Author: A/Prof Patrick Emanuel, Dermatopathologist, Auckland, New Zealand, 2013.
Polymorphic light eruption (PMLE) is a form of photosensitivity, which usually occurs in younger females. It is more common in patients who receive only intermittent sun exposure and typically consists of crops of papules, vesicles or plaques.
Histology of polymorphic light eruption
In polymorphic light eruption, sections show a superficial and deep perivascularlymphocyticinfiltrate (figure 1). There is often impressive papillary dermaloedema (figures 1, 2, 3). When the oedema is massive the lesions may resemble erythema multiforme clinically. The infiltrate is mainly lymphocytic but there may be intermixed eosinophils, neutrophils, and histiocytes (figure 4). The epidermal changes range from being almost normal to showing impressive spongiosis and acanthosis. There are often lymphocytes in the epidermis (exocytosis, figure 3). An interface dermatitis may be seen and associated apoptotic keratinocytes in the epidermis.
None are generally needed. A mucin stain will be negative and confirm dermaloedema rather than mucin (which would suggest a form of lupus).
Differential diagnosis of polymorphic light eruption
Other light eruptions and eczematous reactions – Photoallergic reactions and contact dermatitis can show a dense lymphocyticinfiltrate to resemble PMLE. The clinical presentation and the presence of massive dermaloedema can be helpful features
Lupus – Lupus will also show a superficial and deep dermatitis but there is also often basement membrane thickening and dermal mucin.
Juvenile spring eruption of the ears – Is likely a form of PMLE.
Erythema multiforme – This generally shows more extensive epidermal necrosis, less spongiosis and the infiltrate often exclusively lymphocytic
Pathology of the Skin (Fourth edition, 2012). McKee PH, J. Calonje JE, Granter SR