A report describes a case of a young white woman in her late teens who presented with a 3-year history of pruritic, occasionally painful, erythematous papules and plaques that resolved, leaving behind reticulated hyperpigmented patches over her posterior neck, midback, chest and upper abdomen. She was otherwise healthy and denied dieting or any provoking or exacerbating factors. 

Her interscapular back revealed a symmetric netlike pattern of erythematous papules coalescing into reticulated plaques and hyperpigmented patches of a similar netlike configuration lying below. Her inter- and inframammary chest, upper abdomen and posterior neck also displayed a reticulated pattern of hyperpigmentation. Excoriations accompanied the neck lesions. 

Her nails and oral mucosa revealed no abnormalities, and the laboratory results were normal, including complete blood count, aspartate aminotransferase, alanine transaminase, thyroid-stimulating hormone and herpes simplex viruses 1 and 2. Her antinuclear antibody was weakly positive at 1:80. 

She took previous treatments with calcipotriene, topical tacrolimus and oral and topical steroids, which were unsuccessful. Hematoxylin-eosin staining from the lesion biopsy showed vacuolar changes and many necrotic keratinocytes along the epidermal-dermal junction, with lymphocytes along the junctional zone and in the papillary dermis. There were also scattered eosinophils and neutrophils. The overlying cornified layer showed remarkable parakeratosis. 

Her clinical and histopathologic correlation prompted a diagnosis of prurigo pigmentosa. 

Her treatment included doxycycline, 100 mg BD, which resolved the inflammatory component, yet left the pigmentary alternation.

Source: Corley SB, Mauro PM. Erythematous papules evolving into reticulated hyperpigmentation on the trunk: a case of prurigo pigmentosa. JAAD Case Rep. 2015;1(2):60-2.