Got an itch?: Experts baffled by the urge to scratch

Everyone gets the urge to scratch, but experts struggle to explain why. Now, however, research is pointing the finger at gene mutation

(ALAMY)

We all do it, but why? What is it that makes almost everyone itch at some time, and what is the purpose of itching and scratching? Until recently, it was thought that itching was a mild form of pain. But now, researchers believe that the sensation has its own nerve and brain networks, although much about it still remains a mystery.

"There is no generally accepted therapy for the treatment of itching, and many mysteries, misconceptions, and controversies still surround what is a neglected yet fascinating area," says Dr Ralf Paus of the University of Lübeck. Research suggests that at any point in time, around seven per cent of the population has a chronic itch. More women than men itch, and children are the biggest scratchers.

One of the many puzzles about itching is that it can be triggered by almost anything, from chickenpox to cancer. It can be due to dry skin, head lice, or insect bites, but it can also be a sign of diseases, including diabetes and anaemia, and it can be triggered by allergies and sunburn. It's more common during pregnancy, and during bouts of stress, anxiety and depression, and, just like yawning, is "catching".

Since itching was first defined 340 years ago by Samuel Hafenreffer – "an unpleasant sensation that elicits the desire to scratch" – there have been more theories about itching than causes. It has been suggested that it has evolved as a body-awareness system, a way of constantly checking on key parts of the anatomy. It has been proposed that it is a low-level protection system, a first line of defence, and that it acts as a self-protecting mechanism like other sensations, including touch, to help defend the skin.

Until recently, itching had been thought to follow the same internal communication system as pain. Pain, after all, can stop an itch, which is why we scratch. But our reactions to itching and pain are different – we withdraw from a painful area but touch an itchy one.

While there is some overlap between the two, researchers have now shown the existence of dedicated histamine-sensitive itch nerve fibres, distinct from pain pathways. One reason they had not been found before, say the researchers, is that signals travel along them very slowly, around half a metre a second. Brain scanning has also shown that different areas of the brain are active in pain and itching.

In the latest work, a team of British researchers have found new genetic mutations that can cause the skin to itch, and that give a greater insight into how many chronic itches might be better treated. The King's College team and collaborators focused on an itchy skin disorder, familial primary cutaneous amyloidosis, and showed that a gene mutation was responsible for the condition. They discovered that skin cells with a mutant copy of the gene react differently from molecules that trigger an anti-inflammatory response. The mutant skin cells failed to activate a number of anti-inflammatory genes, and the result was itchy skin.

Professor McGrath, who is president of the European Society for Dermatological Research, said: "This work provides new insight into what can cause itchy skin. We plan to look for abnormalities of this signalling pathway in other itchy skin disorders, and to examine how we can develop new treatments for itching."

Other researchers at Washington University have reported finding an itch gene, known as GRPR, in spinal nerve cells. They say that it may be responsible for relaying itch signals from skin to brain, and that may hold the key to new treatment for itching.

What it is?

Chronic itching comes in three main forms. There is itching that originates in the skin, like dry skin and eczema; itching that is due to nerve-fibre damage as a result of diseases such as kidney failure; and itching based on psychiatric conditions such as depression.

What happens?

Itching may seem basic, but that sensation we feel is a complex interplay involving the skin, nervous system, endocrine system, and immune system. Most itching starts with sensory nerves and itch receptors on the skin. Sensory nerves send messages to the brain allowing changes in heat and other sensations to be monitored. Histamine is released by inflammatory cells near the sensory nerves, affecting the message that goes to the brain. The signal travels along the itch nerve to the spinal cord and to the brain.

"In the brain, the signal is processed by the thalamus and sent to the cerebral cortex, the part of the brain responsible for conscious thought. At this point, a person becomes aware of the 'itchy' feeling. Scratching is a reflex response thought to relieve the itch by simultaneously stimulating pain nerves and interfering with the transmission of the 'itch' signal to the brain,'' says Dr Yvonne Johnston, clinical professor at Binghamton University, New York.

Who itches?

Estimates on the scale of the problem vary. The biggest study, carried out at the University of Oslo, which focused on quality of life and skin complaints in nearly 20,000 people, found the prevalence of itching to be 7.5 per cent among men, 9.2 per cent among women. Those who had experienced negative events and depression had the biggest itching problems.

The causes

Eczema, psoriasis and rosacea; chickenpox and scarlet fever; threadworms and haemorrhoids; fungal infection; candidasis; pregnancy; insect bites and stings; sunburn and dry skin; prickly heat.

Disease

General itching can be a symptom of many conditions, including diabetes (Types 1 and 2), thyroid disease, liver cancer, hepatitis, and anaemia. It is also a symptom of Hodgkin's lymphoma, and it is thought that blood cells release chemicals near the nerves of the skin that then irritate them.

Treatments

*Try pinching the skin near the itch between your thumb and forefinger through your clothing; this is less damaging than actual scratching.

*Have lukewarm showers or baths, and add a little baking soda. Soak the scalp or feet in warm water with baking soda if they are itchy.

*Soaking in a bath in which two cups of rolled oats secured in a sock have been placed can relieve itching.

*Use a cold compress or apply calamine lotion.

*Avoid wearing any irritating clothes and wear cotton if possible.

*Keep away from hot, humid environments.

*Oral antihistamines can help control allergic reactions and itching, and can break the itch-scratch cycle.

*Over-the-counter hydrocortisone cream can help on localised itchy areas.

*Rub-on corticosteroids can be prescribed by your general practitioner.

Source: NHS