Prurigo pigmentosa (PP) is an uncommon dermatosis consisting of a network of erythematous, pruritic papules evolving into reticulated hyperpigmentation with a specific predilection for the trunk. It seen mostly in young adults, mostly in women. Originally described in Japan, there have been an increasing number of reports worldwide. I have diagnosed PP a few times, but admittedly, only after considering other diagnoses such as Gougerot-Carteaud or an allergic contact dermatitis. There are no systemic associations with PP (although a PP-like rash has been reported with adult-onset Still disease). Therapy is with tetracycline antibiotics (minocycline, doxycycline) or dapsone. Topical steroids may help pruritus, but have little effect on the rash itself. Recurrences of PP may occur (1).
PP progresses through several stages of development, commencing as erythematous macules which evolve to urticarial papules and papulovesicles. Subsequently the lesions become crusted or scaly. A few weeks later, lesions spontaneously resolve, leaving behind reticulated pigmentation. The histologic features vary with each stage of lesional morphology. An early urticarial lesion will display a sparse, perivascular and interstitial, predominantly neutrophilic infiltrate involving the superficial dermis with variable papillary dermal edema and focal epidermal exocytosis. Fully developed lesions manifesting as excoriated papules, vesicles, and papulovesicles will demonstrate varying degrees of spongiosis, accompanied by ballooned keratinocytes associated with epidermal necrosis and focal collections of neutrophils. The dermal inflammatory infiltrate at this stage is denser, and often mixed, with increased numbers of lymphocytes, neutrophils and scattered eosinophils. Late lesions demonstrate slight epidermal spongiosis, focal parakeratosis, and variable vacuolar alteration, associated with a mild superficial perivascular lymphocytic infiltrate. Scattered apoptotic keratinocytes and melanophages are readily identified (2).
The cause of PP is unknown. Several triggering factors have been considered: friction from clothing, allergic contact dermatitis, sunlight, and nutritional factors, including dieting, diabetes, and ketonuria (1, 3) Houriet et al raised the question of a possible genetic predisposition for PP, reporting the disorder in 2 monozygotic white twins in their 20s. PP presented 6 years apart in each twin (3).
In an intriguing study, 32 patients with PP were studied histologically A total of 25 patients (78%) had pathological involvement of hair follicles, either as bacterial colonies in the hair follicles (21/32, 66%), folliculitis (8/32, 25%) or perifolliculitis (15/32, 47%). There was a significantly higher proportion of patients with hair follicle involvement compared with control groups with either noninflammatory or inflammatory skin diseases on the back. Minocycline was an effective antibiotic treatment either singly or in combination with steroids (4).
The conclusion by Kafle et al that the presence of bacterial colonies along with sequelae of inflammatory changes on biopsy provides new evidence to support the theory that prurigo pigmentosa is a reactive inflammatory response to bacterial folliculitis. Previously, the benefit of treating PP with antibiotics was attributed to the anti-inflammatory effects of the drugs; while that may be true, perhaps in PP, it is really their antibacterial effect that is responsible for the improvement. Questions remain – what about those cases of PP where folliculitis was not observed? What is the precise follicular bacterium that is potentially pathogenic? What are the true predisposing factors of PP? Are there genetic influences in PP? Although much remains to be learned, from my perspective, the research by Kafle et al brings us within a hair’s breadth of understanding this enigmatic dermatosis.
- Corley SB, Mauro PM. Erythematous papules evolving into reticulated hyperpigmentation on the trunk: A case of prurigo pigmentosa. JAAD Case Rep 2015; 1: 60-2.
- Satter E, et al Prurigo pigmentosa: An under-recognized inflammatory dermatosis characterized by an evolution of distinctive clinicopathological features. J Cutan Pathol 2016; 43: 809-14.
- Houriet C, et al. Prurigo pigmentosa in white monozygotic twins. JAMA Dermatol 2017; Jan 11 [Epub ahead of print].
- Kafle SU, et al. Folliculitis in prurigo pigmentosa: A proposed pathogenesis based on clinical and pathological observation. J Cutan Pathol 2017; 44: 20-7.
*Image from Australian Doctor June 15, 2016