What are the common causes of photosensitivity reactions in patients, and how can I differentiate between them?

Photosensitivity reactions in patients commonly arise from two main mechanisms: phototoxic and photoallergic reactions, often triggered by drugs, chemicals, or endogenous conditions ¹ (Lehmann and Schwarz, 2011). Phototoxic reactions are the most frequent and occur when a photosensitising agent absorbs ultraviolet (UV) light, leading to direct cellular damage; these reactions resemble exaggerated sunburn and typically appear within minutes to hours after sun exposure ¹ (Kutlubay et al., 2014). Photoallergic reactions, in contrast, are immune-mediated delayed hypersensitivity responses that usually develop 24–72 hours after exposure and present as eczematous dermatitis, often spreading beyond sun-exposed areas ¹ (Kutlubay et al., 2014). Common causes include certain medications such as tetracyclines, fluoroquinolones, non-steroidal anti-inflammatory drugs (NSAIDs), and diuretics, as well as topical agents like sunscreens and fragrances ¹ (Lugović-Mihić et al., 2017). Endogenous causes include conditions like lupus erythematosus and porphyrias, which also manifest photosensitivity but through distinct pathophysiological pathways ¹ (Lehmann and Schwarz, 2011).

Differentiation between phototoxic and photoallergic reactions relies on clinical history, timing of onset, morphology, and distribution of lesions. Phototoxic reactions occur rapidly after UV exposure, are dose-dependent, and confined to sun-exposed skin, whereas photoallergic reactions have a delayed onset, are not dose-dependent, and may extend beyond exposed areas ¹ (Kutlubay et al., 2014). Patch testing with photoactivation can help confirm photoallergic reactions ¹. Additionally, drug history is crucial to identify potential photosensitisers, and laboratory tests or skin biopsies may assist in diagnosing underlying endogenous photodermatoses ¹ (Lehmann and Schwarz, 2011). Recent literature emphasises the diagnostic challenge posed by drug-induced photosensitivity and suggests a high index of suspicion and multidisciplinary approach for accurate diagnosis and management (Lugović-Mihić et al., 2017).