Eczema in Psoriatico: An Important Differential Diagnosis Between Chronic Allergic Contact Dermatitis and Psoriasis in Palmoplantar Localization.

Differential diagnosis of palmoplantar non-pustular psoriasis and chronic allergic contact dermatitis (ACD) and the combination of these conditions, termed "eczema in psoriatico" (EIP), is difficult, especially in cases of isolated involvement. A blind re-evaluation of 63 archived formalin-fixed palmoplantar samples, previously diagnosed clinically as either psoriasis or chronic ACD, was performed. Samples were allocated to histopathological diagnoses of psoriasis, contact dermatitis or EIP. Immunohistological stainings were performed for better characterization. Immunochemistry of EIP revealed features that overlapped contemporarily with psoriasis (cytokeratin 17 (CK17), Ki67, interleukin (IL)-8, IL-17, IL-23) and with ACD (CD1a, major histocompatibility complex (MHC) class I, MHC class II, epidermal T-cell subsets). Surprisingly, a significantly much higher number of dermal CD8+ T cells was found in EIP than in ACD and psoriasis. In conclusion, this study provides insight into the immunohistological differentiation of palmoplantar psoriasis, chronic ACD and EIP.