Introduction {#section1-2050313X19882843}
============

Lupus erythematosus is an autoimmune disease that can be skin limited or systemic. Discoid lupus erythematosus (DLE) is the most common variant of cutaneous lupus. While it has a lower chance (5%--10% life time) compared to other subtypes to progress to systemic forms, it has significant morbidity because of its tendency to involve cosmetically important areas such as the face and scalp and resulting disfiguring scarring alopecia.^[@bibr1-2050313X19882843]^ Similar to other subtypes of lupus, DLE is most commonly seen in women and in the third to fourth decade of life.^[@bibr2-2050313X19882843]^ DLE typically presents as erythematous to violaceous papules or plaques that eventually develop scales, follicular plugging, central hypopigmentation and scarring in photo-distributed areas.^[@bibr3-2050313X19882843]^ A localized form of the condition occurs in 80% of the patients with lesions developing solely on the face, ears, and/or scalp and 20% of patients present a disseminated form of the disease defined by the presence of lesions beyond the head and neck area.^[@bibr2-2050313X19882843]^ Rarely, discoid and other forms of cutaneous lupus may present in linear fashion following the lines of embryologic migration (lines of Blaschko).^[@bibr4-2050313X19882843]^ We report, the first to our knowledge, case of linear lupus with *en coup de saber* presentation clinically mimicking acute morphea.

Case report {#section2-2050313X19882843}
===========

A 24-year-old non-smoking middle eastern male presented to dermatology clinic with a history of slowly progressive asymptomatic linear skin eruption. On clinical examination, a sharply demarcated pink to violaceous linear patch was noted extending from the root of his frontal scalp to the nasal tip ([Figure 1(a)--(b)](#fig1-2050313X19882843){ref-type="fig"}). Given the clinical presentation, a Doppler ultrasound and a skin biopsy were performed to rule out vascular etiology and early *en coup de saber* morphea. However, the histological picture was diagnostic of discoid lupus showing characteristic features of basal vacuolation, follicular plugging, lichenoid infiltrate and mucin deposition ([Figure 1(c)](#fig1-2050313X19882843){ref-type="fig"}). Full body examination, review of systems and laboratory investigations did not reveal any features of systemic lupus. The patient was prescribed an alternative day regimen of high potency topical steroid (clobetasol dipropionate 0.05% cream) and calcineurin inhibitor (tacrolimus 0.1% ointment) and strict photoprotection to halt inflammation and prevent scarring. He was seen in follow-up 4 months later, at which time his lesions had mostly cleared.

![(a) Erythematous plaque runs from the root of the frontal scalp down to the glabella. Two red and one purple papules are present in the region of the glabella, as well (white arrows). (b) The erythematous plaque appears to continue, after a brief interruption, down the left dorsum of the nose (black arrows). (c) Periodic acid--Schiff (PAS) staining of skin biopsy shows follicular plugging, vacuolar degeneration of the basal layer and lymphocyte predominant periadnexal inflammation.](10.1177_2050313X19882843-fig1){#fig1-2050313X19882843}

Discussion {#section3-2050313X19882843}
==========

Linear forms have been described in many morphological subtypes of lupus erythematosus, such as subacute, discoid, panniculitis and profunda. Various attempts have been made at explaining the linearity of these lesions as well as their tendency to follow the lines of Blaschko. For example, linear nevoid and linear inflammatory dermatosis both follow Blaschko lines and both show evidence of genetic mosaicism as a potential cause. Therefore, antigenic effects mediated by T-cell activity targeted at the mosaic cells could produce the observed skin lesions.^[@bibr5-2050313X19882843]^ Others have postulated that the linear lesions may be due to microchimerism that specifically targets skin cells in a mechanism like that of graft-versus-hosts disease.^[@bibr5-2050313X19882843]^ However, the predominate view is that the presence of the linear lesions is caused by genetically unique keratinocytes that, when exposed to ultraviolet light, produce an inappropriate cytokine response.^[@bibr5-2050313X19882843]^

Although overlap syndromes that fulfill the diagnostic criteria of both DLE and morphea have been described, the case we present illustrates a case of mimicry.^[@bibr5-2050313X19882843]^ Upon skin biopsy, only classic histological findings of DLE, such as follicular plugging, increased mucin, basement membrane thickening and perivascular lymphocytic inflammation with involvement of adnexal elements were observed.^[@bibr6-2050313X19882843]^ This knowledge had clear implications on our choice of treatment and further emphasizes the importance and near-necessity of performing skin biopsies and subsequent histopathological analysis when using other mainstay methods of diagnosing cutaneous maladies.

Cutaneous lupus erythematosus is an autoimmune skin disease favoring sun exposed skin. The most common form is DLE. Similar to other subtypes of lupus, DLE may rarely follow the lines of Blaschko. We report here, for the first time, a case of linear DLE mimicking *en coup de saber* morphea.

**Declaration of conflicting interests:** The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

**Funding:** The author(s) received no financial support for the research, authorship, and/or publication of this article.

**Informed consent:** Patient consent was obtained for publication of this case report.
