INTRODUCTION
Prurigo nodularis (PN) is a neurocutaneous pruritic disorder that disproportionately affects black individuals.1 Recent advances in research technologies have allowed for granular elucidation of the pathogenesis of the disease. There exists an interplay between the skin, immune, and nervous systems. Here we discuss the current understanding of the clinicopathologic and epidemiological features of PN, cutaneous and non-cutaneous co-morbidities, as well as the psychodermatologic aspects of the disease.
Also known synonymously as chronic nodular prurigo (CNPG), PN is a subtype of chronic prurigo with an extensive global burden on quality of life (QoL).2 According to the European Academy of Dermatology and Venereology Task Force for Pruritus, chronic prurigo is defined as "a distinct disease defined by the presence of chronic pruritus (>6 weeks) and multiple localized or generalized pruriginous lesions."3 It is thought that PN commences with neural sensitization with subsequent development of the itch-scratch cycle.3 Continued itch results in multiple hyperkeratotic papules and nodules that are typically distributed symmetrically along the extensor surfaces of the extremities.4-6 Oftentimes lesions can be found on the upper and lower back, sparing the central back due to an inability of the patient to reach those areas to scratch; this finding is known as the "butterfly sign."7 The PN lesions themselves may also be severely pruritic, leading to the continuation of the itch-scratch cycle. Depending on skin type, PN lesions may appear as red-pink in patients who are White and violaceous papules/nodules in patients with skin of color. On histology, PN lesions demonstrate epidermal hyperplasia, hypergranulosis, spongiosis, compact hyperkeratosis, as well as vertically arranged collagen fibers with an increased number of fibroblasts.8 Compared to matched healthy skin, lesional PN skin also exhibits greater inflammatory cells including lymphocytes, neutrophils, mast cells, and eosinophils.9
Definition and Clinicopathologic Features of PN
Also known synonymously as chronic nodular prurigo (CNPG), PN is a subtype of chronic prurigo with an extensive global burden on quality of life (QoL).2 According to the European Academy of Dermatology and Venereology Task Force for Pruritus, chronic prurigo is defined as "a distinct disease defined by the presence of chronic pruritus (>6 weeks) and multiple localized or generalized pruriginous lesions."3 It is thought that PN commences with neural sensitization with subsequent development of the itch-scratch cycle.3 Continued itch results in multiple hyperkeratotic papules and nodules that are typically distributed symmetrically along the extensor surfaces of the extremities.4-6 Oftentimes lesions can be found on the upper and lower back, sparing the central back due to an inability of the patient to reach those areas to scratch; this finding is known as the "butterfly sign."7 The PN lesions themselves may also be severely pruritic, leading to the continuation of the itch-scratch cycle. Depending on skin type, PN lesions may appear as red-pink in patients who are White and violaceous papules/nodules in patients with skin of color. On histology, PN lesions demonstrate epidermal hyperplasia, hypergranulosis, spongiosis, compact hyperkeratosis, as well as vertically arranged collagen fibers with an increased number of fibroblasts.8 Compared to matched healthy skin, lesional PN skin also exhibits greater inflammatory cells including lymphocytes, neutrophils, mast cells, and eosinophils.9
