What actually makes our skin itch?
Tuesday 24 October 2006
Something is stirring in the world of itching. New treatments and theories are emerging, and the International Workshop for the Study of Itch has held its biggest conference yet, with more than 200 researchers revealing their findings.
Itching, or pruritus, was defined by the German physician Samuel Hafenreffer more than 300 years ago as "an unpleasant sensation that elicits the desire or reflex to scratch".
That definition still holds, but it is recognised that there are acute forms of itching. It is the most common symptom associated with many diseases, but it can also be a symptom of malignancy, infection, and metabolic disorders, and there are many causes, from insect bites, stings, sunburn, allergies and psoriasis, to stress, chicken pox, kidney disease, jaundice, pregnancy, anaemia and fear.
Research by the University of Oslo shows that 7.5 per cent of men and 9.2 per cent of women have a chronic itch problem. One of the latest theories is that itching is not a mild form of pain but has its own superhighway of nerve fibres and chemicals.
This may also help us to understand why stressed or depressed people are more prone to itching, why we scratch our heads when puzzled, why people with more friends and larger social networks suffer less, and why it affects patients with liver and kidney disease. It may also explain why a tickle differs from an itch, and why pain or scratching can take it away.
"Despite around a century of research, there is no generally accepted therapy for the treatment of itching, and many mysteries, misconceptions and controversies still haunt this neglected area,'' says Dr Martin Schmelz, a researcher at the University of Heidelberg.
So why do we itch at all? What is the evolutionary reason for it?
The latest theory is that scratching is a defence system against insects, parasites, infections and allergens, and that it protects the integrity of the skin and other organs. The argument is that having a skin capable of inducing the symptom of itch may have given humans am evolutionary advantage. Studies also show that the itching symptom is the result of a complex interaction between the skin, the nervous system, the endocrine system and the immune system.
There is also a view that it is the brain that itches, projecting the sensation on to the skin. "Pruritus causes the desire to scratch the skin and is experienced as a sensation arising in the skin," says Dr Schmelz. "Like all other skin sensations, itching, strictly speaking, is a product of central nervous system activities.When one senses the difficult-to-control desire to fight itching by self-inflicted painful stimuli, the impression that one's skin itches is nothing but a sensory illusion created by the brain."
One accepted idea has been that pain and itching share a pathway, and that itching is a manifestation of pain. The sensation of itching can, for example, be reduced by the pain of scratching, and painkillers can cause itching.
But the discovery of nerve fibres and cells devoted to itching has led to the theory that pruritus has its own operating system. This has implications for itch sufferers, as it means that treatments can be designed that target the site of the itch and the area of the nervous system where itching is processed. German researchers found that nerve cells involved in itching respond to the hot ingredient in peppers.
Eventually, it may even be possible to stop itch signals in much the same way that pain signals are blocked. Instead of treating the site of the itching, which in many cases does not work, it may be possible to block itch sensations before they reach the skin.
Researchers now believe they can even treatchronic itching ailments such as tinea cruris, known as jock itch. This is a fungal infection of the groin and inner thigh that affects one in five people. Four times as many men as women suffer from it, and it can be a side effect of wearing tight clothes and living in hot climates.
So if researchers can indeed come up with a drug that will cure itching, it would be welcomed as a real breakthrough, not least by the 10 million jock itchers.
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