Itch and motivation to scratch: an investigation of the central and peripheral correlates of allergen- and histamine-induced itch in humans.

Intense itch and urge to scratch are the major symptoms of many chronic skin ailments, which are increasingly common. Vicious itch-scratch cycles are readily established and may diminish quality of life for those afflicted. We investigated peripheral and central processing of two types of itch sensation elicited by skin-prick tests of histamine and allergen solutions. Itch-related skin blood flow changes were measured by laser Doppler in 14 subjects responsive to type I allergens and 14 nonatopic subjects. In addition, this study examined central processing of both types of itch using functional magnetic resonance imaging (fMRI). Itch perception and blood flow changes were significantly greater when itch was induced by allergens compared with histamine. Both types of itch correlated significantly with activity in the genual anterior cingulate, striatum, and thalamus. Moreover, itch elicited by allergens activated orbitofrontal, supplementary motor, and posterior parietal areas. Histamine-induced itch also significantly correlated with activation in the insula bilaterally. The identification of limbic and ventral prefrontal activation in two types of itch processing likely reflects the subjects' desire to relieve the itch sensation by scratching, and these regions have been repeatedly associated with motivation processing. A dysfunction of the striato-thalamo-orbitofrontal circuit is believed to underlie the failure to regulate motivational drive in disorders associated with strong urges, e.g., addiction and obsessive compulsive disorder. The patterns of itch-induced activation reported here may help explain why chronic itch sufferers frequently self-harm through uncontrollable itch-scratch cycles.