Two-point discrimination of itch in patients with atopic dermatitis and healthy subjects

The ability to perceive two pruritic stimuli as separate depending on the distance between them (two-point discrimination of itch) was determined in a single-blind study of 20 patients with atopic dermatitis and 20 healthy subjects. Itch was induced with pairs of histamine injections (0.1 micrograms each) given on the upper arm, either along its axis (longitudinally within a dermatome) or at right angles to it (transversally involving more than one dermatome). The interinjection distances were varied in 3-cm steps until the shortest distance at which the two itching stimuli could be perceived as separate was found. Pairs consisting of one injection of histamine (0.1 micrograms) and one of saline served as controls. The two-point discrimination of itch was significantly better in the atopic dermatitis patients than in the healthy controls, both longitudinally (atopic dermatitis: median 12 cm, range 3-21 cm; healthy controls: 15 cm, range 9-21; p < 0.01) and transversally (atopic dermatitis: median 6 cm, range 3-18; healthy controls: 12 cm, range 6-21; p < 0.001). The two-point discrimination of itch did not correlate with the subject's age, two-point discrimination of touch, or, for the atopic dermatitis patients, with their eczema scores or serum IgE levels. The quality, intensity or spatial summation of itch did not differ significantly between the two groups of subjects.