Some observations on the direction of scratch marks.

This study consists of two parts. In the first we recorded 1696 scratch marks seen on 69 pruritic patients. The scratch marks followed a consistent pattern on the skin, which was independent of the cause of the itching. The most striking feature was a longitudinal alignment on the limbs. Next we tried to relate our findings to experimental work that suggests that itching is extinguished most effectively by counterstimuli applied within the same dermatomal segment. If this is the case, sufferers from itchy skin diseases might be expected to scratch itchy points on their skin using strokes directed along dermatomal lines, thereby gaining maximal relief. We used the data obtained during the first part of our study to test this possibility. Fifty-one percent of the scratch marks analyzed ran at an angle of less than 20 degrees to the nearest interdermatomal line (significantly higher than the 22.5% expected by chance). However the tendency for scratch marks to run along dermatomal lines was confined to the limbs. In addition, the preferred scratch directions of a group of nonitchy subjects, not driven by the need to alleviate actual itching, coincided closely with the pattern of scratch marks seen on the itchy patients. The direction of scratch marks may therefore be determined as much by mechanical factors affecting the ease of scratching as by the distribution of dermatomes.