A case of linear psoriasis in a young patient with Down syndrome and review of the literature

To the Editor, Linear psoriasis is a rare form of psoriasis first described in 1951, being characterized by the distribution of skin lesions along Blaschko's lines. The pathogenesis is still not well understood but genetic mosaicism phenomena are believed to be responsible. Linear psoriasis typically presents in adults with erythematous scaly papules and plaques which tend to progress rapidly along Blaschko's lines. However, true linear psoriasis, in which there are no psoriasiform lesions presenting outside of the Blaschko distribution, is exceedingly rare; hence other sites such as the scalp, nails, palms, and soles are usually involved. This condition is intensely pruritic and usually resistant to antipsoriatic therapies.1-3 The differential diagnosis of linear psoriasis may be wide. Inflammatory linear verrucous epidermal nevus (ILVEN), which also follows Blaschko lines and usually develop in infancy, represent the main diagnostic challenge, followed by lichen striatus. Linear psoriasis is extremely rare in children and only five pediatric cases has been reported so far (Table 1). 10-year-old male 15-year-old-female Here, we present the case of a 2-year-old patient with Down's syndrome who was admitted to our Clinic for the presence of a linear erythematous and scaling plaque on the right thigh. Patient's parents related that this lesion had been present for about 6 months being refractory to topical corticosteroid therapy. The patient did not take any medications and his medical history was unremarkable. There was no family history of psoriasis. Objective dermatologic examination showed the presence of a linear erythemato-desquamative plaque distributed along Blaschko's lines of the right thigh, onychodystrophy of the nails (right and left toe) and desquamation in both retroauricular areas (Figure 1A–C). Dermoscopic examination revealed whitish scales and punctiform vascular patterning (Figure 2). A 3 mm skin biopsy showed the presence of epidermis-coated skin with marked hyperplastic changes, mild spongiosis, hypogranulosis\agranulosis, parachatosis scales with neutrophil granulocyte accumulation, with microaggregate formation at subcorneal site, confirming our clinical suspect of linear psoriasis. Several weeks of treatment with vitamin D derivatives, topical corticosteroids, emollients and/or fixed combination of vitamin D derivatives and topical corticosteroids gave only minor and transient improvement. Patients' parents refused systemic treatment for their child who was then lost to follow-up. Linear psoriasis is a very rare form of the disease. Epidemiologic data are scant, particularly in childhood, with only few cases being reported in the literature for pediatric patients so far.3-7 Particularly, male sex seems to be predominant (83.3%), mean age of onset is 8.3 years with only 16.6% of the cases reporting a positive family history of psoriasis (Table 1). Since its clinical and histological similarity to ILVEN, some cases are probably not reported, contributing to its underdiagnosis. The differentiation of linear psoriasis from other linear dermatoses is not easy. The combination of a thorough history, a careful examination of the skin and histopathology are essential to ensure the correct diagnosis and appropriate treatment. However, its clinical and histologic similarity to ILVEN remains a diagnostic challenge. The distinction between these two entities has been discussed in the literature.8 The late-onset of asymptomatic lesions or mildly itchy lesions, the rapid progression and possible involvement of the nails and/or scalp with a response to antipsoriatic treatment are all features suggesting the diagnosis of linear psoriasis. In contrast, ILVEN presents with lesions that usually appear in the first few months of life, slowly progressive, very itchy, and highly refractory to antipsoriatic treatment. Immunohistochemical studies have been proposed in the literature that could be useful for the diagnosis differentiating between the two diseases. Little expression of keratin 10 was found in psoriasis, whereas its levels remain normal in ILVEN. In addition, involucrin would be detectable in psoriasis, but absent in ILVEN.8 In our case, the presence of toenail onychodystrophy and desquamation at the retroauricular site, together with the histological finding, was diriment for the differential diagnosis. In addition, our patient represents the case with the earliest age of onset reported so far in literature (2 years of age), highlighting the need to collect more data on this disease which is underreported. F.M., A.R., M.M., and V.P. performed the research. F.M., M.M., G.F., and A.R. designed the research study. F.M. and M.M. contributed essential reagents or tools and wrote the paper. G.F., C.B., and V.P. analyzed the data. All authors approved the final version of the paper. Open Access Funding provided by Universita degli Studi di Napoli Federico II within the CRUI-CARE Agreement. [Correction added on 29 November 2022, after first online publication: CRUI-CARE funding statement has been added.] None to declare. The authors confirm that the ethical policies of the jorunal, as noted on journal's author guidelines page, have been adhered to. No ethical approval was required as this is a review article with no original data. The patient's parents gave the consent for photo acquisition and publication.