Unilateral Linear Psoriasis: A Case Report

First described in 1951, linear psoriasis is a rarely seen variant of psoriasis.¹ Clinically, it is characterized by erythematous scaly papules and plaques that follow the lines of Blaschko. True linear psoriasis, in which there are no psoriasiform lesions presenting outside of the Blaschko distribution, is exceedingly rare.² Linear psoriasis typically presents in adults, although there are a few reported cases in children.³ While the pathogenesis is still not well understood, the development of linear psoriasis is thought to be due to somatic changes in a gene that typically predisposes to psoriasis.⁴ Further, these somatic gene changes could plausibly be responsible for the nonhereditary nature of linear psoriasis, as it is not usually seen in families. In addition, linear psoriasis follows a distribution seen in many other mosaic skin disorders. These features suggest a mosaic etiology, resulting in a Blaschkoid distribution secondary to the migrations of mutated cells during early embryogenesis. 

The differential diagnosis for a linear plaque in the adult patient includes linear psoriasis, epidermal nevus, inflammatory linear verrucous epidermal nevus (ILVEN), linear lichen planus, lichen striatus, nevus sebaceous, and blaschkitis. In most cases, the clinical features and histology can easily delineate the difference between diagnoses, as demonstrated in the Table.⁵

The most difficult distinction to make is between linear psoriasis and an ILVEN. ILVEN is an inflammatory condition marked by erythematous, scaly linear papules and plaques following the lines of Blaschko.⁶ Clinical distinction can be made based on onset, symptomatology, and response to treatment. As opposed to linear psoriasis, ILVEN develops in the first month of life and progresses slowly, is pruritic, and typically is resistant to treatment.⁷ Histochemically, ILVEN shows increased keratin 10 staining and lacks involucrin expression, as opposed to the decreased keratin 10 staining and retained involucrin expression in linear psoriasis.^7,8 Neutrophils in the stratum corneum and inflammatory infiltrate favor a diagnosis of linear psoriasis over ILVEN. 

The linear psoriasis form is thought to be responsive to typical therapies for psoriasis vulgaris, although specific reports are lacking for first-line treatments such as topical corticosteroids. More recently, there have been case reports of successful treatment with systemic tumor necrosis factor-alpha blockade as well as IL-12 and IL-23 inhibitors.^9,10 In cases refractory to all treatment, the diagnosis of ILVEN should be reconsidered. 

Case Report
The patient is a 19-year-old white woman with a 4-year history of a rash localized to her left arm and hand. The lesion started as a small plaque on her forearm and gradually spread in a linear fashion from her fingertip to her upper arm. She denied a history of chemical exposure or trauma to the area. Multiple attempts to treat the rash with over-the-counter creams, including topical cortisone and antifungal treatments, failed. Associated symptoms included cyclical bouts of pruritus and erythema. Family history was negative for autoimmune disease and psoriasis. Patient was taking norethindrone oral contraceptive pills at the time of presentation. She was concurrently being treated for hidradenitis supperativa and tinea pedis. 

Dermatologic evaluation was notable for a linear, erythematous, scaly plaque extending from her left third digit proximally to her upper anterior arm. The lesion traversed the olecranon process, following the lines of Blaschko (Figures 1 and 2). The nails were uninvolved. 

A 3-mm punch biopsy was taken from the left upper arm (Figures 3 and 4). Histology by hematoxylin-eosin stain demonstrated parakeratosis with focal areas of neutrophilic scale crust. Prominent psoriasiform epidermal hyperplasia and a patchy granular layer were the main epidermal features. Suprapapillary plate thinning with dilated blood vessels and spongiosis were present in the papillary dermis. The reticular dermis featured a superficial perivascular lymphohistiocytic infiltrate. Periodic acid-Schiff stain was negative. With these findings, a diagnosis of linear psoriasis was made.

The patient cleared after a 1-month course of betamethasone diproprionate 0.05% cream used twice daily. Upon 1-month follow-up, the patient remained clear. 

Conclusion
Linear psoriasis is a rare entity that can be diagnosed with a combination of clinical history, presentation, and histology. Although it follows a benign course and can be treated conservatively with modalities utilized for plaque psoriasis, biologic use may be pursued depending on treatment efficacy and the patient experience. An injury, medication, illness, or other stressor, such as the more common psoriatic variants, likely triggers the onset. It is unknown whether or not patients with linear psoriasis have the same cardiac risk or internal inflammation as patients with more widespread disease.

Dr Zarkhin is a dermatology resident at St. Barnabas Hospital in Bronx, NY. Dr Madden is a Mohs micrographic surgery fellow at Dayton Skin Care in Dayton, OH. Dr Hawley is the medical director of The Derm Institute of West Michigan in Grand Rapids, MI, as well as an associate clinical professor at Michigan State University College of Osteopathic Medicine. Dr Stephenson is an assistant clinical professor for the Beaumont Farmington Hills Dermatology Residency Program and medical director/dermatopathologist at Dermatopathology Laboratory of Central States in Troy, MI.

Disclosure: The authors report no relevant financial relationships.

References
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