In the Chair: Lines That Don’t Lie

When a linear eruption follows Blaschko lines, the differential is narrow—but the distinctions matter. Conditions that share this distribution can look clinically similar enough to be confused yet differ histologically enough to require a biopsy.¹ The wrong call means months or years of misdirected treatment.

This case involves a 15-year-old boy with a 2-year history of progressive linear plaques along his right upper extremity—a presentation that was misdiagnosed repeatedly before reaching a dermatology referral center. The diagnosis that finally fits is rare, specific, and anchored to a set of histopathologic features that distinguish it from its closest mimics.²

Three clinical decision points follow: building the differential from the distribution, interpreting the biopsy, and confirming the diagnosis is cutaneous only. The vaccine history is noted as context; its causal role remains uncertain, and the diagnostic reasoning holds regardless of trigger.

THE PATIENT

A 15-year-old boy presents with a 2-year history of pruritic, erythematous, and scaly plaques on his right upper extremity. The eruption began approximately 1 month after a routine vaccination and started at the injection site on the right arm, gradually spreading in a linear, distal pattern along the lateral arm, the medial and lateral forearms, and the dorsal hand. Lesions have worsened over the past 6 months.

On examination:

• Linear erythematous scaly plaques strictly following Blaschko lines
• Areas of cutaneous atrophy
• Dyspigmentation
• Localized hair loss within the affected skin
• No nail involvement
• No photosensitivity
• No systemic symptoms (no fever, arthralgia, myalgia, or muscle weakness)
• No mucosal lesions
• Preserved sweating; no sensory or neural deficits

He has tried multiple topical therapies without a satisfactory response.

Clinical Decision Point 1: Building the Differential

The distribution is the first and most important clinical signal: unilateral, linear, strictly following Blaschko lines on the arm and hand. The lesions are erythematous and scaly with atrophy and localized hair loss. No comedones, no violaceous hue, no Auspitz sign. Pruritus is present but not the dominant feature.

📋 WHAT WOULD YOU DO?

Clinical Decision Point 2: Interpreting the Biopsy

A skin punch biopsy is performed. Histopathological analysis returns the following findings:

• Lichenoid interface dermatitis with epidermal acanthosis
• Focal parakeratosis
• Follicular plugging
• Basal vacuolar degeneration with Civatte bodies
• Vacuolar changes of follicular structures
• Deep periadnexal lymphoplasmacytic infiltrate
• Prominent dermal mucin deposition

PAS and ZN stains are not performed because the clinical picture does not suggest fungal or mycobacterial etiology. Lupus band testing is not available at this center.

⚠ Histopathologic Clue

Dermal mucin deposition is a key feature of lupus erythematosus, not typically seen in linear lichen planus (LP), blaschkitis, linear psoriasis, or inflammatory linear verrucous epidermal nevus (ILVEN).

Its presence, alongside lichenoid interface dermatitis, follicular plugging, and Civatte bodies, is the pivotal finding that separates linear cutaneous discoid lupus erythematosus (LCDLE) from its closest histologic mimics.

📋 WHAT DO YOU THINK?

Clinical Decision Point 3: Ruling Out Systemic Involvement

LCDLE is confirmed. Before finalizing the treatment plan, you need to determine whether this presentation is purely cutaneous or warrants a systemic lupus erythematosus (SLE) workup.

The patient has no systemic symptoms:

• No fever
• No arthralgia
• No photosensitivity
• No oral ulcers
• No fatigue

📋 WHAT WOULD YOU DO?

Clinical Takeaways

• A linear eruption following Blaschko lines is a distribution, not a diagnosis.
• Biopsy is required to differentiate LCDLE from blaschkitis, linear LP, lichen striatus, linear psoriasis, and ILVEN.
• Dermal mucin deposition is the single most important histopathologic feature distinguishing LCDLE from linear LP.
• Follicular plugging and deep periadnexal lymphoplasmacytic infiltrate further support LCDLE over linear LP.
• Clinical clues favoring LCDLE include cutaneous atrophy, dyspigmentation, and localized hair loss.
• LCDLE does not progress to SLE, but systemic exclusion workup remains mandatory.
• G6PD screening should be performed before initiating hydroxychloroquine.
• LCDLE generally responds favorably to antimalarials combined with topical corticosteroids.

The Takeaway

This patient’s diagnosis was available from the beginning: a unilateral linear eruption following Blaschko lines, with atrophy, dyspigmentation, and localized hair loss, progressing distally over 2 years without systemic involvement.

What was missing was the structured differential and the biopsy to resolve it.

Blaschko line distribution narrows the field significantly; atrophy and hair loss narrow it further. Dermal mucin on histopathology closes it.

For dermatology clinicians, the practical lesson is in the sequence:

1. Distribution
2. Biopsy
3. Systemic workup

LCDLE is rare enough that it may not surface immediately on the differential, but the histopathologic feature that confirms it, dermal mucin deposition, is specific enough that it should be recognized when it appears.

And for a patient about to start hydroxychloroquine, G6PD screening is a nonnegotiable step in the workup, regardless of diagnostic confidence.

The misdiagnosis in this case lasted 2 years. The biopsy that ended it took a week.

References

1. Jackson R. The lines of Blaschko: a review and reconsideration: observations of the cause of certain unusual linear conditions of the skin. Br J Dermatol. 1976;95(4):349-360. doi:10.1111/j.1365-2133.1976.tb00835.x
2. Saberi F, Ghanadan A, Razavi Z, Azhari VS, Akhdar M, Al-Zahawi S. The first case of linear cutaneous lupus erythematosus following Covid-19 vaccination: a case report. Clin Case Rep. 2026;14:e72703. doi:10.1002/ccr3.72703