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Group of disorders where skin reactions are triggered by light. Rare in children, often with different etiologies than adults. Photosensitivities may be classified as:
- Chemical and drug-induced photosensitivity: substances altered by light into toxins
- Genetic disorders: abnormal DNA repair of normal light induced damage, for example, xeroderma pigmentosum
- Photoaggravated dermatoses: skin diseases exacerbated by light, for example, systemic lupus erythematosus (SLE)
- Idiopathic/immunologically mediated photodermatoses: Polymorphous light eruption (PMLE), most common photosensitivity, is thought to have an immunologic basis.
- Photosensitivity resulting from pigment loss: for example, albinism
- Increased incidence of photosensitivity disorders in some populations, for example, Bloom syndrome in Ashkenazi Jews, actinic prurigo (AP) in American Indians and Hispanics
- Age of onset is variable for each disorder.
- Onset in infancy: congenital erythropoietic porphyria (CEP), neonatal lupus erythematosus
- Rarely, CEP is discovered when neonates with this disorder are given phototherapy for jaundice and develop severe blistering skin lesions.
- Onset in childhood: albinism, hydroa aestivale, hydroa vacciniforme, certain porphyrias, xeroderma pigmentosum, Hartnup disease, poikiloderma congenitale, Bloom, Rothmund-Thomson, and Cockayne syndromes
- Onset in adults, but can occur in childhood: vitiligo, drug-induced photosensitivities, PMLE, SLE, porphyria cutanea tarda, and pellagra
- Family history
- Positive in 3–56% of PMLE
- Preexisting skin disease, for example, atopic dermatitis, SLE
- Exposure to plants, drugs, or toxins
Genetic disorders include the porphyrias, DNA repair defects, and other biochemical disorders.
- Most genetic disorders are autosomal recessive. The various porphyrias have variable inheritance patterns.
Findings are diverse for the different disorders and rarely diagnostic.
- Photosensitivity may be due to multiple mechanisms.
- Phototoxicity: Light alters chromophore substances in the skin into a toxic-reactive form. Skin reaction may occur on first exposure.
- Photoallergy: Light alters substances in the skin to become antigenic. Skin reaction occurs only after multiple exposures.
- Can be type 1 histamine-mediated and occurs immediately after exposure, for example, solar urticaria (SU)
- Can be type 4 cellular immune-mediated and occurs hours to days after exposure, for example, PMLE
- Inability to repair DNA damage, for example, xeroderma pigmentosum
- Loss of natural ultraviolet (UV) skin protection (melanin), for example, albinism, vitiligo, Hermansky-Pudlak syndrome
- Although light induces the skin abnormalities in photosensitivity, in some instances, repeated light exposure may suppress the skin reaction.
- This may be due to the immunosuppressive effect of light exposure.
- Can occur spontaneously. Skin eruption may occur in early spring with first sun exposure but diminish with repeated exposure, for example, juvenile spring eruption (JSE).
- This phenomenon can be used therapeutically by administering UV light to patients with PMLE and others.
- Combination of sunlight with some abnormality in the skin such as loss of pigment, a chemical agent, a metabolic product, another skin disorder, a genetic disease, or an unknown factor produces a cutaneous abnormality.
- Specific wavelengths of the radiant energy emitted by the sun and reaching the earth are usually responsible for each photosensitivity disorder, most commonly UVB (290 to 320 nm), or UVA (320 to 400 nm). Commercially available sunscreens target these wavelengths. Some disorders, such as the porphyrias, are triggered by visible light (400 to 800 nm), thus are difficult to prevent with sunscreens. β-carotene has been used as a protective agent in some of the porphyrias due to its ability to absorb in the visible light spectrum.