Pityriasis Lichenoides et Varioliformis Acuta

Case Overview

By Dr. Danielle Wehle, Sagis Diagnostics
Educational dermpath case series for dermatology residents

Patient: 20-year-old female
Lesion Location: Trunk

A biopsy was performed of a diffuse rash on the trunk of this young woman. Take a moment to review the histologic images below to see how Dr. Wehle arrived at this diagnosis.

(Low-power histologic view of diffuse rash biopsy)

Key Histologic Findings

On examination, several defining features stand out:

• Wedge-shaped superficial and mid-dermal perivascular lymphocytic infiltrate
• Necrotic keratinocytes in the epidermis with overlying parakeratosis
• Erythrocyte extravasation into the epidermis and superficial dermis

(Medium power showing wedge-shaped perivascular lymphocytic infiltrate)

(High power showing necrotic keratinocytes and erythrocyte extravasation into the epidermis)

Differential Diagnosis: Three Common Mimickers

When evaluating a papular rash with a wedge-shaped infiltrate like this, several differentials come to mind. Let’s walk through the three most likely contenders and highlight what sets them apart.

1️⃣ Fixed Drug Eruption

Etiology: Hypersensitivity reaction to a medication; reproducible at the same anatomic site with re-exposure

Histology:

• Band-like (lichenoid) interface dermatitis at the dermoepidermal junction
• Melanin pigment (melanophages) in the papillary dermis — a key distinguishing feature
• Necrotic keratinocytes may be present
• No significant erythrocyte extravasation into the epidermis

Key Distinction:

Fixed drug eruption can be confused with PLEVA due to overlapping interface changes, but the presence of prominent band-like interface dermatitis and melanin pigment deposition are characteristic of FDE. Critically, erythrocyte extravasation into the epidermis — a hallmark of PLEVA — is absent in fixed drug eruption.

2️⃣ Arthropod Bite Reaction

Etiology: Local hypersensitivity reaction to insect or arthropod venom/saliva

Histology:

• Wedge-shaped superficial and deep perivascular infiltrate
• Numerous eosinophils — often a prominent and diagnostically important component
• Variable plasma cells and lymphocytes
• No significant epidermal necrosis or necrotic keratinocytes

Key Distinction:

Both arthropod bite reactions and PLEVA display a wedge-shaped perivascular infiltrate, making them architectural mimickers. However, arthropod bites show numerous eosinophils and do not have necrotic keratinocytes in the epidermis. In PLEVA, eosinophils are not a significant feature, and epidermal damage is defining.

3️⃣ Lymphomatoid Papulosis (LyP)

Etiology: Recurrent, self-healing CD30+ T-cell lymphoproliferative disorder; low-grade malignancy with benign behavior

Histology:

• Wedge-shaped perivascular and interstitial infiltrate
• Large atypical lymphoid cells with hallmark CD30 positivity
• Background mixed inflammatory infiltrate (lymphocytes, histiocytes, eosinophils, neutrophils)
• Epidermal ulceration may occur; less pronounced epidermal necrotic keratinocytes than PLEVA

Key Distinction:

Lymphomatoid papulosis shares a wedge-shaped infiltrate and self-limited papular course with PLEVA, making histologic distinction critical. The key differentiator is the presence of large atypical cells that are CD30 positive in LyP — absent in PLEVA, which is comprised predominantly of CD8 positive T-lymphocytes. Furthermore, LyP does not show the prominent epidermal changes of PLEVA. Immunohistochemistry is essential when this differential is on the table.

The Final Diagnosis:

Pityriasis Lichenoides et Varioliformis Acute (PLEVA)

The combination of a wedge-shaped perivascular lymphocytic infiltrate, necrotic keratinocytes in the epidermis with overlying parakeratosis, and erythrocyte extravasation into the epidermis and superficial dermis is diagnostic of PLEVA. This triad, together with the clinical picture of a young patient with a diffuse papular rash on the trunk, clinches the diagnosis. PLEVA commonly affects patients under 30 years of age, with lesions involving the trunk and proximal extremities presenting as red-brown, mildly pruritic papules that may form blisters, crusts, or ulcers.

Key Takeaways for Residents:

• Always evaluate papular rashes with a wedge-shaped infiltrate at high power — the epidermal changes make the diagnosis.
• Erythrocyte extravasation into the epidermis is the key feature that separates PLEVA from most mimickers — look for it specifically.
• When LyP is in the differential, order CD30 immunohistochemistry — positivity in large atypical cells confirms LyP over PLEVA.
• PLEVA is self-limited in most cases; accurate diagnosis prevents unnecessary workup for lymphoma or systemic disease.

📚 Quick Summary

💬 Final Thought

PLEVA is a classic dermatopathology “wedge-shaped mimic” that rewards careful microscopic examination at every power level. Remember: the triad of wedge-shaped infiltrate + epidermal necrosis + erythrocyte extravasation tells the story — and immunohistochemistry closes the case when LyP is in the differential.

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