Diagnosis: Polymorphous light eruption
Biopsy findings
The biopsy showed extensive spongiosis and edema of the dermis with a deep lymphohistiocytic infiltrate, consistent with polymorphous light eruption—an idiopathic, delayed type hypersensitivity to UVA and UVB light.1-3
Characteristics
Polymorphous light eruption may affect up to 10% of the population, with a predilection for females.3,4 The prevalence increases in northern latitudes. All skin types may be affected, but those with Fitzpatrick skin types I or II are most frequently affected.4
Polymorphous light eruption may appear spontaneously at any age and may manifest as plaques (as was the case with our patient), papules, or rarely, vesicles (Figure 2). It tends to present early in the spring and summer, with the first significant UV exposure of the year. The rash typically develops 1 to 4 days after sun exposure, and with time, remits spontaneously. Occasionally, though, it will last as long as there is significant sun exposure.
FIGURE 2
Polymorphous light eruption
A condition that mimics a phototoxic drug reaction
The differential diagnosis for polymorphous light eruption includes phototoxic drug reaction, systemic lupus erythematosus, and porphyria cutanea tarda.
A phototoxic drug reaction is a non-immunologic reaction that manifests 2 to 6 hours after exposure to sunlight.1-3 It can be difficult to differentiate from polymorphous light eruption without a biopsy. A phototoxic drug reaction can be likened to sunburn, with a mild form causing slight erythema and a severe form causing vesicles or bullae.
Our patient certainly used medicines known to cause phototoxic reactions (ibuprofen, fluphenazine). However, she’d had no reactions with prior sun exposure. The significant dermal edema (hence the plaque) without any vesicles or bullae made a phototoxic drug reaction diagnosis less likely. The biopsy clarified the issue.
Systemic lupus erythematosus
SLE was also a possibility with our patient. Sunlight can precipitate a lupus rash, so the photodistribution made lupus plausible. Its abrupt onset and marked pruritus spoke against it, though, as did the negative serum antinuclear antibody test, which had been ordered.