The remarkable paradigm of segmental (mosaic) drug eruptions.

Sir,

Vetrichevvel et al. recently described a case of linearly arranged fixed drug eruption (FDE) co-existing with non-segmental lesions.[1] I wish to highlight that such linear pattern is not zosteriform/dermatomal as claimed by the Authors, but rather Blashko-linear as clearly evident from its S-shaped fashion (typical of antero-lateral thoracic Blashko lines). Hence, the diagnosis should be redirected towards “superimposed segmental FDE”. In fact, according to a recently proposed paradygm, segmental manifestations of acquired polygenic skin disorders, such as drug-induced inflammatory dermatoses, can manifest as either “isolated” (segmental lesions as the sole manifestation of a dermatosis) or “superimposed” (segmental lesions co-existing with non-segmental, otherwise typical manifestations of a dermatosis).[2] Both forms originate from a postzygotic mutational event but do not imply nor exclude any specific type of such mutation (e.g. loss of heterozygosity).[2]

Vetrichevvel et al.'s case adds up to already reported, yet exceptional instances of drug eruptions occurring in a mosaic form, which include, beside FDE,[13-5] lichenoid dermatitis,[6-10] spongiotic dermatitis[11-14] and pigmented purpuric dermatosis [Table 1].[1516] Segmental lesions were distributed along with Blashko lines in all cases. The superimposed segmental pattern was – somehow surprisingly – frequent (33.3% vs. the 66.7% of the isolated segmental one).

Table 1

Reported cases of segmental drug.induced eruptions

Dermatoses due to drugs are a remarkable model to study the interaction between the environment (i.e. drugs) and genotype (i.e. the [immuno] genetic predisposition to develop a cell-mediated hypersensitivity reaction). Although rare, the occurrence of acquired, non-mendelian skin disorders in a segmental (mosaic) fashion represents an experimentum naturae showing that: 1) such disorders depend not only on environmental factors but also on gene defects; 2) among the allegedly pre-disposing postzygotic mutations, at least some are harbored within resident skin cells.