A possible explanation can be seen in the hypothesis put forward by Paus et al., The author has described four cases in which unrelated dermatoses (viral exanthem, acute generalized exanthematous pustulosis, polymorphous light eruption, and irritant contact dermatitis) spared sites affected with tinea versicolor.
The authors speculate that the presence of Malassezia spp.
The reason for sparing could be destruction of the dermal papillae within the injured tissue that prevent the epidermal hyperplasia of psoriasis.
There are also case reports of this phenomenon occurring in leucocytoclastic vasculitis where there was spontaneous repigmentation of the vitiligo patches distant from those treated by autologous punch grafts.
It is believed that the development of skin toxicity, which is a major adverse effect of EGFR inhibitors, is due to the inhibition of homodimeric EGFR on normal skin cell surface.
The authors propose that this sparing may be either due to decrease in the population of EGFR expressing cells or decrease in the EGFR expression in the irradiated area.
This is in contrast with dermatomyositis, where the violaceous erythematous papules typically involves the knuckles.The author also attempts to untangle the confusion that has risen in connection with these signs.White or graying hair follicles are commonly spared in AA, and regrowing hair shafts are usually white before they become repigmented.The sign manifests because of the unique combination of the anatomy of the hand and the sweeping movement the hand makes during application.
There are reports of cancer patients who on treatment with recombinant monoclonal antibody against human EGFR, such as cetuximab and erlotinib, develop skin rashes that spare the previously irradiated areas.
The classic description of a primary irritant contact dermatitis is that of a confluent erythema of the convex surfaces in close contact with the diaper, i.e., the buttocks, the genitalia, the lower abdomen, the pubic area, and the upper thighs, with sparing of the deeper parts of the groin flexures.