Contact dermatitis: relevant differential diagnoses, simulators, and variants.

Similar to all eczematous disorders, irritant and allergic contact dermatitis are primarily defined by their morphology. In addition, the diagnosis of this specific reaction pattern on the skin (and mucous membranes) also requires prior exposure (direct or indirect) to an allergen or irritant. While it is quite easy to give a textbook description of the typical features of eczema, its clinical manifestations in daily practice are diverse and frequently uncharacteristic. Contact reactions may present as lichenoid, lymphomatoid, granulomatous, pigmented, purpuric, and erythema multiforme-like lesions, thus lacking the typical eczematous appearance and broadening the spectrum of differential diagnoses to be considered. Moreover, a considerable number of agents, acting by mechanisms other than contact, may trigger adverse reactions mimicking the eczematic morphology and distribution pattern of contact dermatitis. Increasingly used in oncology, this is especially true for drugs such as kinase inhibitors. Knowledge of these associations is crucial for the adequate diagnostic and therapeutic care of patients with supposed or actual contact dermatitis.