People with a chronic itchy skin condition often experience itchy symptoms in response to heat, electricity or other normally painful stimulation, results of a new study suggest.

For people with chronic itch, the problem is more than skin deep, the study's lead author told Reuters Health.

"Chronic itch changes the processing of sensory information in the spinal cord such that normally non-itching stimuli become itching," according to Dr. Martin Schmelz of the University of Heidelberg in Germany. The altered processing "even changes painful sensations which normally suppress the itch into the sensation of itch."

This finding is of "crucial importance" to people with atopic dermatitis, according to Schmelz. Correcting this glitch in how the body processes sensations may lead to better treatment for atopic dermatitis, the most common form of the skin condition eczema.

In fact, it confirms the age-old advice that scratching a chronic itch only makes it worse.

"When they scratch their eczema to suppress the itch, that will intensify the itch instead, leading to the well known itch-scratch cycle," Schmelz said.

Atopic dermatitis causes red and itchy spots on the skin. The condition is most common in childhood, affecting 10 percent to 15 percent of children and adolescents in Western countries.

Itching is often caused by a compound called histamine that is released by the immune system during an allergic reaction, but antihistamine medications, which block histamine, are often ineffective in reducing itch caused by atopic dermatitis.

Based on some evidence that atopic dermatitis causes painful sensations to be perceived as itchy, Schmelz's team set out to compare the responses of people with atopic dermatitis with people with psoriasis - another chronic skin condition - and healthy people.

The study, results of which are reported in Tuesday's issue of the journal Neurology, included 25 people with atopic dermatitis, 9 with psoriasis and a "control" group of 20 people with healthy skin.

Participants were exposed to several types of normally painful stimulation, including pinpricks, mild electric shocks, heat and injection of an acidic solution.

As expected, the control group and people with psoriasis experienced pain when exposed to the stimuli, the researchers report. In contrast, people with atopic dermatitis experienced itching instead of pain.

Schmelz and his colleagues are now working on learning more about how sensory processing goes awry in people with atopic dermatitis.

The eventual goal, he said, will be to find drugs that normalize "the central misinterpretation" that causes people with atopic dermatitis to feel an itchy sensation in response to pain.

SOURCE: Neurology, January 27 2004.

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