Extra-palmoplantar lesions associated with palmoplantar pustulosis

Palmoplantar pustulosis (PPP) is a chronic inflammatory disorder characterized by sterile pustules predominantly involving the palms and soles of middle-aged women. In contrast, regions other than the palms and soles are occasionally affected, manifesting scaly erythemas which resemble psoriasis, and solitary pustules are also seen. Some of these extra-palmoplantar lesions are induced by the Koebner phenomenon or occur after focal infections like tonsillitis. The tenderness and inflammation of the extra-palmoplantar lesions in PPP are milder than in psoriasis. Histological features show mild acanthosis of the epidermis with parakeratosis and mild infiltration of inflammatory cells in the upper dermis. On the other hand, severe pustular lesions are occasionally seen in the palms and soles of the patients with pustular psoriasis. These findings suggest a close relationship between PPP and psoriasis; however, different genetic, environmental, and immunological factors are likely to be involved. Recently, understanding of psoriasis pathophysiology has greatly progressed, and the concept of psoriasis pathogenesis is currently viewed as complicated responses between infiltrating leucocytes and the resident skin, via a number of inflammatory cytokines, chemokines, and mediators produced in the skin under regulation of cellular immune systems. By contrast, the pathogenesis of PPP has been poorly investigated. This paper reviews findings of the clinicopathophysiology of PPP, making a focus on the extra-palmoplantar lesions.