Differential diagnosis of photoallergic skin diseases

Percy Lehmann

Department of Dermatology, Allergology and Environmental Medicine, HELIOS Klinikum Wuppertal, University of Witten-Herdecke, Germany

A vast variety of dermatoses must be taken into consideration in order to encompass the differential diagnosis of photoallergy (PA). Inflammatory skin diseases occurring on sun exposed sites are to be considered and differentiated by the clinical picture, history, histology, and phototesting. Classical photoallergic dermatitis is characterized by a pruritic eczematous reaction on sun exposed body areas. Photopatch testing is decisive to detect the culprit photoallergen. Systemic photoprovocative testing may be employed, when a photosensitive drug is suspected to induce the reaction.

Primary and secondary photodermatoses must be ruled out. Polymorphic light eruption (PLE) is characterized by plaques, papules and vesicles, but not classical eczema. Furthermore, PLE occurs only in certain exposed sites and rarely in all sites, unlike PA. Also, PLE tends to ameliorate during the summertime and photopatch testing is negative. Other primary photodermatoses present different clinical pictures: In solar urticaria (SU) urticarial plaques develop within minutes after sun exposure and vanish after cessation of the irradiation in a short period of time. Phototesting leads here quickly to the correct diagnosis. Hydroa vacciniforme occurs in children, while PA does not, and the clinical picture is characterized by a vesicular reaction followed by the development of scars. Actinic prurigo occurs at any age, starting in childhood. It is characterised by a very pruritic reaction, undistinguishable from classical prurigo. Photopatch test is negative. Chronic actinic dermatitis affects mostly old men, sometimes starting with a classical PA. The eczema may generalize and affect also covered body regions. In later stages lichenification is a hallmark of this immensely torturing disease.

The main differential diagnosis in the group of secondary photodermatoses is photoprovoked atopic dermatitis, since here one encounters also an eczematous reaction in sun exposed areas. Here, history and also other characteristic clinical and laboratory signs of atopy lead to the correct diagnosis. Many patients of this group start with a classical atopic dermatitis and develop the photosensitivity only in later stages. Other secondary dermatoses, which can easily be differentiated by history, clinical picture, histology, and phototesting include lupus erythematosus, erythropoetic protoporphyria, dermatomyositis and rosacea. Airborne contact dermatitis (ABCD) may be undistinguishable from PA. Here, history, photopatch test and patch test including plant allergens, will enable the correct diagnosis.

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© Percy Lehmann (text) and Radoslaw Spiewak (source code).
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Document created: 30 August 2009, last updated: 31 August 2009.