Is the skin an excitable medium? Pattern formation in erythema gyratum repens

Erythema gyratum repens (EGR) is a rare, inflammatory dermatosis of unknown aetiology. The morphology of the eruption is striking and displays rapidly evolving circinate and gyrate bands of erythematous and scaly skin. Although the aetiology of the pattern is unknown, it has previously been noted that the eruption shares morphologic features with the patterns of spatio-temporal chemical concentration profiles observed in the Belusov-Zhabotinski (BZ) reaction. Yet this morphologic correspondence has not been investigated further. Here we apply a simple non-linear reaction–diffusion model, previously used to describe the BZ reaction, as a template for pattern formation in EGR, and show how the mechanism may provide a biochemical basis for many of the dynamic and morphologic features of the rash. These results are supported by the results of a cellular automaton simulation approximating the dynamics of oscillatory chemical systems—the Hodgepodge machine—where the spatio-temporal patterns developed show astonishing similarities to the morphology of EGR.