Unravelling the spectrum: Isolated Blaschko linear psoriasis – A rare clinical entity

INTRODUCTION

Blaschko linear psoriasis, an extremely rare and distinct variant of psoriasis, is characterized by the distribution of psoriatic lesions along the lines of Blaschko. It is clinically and histologically similar to chronic plaque psoriasis, but its unique pattern of distribution sets it apart from all other forms of psoriasis. It may present as the sole manifestation of psoriasis (type I, or isolated type) or appear alongside classic disseminated lesions of psoriasis vulgaris. In the latter case (type II, or superimposed type), lesions present along the lines of Blaschko become more apparent as the classical psoriatic lesions diminish with antipsoriatic treatment. This unique presentation often leads to diagnostic confusion with other linear dermatoses like inflammatory linear verrucous epidermal nevus (ILVEN) or Koebner’s response of psoriasis over verrucous epidermal nevus, complicating its recognition and necessitating a thorough diagnostic assessment.¹ This case report highlights the unique presentation of isolated Blaschko linear psoriasis and differentiates it from other linear dermatoses through comprehensive clinical and histopathological evaluation, contributing to the limited literature on this rare condition.

CASE REPORT

A 12-year-old male presented in the dermatology department with complaints of reddish, raised, scaly lesions arranged in a linear fashion over the left arm, forearm, and hand since 3 years. The lesions initially started over the left forearm and gradually progressed to involve his left arm proximally and left hand distally in a span of 2 years. The lesions were associated with mild itching. There was a history of seasonal variation, with the lesions typically worsening in the winter season. A history of similar lesions anywhere else on the body was not present. No history of any trauma or drug intake prior to the onset of lesions was present. The patient’s father did not elicit a family history of similar lesions.

Examination revealed multiple well-defined erythematous scaly papules and plaques arranged in a linear fashion along the lines of Blaschko, originating from the lateral aspect of the dorsum of the left arm, extending across the dorsum of the forearm, and reaching the dorsum of the left index finger till the proximal nail fold. The scales were thick, whitish, and adherent to the underlying plaques [Figures 1 and 2]. The Auspitz sign was positive, indicating the presence of pinpoint bleeding upon removal of the scales. Based on these clinical findings, a differential diagnosis of Blaschko linear psoriasis and ILVEN was considered.

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Figure 1:

12-year-old male presented with erythematous scaly papules and plaques present along the lines of Blaschko, originating from the lateral aspect of the dorsum of the left arm, extending across the forearm, and reaching till the dorsum of the left index finger.

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Figure 2:

Well-defined erythematous papules and plaques with overlying thick, whitish, adherent scales present over dorsum of left forearm, left hand, left index finger (including its proximal nail fold).

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Dermoscopy revealed white superficial scales and red globules/dotted vessels arranged in a homogenous pattern on a light red background [Figure 3]. On histopathology, the epidermis revealed acanthosis, parakeratosis, elongation of rete ridges, Munro’s microabscess, and suprapapillary thinning accompanied by lymphocytic infiltrates in the dermis [Figure 4].

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Figure 3:

Dermoscopy showing whitish scales (black arrow) and red globules (red arrow) on a light red background (De10x, polarized mode).

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Figure 4:

Acanthosis, parakeratosis, elongated rete ridges, Munro’s microabscess (black arrow), suprapapillary thinning, and lymphocytic infiltrates in the dermis (Hematoxylin and Eosin stain, 10x).

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Based on clinical features, dermoscopic findings, and histopathology, a diagnosis of isolated Blaschko linear psoriasis was made. The patient was treated with daily application of a combination of 0.05% clobetasol propionate and 3% salicylic acid cream, following which the lesions started resolving.

DISCUSSION

Blaschko linear psoriasis is a rare variant of psoriasis characterized by the distribution of psoriatic lesions in a segmental fashion along the lines of Blaschko. Its isolated variant presents alone with no lesions elsewhere on the body, while its superimposed variant occurs in association with nonsegmental lesions of chronic plaque psoriasis. Isolated variant is exceptionally uncommon, with only a few cases documented in the literature.¹ Segmental lesions develop at a young age and are often more severe and resistant to treatment than associated non-segmental lesions of plaque psoriasis.² Segmental manifestations are believed to indicate genetic mosaicism arising by postzygotic loss of heterozygosity occurring in a somatic cell during early embryogenesis, which results in either homo- or hemizygosity for one of the genes predisposing to psoriasis.³

A study by Onoufriadis A et al.⁴ investigated the gene expression profiles of Blaschko linear psoriasis using transcriptomic analysis and compared them to psoriasis vulgaris. The study found that both Blaschko linear psoriasis and psoriasis vulgaris share common molecular pathways related to immune system activation, inflammation, and skin barrier dysfunction, specifically involving cytokines like TNF-α and IL-17. However, Blaschko linear psoriasis also displays unique gene expression patterns related to skin development and cellular adhesion, suggesting distinct mechanisms of skin cell movement and organization compared to psoriasis vulgaris.⁴ This variant of psoriasis may be mistaken for ILVEN or Koebner’s response of psoriasis over verrucous epidermal nevus. Clinical distinction between ILVEN and Blaschko linear psoriasis can be made based on onset, symptomatology, and response to treatment, summarized in Table 1.^5,6 Psoriasis overlying an epidermal nevus has extremely pruritic lesions, and patients generally develop typical psoriatic lesions elsewhere in the body.⁷ Lesions in our patient developed later in life and were not present at birth. They were unilateral, mildly itchy, and progressed rapidly along the lines of Blaschko. The patient did not develop similar lesions over other body parts. In addition, dermoscopy and histopathology revealed features of psoriasis, and the lesions responded well to anti-psoriatic topical treatment. These findings contributed to our diagnosis of isolated Blaschko linear psoriasis. Currently, there are no established treatment guidelines for Blaschko linear psoriasis, so it is reasonable to follow the protocols used for classical psoriasis. However, treatment can be complex, as patients may exhibit varying responses to both traditional and biologic therapies. This variation is likely due to the loss of heterozygosity in the affected cells, which leads to differences in the proliferation and differentiation of keratinocytes.¹ Topical corticosteroids and vitamin D analogues are the first-line treatments for localized Blaschko linear psoriasis. In cases that are extensive or unresponsive to topical therapies, options such as narrow-band ultraviolet B phototherapy, systemic agents like methotrexate, and biologics may be considered.⁸ There have been studies of favorable responses to some biological, such as ixekizumab (IL-17A inhibitor).^1,9 In a study by Chen et al. assessing the efficacy of ixekizumab, three patients with type 1 disease and one with type 2 disease were treated, all of whom experienced favorable outcomes. However, the same study observed that patients with type II disease were more resistant to antipsoriatic treatments than type I.¹ In another study by Ghoneim Sara et al., after receiving eight doses of ixekizumab, the patient showed almost complete resolution of skin lesions, indicating the effectiveness of IL-17A inhibitors in treating isolated Blaschko linear psoriasis.⁹ A study by Colombo et al. concluded that biologic therapies, including TNF inhibitors such as etanercept, demonstrated limited effectiveness in treating superimposed Blaschko linear psoriasis, emphasizing the need for adjunctive or alternative treatments for this variant of psoriasis.¹⁰

CONCLUSION

This case report highlights the clinical importance of Blaschko linear psoriasis, an uncommon presentation of a common dermatological condition, and differentiates it from other conditions like ILVEN through comprehensive clinical and histopathological evaluation. It contributes to the expanding understanding of this rare psoriasis variant and emphasizes the need for a detailed assessment to ensure optimal diagnosis and treatment.