A Chronic and Recurring Unilateral Linear Rash in a Prepubescent Boy

Case A 13-year-old boy presented to the dermatology clinic with a 3-year history of linear erythematous nonitchy rashes on his right upper and lower limbs. He had been treated previously for childhood eczema since he was a toddler. On physical examination, there were pink-to-erythematous, well-defined plaques topped with thick silvery scale arranged linearly over the right lateral leg [Figure 1a] and right proximal arm [Figure 1b] extending up to the first web space of his right hand associated with nail pitting of his right thumb and index finger [Figure 1c]. Lesional skin-biopsy from the right calf showed confluent parakeratosis with collections of neutrophils within the stratum corneum, hypogranulosis, regular psoriasiform hyperplasia, and dilated capillaries in the papillary dermis [Figure 2]. The patient was initially treated with emollients and a range of topical corticosteroids, including clobetasol propionate 0.05% cream, betamethasone dipropionate with salicylic acid ointment (Diprosalic), mometasone furoate 0.1% ointment, and calcipotriol with betamethasone gel (Daivobet). Despite this regimen, clinical improvement was minimal. In view of the suboptimal response to topical therapy, second-line treatment options–namely phototherapy, oral retinoids, and methotrexate–were subsequently discussed with the patient’s parents, who expressed a preference for initiating oral retinoid therapy. He is currently being treated with oral acitretin 40 mg/day and topical betamethasone 0.1% + salicylic acid 2% ointment, with marked improvement of his lesions [Figure 3a and b].Figure 1: Patient’s cutaneous examination showing pink-to-erythematous, well-defined plaques topped with thick silvery scale arranged linearly over the: (a) Right leg, front view; (b) Right leg, lateral view; (c) First web space of his right hand associated with nail pitting of his right thumb and index fingerFigure 2: Lesional skin-biopsy showed confluent parakeratosis with collections of neutrophils within the stratum corneum, hypogranulosis, psoriasiform hyperplasia, and dilated capillaries in the papillary dermisFigure 3: Patient during his recent follow-up showing marked improvement of his lesions over his right arm (a) and right leg (b) What is the diagnosis? Linear verrucous epidermal nevus Linear psoriasis Lichen striatus Linear lichen planus Which of the following favors linear psoriasis over ILVEN? Onset in early infancy Severe itching and slow course Later onset with nail or scalp involvement No response to anti-psoriatic treatment Which immunohistochemical marker is present in psoriasis but absent in ILVEN? Ki67 Involucrin Keratin 10 CD4 Answers: B C B Diagnosis: Linear psoriasis The clinical presentation with chronic pink-to-erythematous, well-defined plaques topped with thick silvery scale in a linear pattern is consistent with linear psoriasis. This is corroborated by histopathological features. Discussion Psoriasis is a chronic skin disease affecting approximately 2% of the population.[1] There are reported atypical forms of psoriasis, such as linear, zonal, regional, verrucous, follicular, lichenoid, and seborrheic psoriasis. Linear psoriasis is characterized by the distinct distribution of lesions following Blaschko lines.[2] The pathogenesis of linear psoriasis is still unclear but could be explained by the concept of genetic mosaicism. Happle proposed the pathogenesis as somatic recombination of a gene predisposing to psoriasis, leading to segmental mosaicism. This hypothesis offers a reasonable explanation for linear psoriasis being a nonhereditary trait and the linear distribution, like many other mosaic skin disorders.[3] The usual diagnostic dilemma is differentiating linear psoriasis from inflammatory linear verrucous epidermal nevus (ILVEN), since the two diseases share clinical and histologic similarity. Factors that point toward the diagnosis of linear psoriasis include later onset, more rapid progression, possible involvement of other classical sites of psoriasis such as the scalp and nails, positive family history of psoriasis, and good response to anti-psoriatic treatment.[4] On the other hand, ILVEN tends to occur during the first few months of life, has a slow progression, is usually accompanied by severe pruritus, and is unresponsive to anti-psoriatic treatment.[5] Other differential diagnoses to consider are linear lichen planus, lichen striatus, and “Blaschkitis.” Histologically, the two entities share similar findings, and the coexistence of ILVEN and psoriasis has also been reported.[5] The classic histologic findings of psoriasis are parakeratosis, hypogranulosis, regular acanthosis, suprapapillary thinning, and formation of Munro and Kogoj microabscesses in the corneal layer and superficial epidermis. These histological findings can also be observed in ILVEN lesions.[4] Although not routinely done, immunohistochemical studies may be helpful in distinguishing ILVEN from linear psoriasis. In psoriatic lesions, there is an increase in the number of cells positive for Keratin 10, whereas in ILVEN lesions, there tends to be a reduction of Ki67-positive nuclei. Involucrin is another marker that is also detectable in psoriasis but absent in ILVEN.[6] Response to anti-psoriatic treatment tends to be less effective in linear psoriasis when compared to classical psoriasis. These findings were seen in cases reported in the literature in which classical psoriatic lesions were cleared by topical steroids, and linear lesions were less sensitive to topical therapy and generally recurred after treatment cessation.[7] Other treatment that have noted significant improvement includes ixekizumab,[8] ustekinumab,[9] and narrowband ultraviolet (UV) therapy.[10] There are no current guidelines for the treatment of linear psoriasis, and larger studies are needed to determine optimal therapy for this rare variant. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.