A patient presented to the dermatology department with a skin eruption over a new tattoo site (fig 1⇓). What is the name of the phenomenon in this picture, and what is the underlying skin condition?

This patient has chronic plaque psoriasis. The development of psoriatic lesions over a new tattoo site is known as the Koebner phenomenon.

Discussion

Localised skin eruption after cutaneous injury, in this case the development of psoriasis over a recent tattoo site, is described as the Koebner phenomenon.1 It was first recognised by Heinrich Koebner in 1876, and is synonymous with the isomorphic response. Koebner himself drew a picture, published in 1883, of psoriatic Koebnerisation over a tattoo.2

Other types of cutaneous trauma known to provoke the Koebner phenomenon include: excoriation (scratching), sunburn, friction, pressure, lacerations, bites, and vaccinations.1 Around 25% of patients with psoriasis will experience Koebnerisation, and it is therefore beneficial to warn them of its existence.1

This patient has clear evidence of a psoriatic plaque over the extensor aspect of the elbow (fig 2⇓). In cases where the diagnosis of psoriasis is not entirely clear, as well as looking for evidence over areas of predilection such as the scalp, umbilicus, elbows, and knees, searching for Koebnerisation over trauma sites can provide key clues.

Fig 2 Koebnerisation of psoriasis over the tattoo associated with a typical psoriatic plaque on the elbow

Vitiligo, a disorder characterised by focal areas of depigmentation, and lichen planus, an inflammatory condition featuring pruritic papules, also both commonly display Koebnerisation. There are many other skin diseases in which it is reported.3

In psoriatic Koebnerisation, the latency period from skin injury to disease is normally between 10 and 20 days, but can be up to two years.1 The pathogenic mechanisms are not fully understood, however studies show an increased production of cytokines, CD4 lymphocytes, and adhesion molecules following epidermal and dermal injury.134 Patients are more at risk of Koebneristation when their skin disease is flaring, during winter months, and at times of emotional stress.34 An area of Koebnerisation is treated in the same way as the associated skin disease.4

Cosmetic tattoos are becoming increasingly popular—36% of young adults in the United States have at least one.5 Other skin problems that are induced by tattoos include infection, allergic contact dermatitis, and photosensitising reactions.4