What is Photosensitivity?
Photosensitivity is an abnormal cutaneous reaction to ultraviolet radiation (UVR) or visible light that occurs at lower doses than would normally cause damage in non-photosensitive individuals. 1, 2
Clinical Definition and Mechanisms
Photosensitivity encompasses two distinct pathophysiologic mechanisms:
Phototoxicity: A non-immunologic reaction where UV radiation activates a photosensitizing substance in the skin, causing direct cellular damage through free radical formation and DNA damage. This is dose-dependent and can occur on first exposure. 3
Photoallergy: An immunologic reaction requiring prior sensitization, where UV radiation converts a chemical into an antigen that triggers a delayed-type hypersensitivity response. This is much less common than phototoxicity. 3
Clinical Manifestations
The presentation varies widely depending on the underlying cause:
Acute phototoxic reactions: Burning, tingling sensation, and erythema resembling exaggerated sunburn in sun-exposed areas, typically appearing within hours of exposure 4, 5
Photoallergic reactions: Eczema-like rash, pruritus, and vesiculation that may extend beyond sun-exposed areas, appearing 24-72 hours after exposure 1, 5
Other patterns: Urticaria (solar urticaria), pigmentation changes (blue-grey discoloration with amiodarone), onycholysis, and blistering 4, 1, 5
Categories of Photosensitive Disorders
Primary (Idiopathic) Photodermatoses
These conditions are entirely caused by solar exposure: 1, 2
- Polymorphic light eruption (PLE): Recurrent pruritic papulo-vesicular eruption appearing within hours of sun exposure, most common primary photodermatosis 1
- Chronic actinic dermatitis (CAD): Persistent eczematous reaction in chronically sun-exposed skin 6, 1
- Solar urticaria: Rapid onset of urticaria after sun exposure 6, 1
- Actinic prurigo: Excoriated papules and nodules on face and limbs, most prominent distally 1
- Hydroa vacciniforme: Discrete erythematous macules evolving into blisters within days of exposure 1
Drug- and Chemical-Induced Photosensitivity
Several hundred substances can invoke photosensitive reactions: 3, 5
- Common culprits: Tetracyclines, fluoroquinolones, NSAIDs, thiazide diuretics, cardiovascular medications (especially amiodarone), and antidepressants 7, 4
- Amiodarone-specific: Causes photosensitivity in over 50% of treated patients, with potential for blue-grey pigmentation in 1-2% with extended sun exposure 4
Photo-Aggravated Dermatoses
Pre-existing skin conditions worsened by UV exposure: 2, 8
Genodermatoses with Photosensitivity
Inherited disorders with deficient photoprotection: 2, 8
Metabolic Photodermatoses
Accumulation of endogenous chromophores: 8
- Porphyrias (congenital enzymatic disorders) and pellagra (acquired) 8
Diagnostic Approach
A detailed history focusing on timing and pattern of reactions, specific triggers, distribution of affected areas, associated symptoms, and medication review is essential. 6
Key diagnostic elements include:
- Phototesting: Minimal erythema dose (MED) determination for UVB and minimal phototoxic dose (MPD) for UVA, performed by dermatology specialists 9, 6
- Laboratory evaluation: Complete blood count, comprehensive metabolic panel, antinuclear antibodies if lupus suspected, porphyrin levels if porphyria suspected 6
- Physical examination: Document distribution and morphology of lesions, signs of chronic photodamage, and ocular involvement 6
Clinical Context in Phototherapy
Understanding photosensitivity is critical when administering UV-based treatments:
- Skin type assessment: Subjective Fitzpatrick classification is not always a reliable predictor of objective erythema sensitivity obtained from MED testing 9
- Individual variability: MED varies by a factor of four in white individuals, while MPD for PUVA varies by at least 10-fold between individuals 9
- Delayed reactions: PUVA erythema peaks at 48-96 hours (or later), requiring treatment intervals of at least 2-3 days 9, 7
Common pitfall: Do not rely solely on skin phototyping to determine starting doses for phototherapy—objective MED or MPD testing is more reliable for preventing severe burns. 9