve stimulation (TENS), muscle relaxants and other drugs, and botulinum toxin (Botox) injections. Exercises that increase range of motion in the shoulder and strengthen back muscles may also help.
Tips for Treating Itchy Skin
These self-care steps can help relieve ordinary itches that stem from dry skin, insect bites, contact allergies, or fungal infections.
Itching for answers
If you don’t know what’s making you itch, you should suspect soaps, detergents, cosmetics, or an allergy to nickel (found in most costume jewelry). Some people develop sensitivities to certain fabrics. Try to eliminate whatever you think the culprit is—even if that may mean giving up your favorite wool sweater or blanket.
This article first appeared in the UC Berkeley Wellness Letter.
Also see Hives: Causes and Treatments.
Published February 06, 2020