Phototoxicity and Photoallergy

Photoreactions: Basics - Sun's Double Trouble

• Phototoxicity
  • Non-immunologic; direct cell damage by UV-activated agent.
  • Dose-dependent (agent & UV). Rapid onset (min-hrs).
  • Resembles exaggerated sunburn.
  • Agents: Tetracyclines, Amiodarone, Psoralens.
• Photoallergy
  • Immunologic (Type IV); UV forms allergen from agent.
  • Requires prior sensitization. Delayed onset (24-72 hrs).
  • Eczematous, itchy; may spread beyond exposed areas.
  • Agents: Topical NSAIDs, sunscreens, fragrances.

⭐ Phototoxic reactions are non-immunological and dose-dependent, while photoallergic reactions are immunological (Type IV hypersensitivity) and require prior sensitization.

Phototoxicity Details - Rapid Rash Reaction

• Mechanism: Non-immunologic; light (esp. UVA) activates a chemical, causing direct cellular damage.
  • No prior sensitization required.
• Onset: Rapid, within minutes to hours of sun exposure.
• Clinical Features: Resembles exaggerated sunburn.
  • Erythema, edema, pain, burning.
  • Vesicles or bullae in severe cases.
  • Strictly confined to sun-exposed skin.
• Dose-Related: Severity depends on drug/chemical dose and light intensity.
• Outcome: Heals with hyperpigmentation; no immunologic memory or cross-reactions.

⭐ Common phototoxic drugs include tetracyclines (especially doxycycline), NSAIDs, amiodarone, and psoralens (PUVA therapy). Phytophotodermatitis (e.g., from lime juice containing furocoumarins) is a classic example.

Photoallergy Details - Itchy Delayed Reaction

• Mechanism: Type IV (delayed) hypersensitivity. UV radiation (UVA > UVB) transforms a chemical (hapten) into an allergen. Requires prior sensitization.
• Onset: Delayed, 24-72 hours post sun exposure.
• Clinical Features:
  • Intensely itchy (pruritic) eczematous eruption.
  • Lesions: Papules, vesicles, scaling; may lichenify chronically.
  • Distribution: Sun-exposed areas, may spread to non-exposed sites.
• Morphology: Resembles allergic contact dermatitis.
• Common Photoallergens:
  • Sunscreens: PABA derivatives, oxybenzone, cinnamates.
  • Fragrances: Musk ambrette, sandalwood oil.
  • Topical NSAIDs: Ketoprofen, piroxicam.
  • Antimicrobials: Sulfonamides, FQs.
  • 📌 Mnemonic: "Some Fine New Agents" (Sunscreens, Fragrances, NSAIDs, Antimicrobials)
• Diagnosis: Photopatch testing (gold standard).
• Management: Identify & avoid allergen; topical/systemic corticosteroids.

⭐ Photopatch testing is the gold standard for diagnosing photoallergy, identifying the specific photoallergen. Common photoallergens include fragrances, sunscreens (e.g., PABA derivatives, oxybenzone), and topical NSAIDs.

Phototoxicity vs. Photoallergy - Twin Teardown Table

High‑Yield Points - ⚡ Biggest Takeaways

• Phototoxicity: Non-immunologic, rapid onset (minutes-hours), like exaggerated sunburn, often painful.
• Photoallergy: Type IV hypersensitivity (immune), delayed onset (24-72h), eczematous, itchy rash.
• Phototoxicity is dose-dependent (drug & UV) and affects anyone; no prior sensitization needed.
• Photoallergy requires prior sensitization; rash can spread beyond sun-exposed areas.
• Key phototoxic drugs: Doxycycline, Amiodarone, Psoralens, Thiazides, Piroxicam.
• Key photoallergic agents: Topical Ketoprofen, sunscreens (PABA), fragrances, Phenothiazines.