Dermatology · Drug-Induced Photosensitivity
The facts most likely to be tested
Phototoxic reactions occur when a drug absorbs UV radiation and releases energy into the skin, causing direct cellular damage that mimics an exaggerated sunburn.
Photoallergic reactions are Type IV delayed hypersensitivity responses where UV light alters a drug to act as a hapten, triggering a pruritic, eczematous eruption.
Tetracyclines (especially doxycycline) are the most common cause of phototoxic reactions, presenting with erythema and vesicles in sun-exposed areas.
Fluoroquinolones and sulfonamides are high-yield culprits for drug-induced photosensitivity that appear rapidly after sun exposure.
Amiodarone is a classic cause of persistent photosensitivity that can lead to a characteristic slate-gray or blue-gray skin discoloration.
Thiazide diuretics and NSAIDs are frequently tested medications associated with lichenoid or erythematous photosensitive eruptions.
Sun-exposed areas such as the face, V-neck of the chest, and dorsal hands are the classic distribution for photosensitivity, while submental and retroauricular areas are typically spared.
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A 54-year-old male presents to the clinic with a pruritic, erythematous rash on his face, neck, and the back of his hands. He recently started a new medication for his hypertension and has been spending significant time gardening. Physical examination reveals erythema and edema on the sun-exposed areas of his skin, with sparing of the skin under his chin. He denies any systemic symptoms or recent travel.
Which of the following medications is the most likely cause of this patient's condition?
Hydrochlorothiazide
The patient's presentation of an exaggerated sunburn in sun-exposed areas is classic for a phototoxic reaction, and thiazide diuretics are a high-yield, commonly tested cause of this condition.
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High yield triage
Etiology / Epidemiology
Common in fair-skinned individuals and patients on photosensitizing medications or with porphyria.
Clinical Manifestations
Exaggerated sunburn, sun-exposed areas, sparing of skin folds (shadow areas).
Diagnosis
Phototesting and photopatch testing are the gold standards for diagnosis.
Treatment
Sun avoidance, broad-spectrum sunscreen, and topical corticosteroids for acute inflammation.
Prognosis
Generally self-limiting; chronic exposure increases risk of squamous cell carcinoma.
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Epidemiology & Etiology
Incidence is highest in patients with Fitzpatrick skin types I-II. Primary triggers include tetracyclines, thiazides, and NSAIDs. Underlying metabolic disorders like Porphyria Cutanea Tarda are critical considerations in refractory cases.
Pertinent Anatomy
Reactions are strictly limited to sun-exposed skin (face, V-neck, dorsal hands). The nasolabial folds and submental areas are typically spared due to anatomical shadowing.
Pathophysiology
Phototoxicity occurs when a drug absorbs UV radiation, forming reactive oxygen species that damage cell membranes. Photoallergy is a Type IV hypersensitivity reaction requiring prior sensitization. Both result in keratinocyte apoptosis and inflammatory cytokine release.
Clinical Manifestations
Patients present with erythema, edema, and vesicles appearing minutes to hours after exposure. Sun-exposed areas show sharp demarcation. Red flags include systemic symptoms or blistering covering >20% of body surface area.
Diagnosis
Diagnosis is primarily clinical. Phototesting (reproducing lesions with UV light) is the gold standard to confirm sensitivity. Photopatch testing is used to distinguish between photoallergic and phototoxic etiologies.
Treatment
Immediate management requires sun avoidance and discontinuation of the offending agent. Topical corticosteroids (e.g., triamcinolone) reduce inflammation. Avoid systemic NSAIDs if severe blistering is present. Use broad-spectrum sunscreen (SPF 50+) daily.
Prognosis
Acute reactions resolve within days of trigger removal. Chronic UV damage leads to premature aging and a significantly elevated risk of squamous cell carcinoma.
Differential Diagnosis
Porphyria Cutanea Tarda: blistering and scarring on hands
Systemic Lupus Erythematosus: malar rash sparing nasolabial folds
Polymorphous Light Eruption: delayed papular/vesicular rash
Phytophotodermatitis: linear streaks from plant contact
Solar Urticaria: wheals appearing within minutes