Photosensitivity is the amount to which an object reacts upon receivingphotons, especiallyvisible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished,photoallergy andphototoxicity.[1][2] Thephotosensitive ganglion cells in the mammalian eye are a separate class of light-detecting cells from thephotoreceptor cells that function in vision.
Sensitivity of theskin to a light source can take various forms. People with particular skin types are more sensitive tosunburn. Particular medications make the skin more sensitive to sunlight; these include most of thetetracycline antibiotics, heart drugsamiodarone, andsulfonamides.Some dietary supplements, such asSt. John's Wort, include photosensitivity as a possible side effect.
Particular conditions lead to increased light sensitivity. Patients withsystemic lupus erythematosus experience skin symptoms after sunlight exposure; some types ofporphyria are aggravated by sunlight. A rare hereditary conditionxeroderma pigmentosum (a defect in DNA repair) is thought to increase the risk of UV-light-exposure-related cancer by increasing photosensitivity.
Photosensitivity occurs in multiple species includingsheep,bovine, andhorses. They are classified as primary if an ingested plant contains a photosensitive substance, likehypericin inSt John's wort poisoning and ingestion of biserrula (Biserrula pelecinus)[3] in sheep, orbuckwheat plants (green or dried) in horses.[4]
Inhepatogenous photosensitization, the photosensitzing substance isphylloerythrin, a normal end-product ofchlorophyll metabolism. [5] It accumulates in the body because of liver damage, reacts with UV light on the skin, and leads to free radical formation. These free radicals damage the skin, leading to ulceration, necrosis, and sloughing. Non-pigmented skin is most commonly affected.
