Circumscribed Juvenile Pityriasis Rubra Pilaris (Type 4) Koebnerising after a Hot Water Burn: Mild Disease with Maximum Koebner Response.

Dear Editor, Pityriasis rubra pilaris (PRP) is a chronic, inflammatory, papulosquamous skin disorder that is characterized by follicular hyperkeratosis and reddish-orange, scaling dermatitis with islands of normal skin (1,2). PRP is classified into 5 groups based on clinical features. Type 4 PRP is characterized by well-demarcated, hyperkeratotic erythematous plaques localized on the elbows and knees with palmoplantar keratoderma (1,2). An 8-year-old girl presented to our clinic with erythematous plaques on both elbows, the legs, and the knees. Plantar keratoderma was noticed on clinical examination. The lesions had started on the elbows and knees about a year ago. The lesions on the leg were surrounded by an irregular, hyperpigmented border. On close inspection, the plaques were formed by follicular papules and mild desquamation was noticed. Upon questioning, it was learned that the lesions on the leg with hyperpigmented borders had emerged after a hot water burn three months ago and that they were localized exactly on the burned areas of the skin (Figure 1). A biopsy was performed on the new lesions, and histopathological evaluation revealed parakeratosis with alternating orthokeratosis, irregular hyperkeratosis, keratotic plugs, and a mild perivascular lymphocytic infiltration around the blood vessels (Figure 2). A diagnosis of PRP was established. The Koebner phenomenon (KP) is described as the development of lesions in previously normal skin after exposure to internal or external trauma such as surgical incisions, burns, friction, insect bites, and allergic and irritant reactions (3). The pathogenesis of KP is not fully understood, but epidermal cell injury and dermal inflammation have been proposed as having a role in the pathophysiology (4). Experimental studies on the mechanism of KP have been performed mostly on patients with psoriasis (3). Disease severity, early age of disease onset, and multiple previous therapies have been found to be associated with KP (5,6). KP has previously been reported after injury with the sharp end of a stick in type 3 PRP, a generalized PRP form (7). However, our patient was diagnosed with type 4 PRP, which is a localized form of the disorder. Griffiths reported type 4 PRP does not evolve to generalized forms (8). In this respect, our case was interesting as maximum Koebner response was observed despite the mild PRP. We therefore believe that disease severity is not a determining factor in KP and that the severity of skin damage plays a crucial role. We also think that changes in the cytokine milieu in the burn area may be responsible for KP, as levels of IL-17 and IL-22, which have been shown to be upregulated in burns, also play a role in PRP pathogenesis (9,10). The disease onset at an early age might have also had a contributing role in the Koebner response in this patient. The hyperpigmented borders of the Koebnerized plaques were also notable as they were spared from KP. Some spared areas were also seen within the Koebnerized plaques themselves. A threshold level of trauma is thought to be necessary for inducing KP (3). The clinical picture of our patient may indicate that the skin damage was much less severe in some areas of the burn, especially in the periphery, and that KP was therefore not observed in these areas. Our case clearly demonstrates that the Koebner response is not related to disease severity. We believe that the type of trauma is an important factor in determining the severity of skin damage and the changes in the cytokine milieu in the involved skin. Early disease onset also seems to contribute to the development of KP. Further studies investigating the mechanism of KP in various skin disorders are necessary. As far as we are aware, this is the first case reporting Koebnerization in the circumscribed juvenile form of PRP.