Random Dermatology MCQ - Purpuric Contact Dermatitis

A 65-year-old man develops a sharply demarcated, non-blanching, purpuric eruption on his lower legs exactly corresponding to the area where he had applied a new topical analgesic cream containing benzocaine. The rash is asymptomatic and there is no associated scaling or weeping. Which of the following is the most likely diagnosis?

A) Purpuric contact dermatitis
B) Stasis dermatitis
C) Leukocytoclastic vasculitis
D) Senile purpura
E) Lichen aureus

Correct Answer: A) Purpuric contact dermatitis

Explanation

This presentation is classic for purpuric contact dermatitis, a distinct variant of contact dermatitis where the primary manifestation is purpura (extravasated red blood cells) rather than spongiotic vesicles or eczema.

Key Clinical Features of Purpuric Contact Dermatitis:

• Morphology: Non-blanching, purpuric macules and patches (due to petechiae and ecchymoses). It is often non-pruritic and non-tender.

• Distribution: Sharply demarcated to the exact area of contact with the offending agent. The shape often mimics the application pattern.

• Lack of Inflammation: Unlike classic allergic contact dermatitis, there is typically no scaling, weeping, or significant erythema.

• Common Allergens: Textile dyes (especially black rubber and azo dyes), topical medications (like benzocaine in this case), and certain plants.

Pathophysiology:

• It is considered a non-eczematous, allergic contact dermatitis.

• The allergen causes a T-cell-mediated (Type IV) hypersensitivity reaction that targets the dermal blood vessels.

• This leads to increased vascular permeability and erythrocyte extravasation without the typical epidermal spongiosis seen in classic contact dermatitis.

Why Not the Other Options?

• (B) Stasis dermatitis: Presents with erythema, scaling, weeping, and hyperpigmentation on the lower legs in the context of chronic venous insufficiency. It is not sharply demarcated or purely purpuric.

• (C) Leukocytoclastic vasculitis: Presents with palpable purpura (raised lesions) due to inflammation of small vessels. A biopsy would show neutrophilic infiltration and fibrinoid necrosis, which is absent in purpuric contact dermatitis.

• (D) Senile purpura: Caused by fragile blood vessels and solar elastosis in the elderly. Presents with ecchymoses on the extensor forearms and hands, not in a sharply demarcated pattern corresponding to a topical application site.

• (E) Lichen aureus: A form of pigmented purpuric dermatosis characterized by rusty-brown or golden-purple patches, often on the lower legs. It is persistent and not linked to a specific contactant.

Diagnosis and Management:

• Diagnosis: Confirmed by patch testing, which will reproduce the purpuric reaction.

• Management: Identification and avoidance of the causative allergen is the cornerstone of treatment. The purpura will resolve slowly over weeks as the extravasated blood is reabsorbed.

Note: Purpuric contact dermatitis is a great mimicker. The key diagnostic clue is the perfect correspondence between the purpuric eruption and the shape of the contacted object or area of application. It should be suspected in any case of localized, unexplained purpura.