Is this psoriasis or something else?

Question

What is your diagnosis?

Diagnosis

Porokeratosis ptychotropica.

Discussion

Porokeratosis ptychotropica represents a novel, rare, under-recognized and a distinct form of porokeratosis. The term “ptychotropica” is derived from the Greek words ‘ptyche’ denoting fold and ‘trope’ denoting turning, and alludes to flexural localization of the disorder. For mysterious reasons, porokeratosis ptychotropica has a predilection for adult males.¹ Nevertheless, it has also been documented in children.² The majority of reported cases are sporadic. However, familial cases, suggesting an autosomal dominant inheritance, have been described.³

Classical clinical presentation of porokeratosis ptychotropica consists of slowly enlarging, pruritic, bilaterally symmetrical, verrucous plaques in a butterfly motif, usually involving the natal cleft and buttocks. The lesions tend to coalesce centrally and expand gradually over time, with the formation of peripheral satellite, red-brown hyperkeratotic papules. Occasionally, verrucous papules and plaques may extend to involve the penis, scrotum, vulva and thighs. Itching and discomfort are the most important complaints elicited.^1,3

The definitive diagnosis of porokeratosis ptychotropica is a challenge and may be delayed for several years or even decades.¹ We believe that, owing to misdiagnoses, the actual prevalence of the disorder may be much higher than suggested by the relevant literature. In our case, the disorder simulated psoriasis and neurodermatitis, and definitive diagnosis could only be established 6 years after the onset. This case taught us that persistent psoriasiform and verrucous gluteal plaques should be biopsied in order to exclude porokeratosis ptychotropica.

Skin biopsy is the gold standard for diagnosis. Histologically, porokeratosis ptychotropica harbours multiple cornoid lamellae, responsible for the hyperkeratotic and verruciform clinical appearance. The presence of numerous cornoid lamellae also distinguishes this disorder from the classical porokeratosis of Mibelli, where the cornoid lamellae are only a few in number and exist only at the periphery.^1,3

The etiology of porokeratosis ptychotropica is an enigma. The role of human papillomavirus infection has not been proven.⁴ Both genetic and environmental factors have been implicated, including ultraviolet light exposure, infections, drugs, immunosuppression and repeated trauma (friction with clothing, scratching, sweating). Trauma may exacerbate the disorder by inducing a Koebner-like effect, with the development of look-alike lesions. ¹ It has been conjectured that porokeratosis ptychotropica is the end stage of a local proliferation of porokeratosis of Mibelli-like lesions, prompted by koebnerization and/or lichenification.⁵ Recently, the first case of porokeratosis ptychotropica transforming into invasive squamous cell carcinoma has been reported by Mazori et al.⁴ Because of such an impending risk, close long-term follow up of affected patients has been recommended.^3,4

Treatment options in porokeratosis ptychotropica are generally impeded by temporary, disappointing or futile outcomes, and are eventually followed by recurrences.¹ The favorable effects, if ever attained, are only partial and comprise symptomatic relief and flattening of the lesions. Our patient has not responded to oral acitretin and topical tazarotene. So far, complete remission has only been observed with dermatome slicing.¹ Surgical excision might be a convenient option in small lesions.^2,3

There is increasing evidence implicating that porokeratoses are related to aberrant terminal differentiation of keratinocytes, connected to postzygotic mosaic somatic mutations in enzymes of the mevalonate pathway. The resultant defect is in cholesterol biosynthesis which might judiciously be treated with topical or systemic inhibitors of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase enzyme.² Thus, the future holds promise for therapeutic optimism in porokeratoses.