Evaluating Photosensitivity: History, Exam, Labs, and Phototesting

Conclusions

• A careful history and physical examination are usually sufficient to diagnose photosensitivity, so phototesting is not needed in most cases.
• The most useful history focuses on timing after exposure, duration, symptoms, distribution, medication and supplement exposures, seasonal variation, and associated systemic features.
• Photosensitive eruptions can be categorized into broad groups such as idiopathic photodermatoses, drug-induced reactions, connective tissue disease, porphyrias, genetic disorders, and photoaggravated dermatoses.
• Photodistribution and morphology are key diagnostic clues, because the pattern of the rash often helps distinguish among PMLE, solar urticaria, drug photosensitivity, chronic actinic dermatitis, and other causes.
• A skin biopsy cannot confirm photosensitivity itself, but it can help classify the underlying pattern or suggest a specific disorder.
• Targeted laboratory testing such as ANA/autoimmune serologies, porphyrin studies, and organ-specific tests is important when the history suggests connective tissue disease, porphyria, or metabolic involvement.
• Phototesting is most useful when the diagnosis is uncertain, when the degree of photosensitivity needs quantification, or when the action spectrum must be determined.
• Phototesting can also be used to provoke and reproduce eruptions such as solar urticaria or PMLE, or to perform photopatch testing for suspected photocontact allergy.
• Different light sources can be used for testing, including daylight, phototherapy units, and visible-light sources such as slide or data projectors.
• Drug photosensitivity often shows reduced MED to UVA or UVB with little or no visible-light reaction, and stopping the culprit medication can lead to gradual resolution.
• Solar urticaria can be demonstrated by visible-light or UV provocation and may require repeated or formal dosing to capture the reaction.
• Chronic actinic dermatitis is associated with marked sensitivity on phototesting, often especially to UVB, and presents as persistent eczematous or lichenified sun-exposed dermatitis.
• Formal phototesting can help explain severe or unusual cases, guide classification, and sometimes uncover an overlooked cause such as hydrochlorothiazide exposure.
• Severe photosensitivity can substantially impair quality of life and may even contribute to suicidal distress, so clinicians should assess psychosocial impact as part of evaluation.
• Overall, the presentation concludes that history, examination, and targeted tests should come first, with phototesting reserved for difficult or confirmatory cases.