Answer to Image of the Month January 2016

Submitted by Poonam Panjwani and Inchara YK


Pustular dermatitis

The onset of lesions followed drug intake. On clinicopathologic correlation, this was consistent with a pustular drug eruption.

Microscopy: The epidermis shows normal basket-woven stratum corneum, spongiosis with formation of vesicles. Collections of neutrophils are noted in the subcorneum. Focal vacuolar alteration of basal keratinocytes is noted with scattered necrotic keratinocytes. The dermis shows a moderate perivascular and interstitial infiltrate of lymphocytes and neutrophils. Marked extravasation of RBCs noted along with mild papillary dermal oedema. The PAS stain showed no fungal organisms. Direct IF was negative.
Explanation: The clinical differentials offered in this case were drug reaction, IgA pemphigus and linear IgA disease.
The spectrum of histopathological changes seen in drug reaction is broad and confusing as it has much overlap with many inflammatory diseases. Several histologic criteria and clues have been described to aid a diagnosis of drug reaction.
These are:
1. Signs of acuteness:
-Normal stratum corneum
-Papillary dermal oedema
-Dilated dermal blood vessels clogged with neutrophils
-Extravasated RBCs
2. Vacuolar interface dermatitis
3. Mixture of histopathological patterns on biopsy
4. Constellation of findings not corresponding to any well defined disease.

The most important differential for a pustular drug eruption is pustular psoriasis. Pustular psoriasis differs from the other types of psoriasis by lack of epidermal hyperplasia and commonly has a few eosinophils. Criteria that favour drug eruption over pustular psoriasis are:
- Many eosinophils (especially when present within the pustules). Some eosinophils may also be seen in pustular psoriasis. This case was unusual in that there were no eosinophils.
- Necrotic keratinocytes
- Focal leucocytoclasia
- Deep extension of infiltrate
- Absence of tortuous and dilated dermal blood vessels.

IgA pemphigus is another close mimic and can be differentiated by IF, which shows intercellular deposits of IgA.

Key message: The diagnosis in this case was established on the basis of clinicopathologic correlation. It was further bolstered by the fact that similar lesions reappeared upon re-exposure to the drug.

Suggested further reading:
1. Weyers W, Metze D. Histopathology of drug eruptions-general criteria, common patterns, and differential diagnosis. Dermatol Pract Concept. 2011;1(1):9.
2. Kardaun SH, Kuiper H, Fidler V, Jonkman MF. The histopathological spectrum of acute generalized exanthematous pustulosis (AGEP) and its differentiation from generalized pustular psoriasis. J Cutan Pathol 2010;37(12):1220-29.