Itching Affects Different Parts of the Brain

Jan. 3 (iVillage Total Health) -- The brain reacts differently to allergen- and histamine-induced itch, according to a recent study.

Pruritus is the medical term used to describe an itching or tingling sensation on the skin that provokes the desire to scratch or rub. Skin conditions such as hives or eczema, or a skin rash may cause itching. In other cases, an internal disease such as an allergic reaction may cause pruritus.

Researchers in Great Britain from Oxford University and Amersham Hospital studied brain reactions caused by the common allergy triggers allergens and histamine. .An allergen is any substance that the body mistakenly perceives as a threat, triggering a specific chain of events called an allergic cascade. Histamine is a chemical released into the bloodstream by white blood cells in response to an allergen.

During the study, two groups were examined, one with laser Doppler measurements and the other with functional magnetic resonance imaging (MRI). The researchers tested 28 women for allergies to grass pollen and dust mites. Of these women, 14 tested positive for grass pollen and/or dust mite allergies and 14 tested negative. Over three consecutive days the women who tested positive were given a skin prick to the forearm with their specific allergen, histamine or saline (a control). Non-allergic participants were challenged with histamine and saline over two consecutive days. The women rated itch intensity continuously on a scale of 0 to 10. Itch-related skin blood flow changes were also measured with laser Doppler.

A second group of 16 allergic and non-allergic men and women was also studied. Members of this group were asked to lie down in an MRI scanner. One minute after the participants reclined, their toes were stimulated. The subjects then rated the itch they felt on a rating scale of 0 to 5.

After reviewing the results, the researchers found a number of differences between allergen- and histamine-induced itches. Allergen-induced itches produced different perceptions, greater intensity and differences in blood flow. The sensations were delayed more and perceived to last longer.

The imaging technologies used indicated that the allergen-induced itches activated different parts of the brain than histamine-induced itches. The results showed greater allergen-itch response in certain areas of the brain, and brain activity similar to that produced by compulsive behaviors. These responses may demonstrate why some eczema patients scratch compulsively and may harm themselves.

The study appears in the January 2007 edition of the Journal of Neurophysiology .