Introduction


Photosensitivity or photo contact dermatitis is an intensification in the skins reactivity to sunlight. It is an eczematous condition caused by an interaction between light and a substance present on the skin; it is a light induced contact dermatitis. It may arise as a feature of a systemic disease as seen in lupus erythematosus or come as an unwanted side effect of medications and skincare products. Photosensitivity incurs a variety of localised skin reactions that manifest as cutaneous effects such as redness, lesions, blisters and rash. The epidermis and outer dermal layers of the skin are affected. Photosensitivity divides itself into two different forms- phototoxicity and photoallergy. On the whole the condition is poorly understood.

This image is an example of a skin reactions that can occur with photo sensitivity.
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Phototoxicity


Phototoxicity is an adverse chemical reaction of the skin induced by light. It may be photodynamic (requires oxygen) or non photodynamic. The causative toxin enters the skin systemically or topically and an example of potential causative agents are tetracyline antibiotics, essential oils and plant species such as the Western Poison Oak. A phototoxic substance is rendered toxic by light exposure and symptoms usually manifest within a few minutes or hours of exposure. Phototoxicity precipitates cutaneous effects such as exaggerated sunburn, blistering, cutaneous edema and lesions. It occurs primarily in humans but also to lesser extents in certain animals and plants.

This image shows a skin reaction to a phototoxic compound
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Light of the UVA (320-400nm) range is more likely to cause a photosensitivity reaction. Phototoxic compounds typically contain an aromatic ring or a resonating double bond, both of which demonstrate a capacity to absorb radiant energy. When a compound is photo activated electrons gain energy and are elevated to a higher energy state, as they revert to a more stable configuration they release energy and this results in the formation of reactive oxygen intermediates such as hydrogen peroxide. Reactive oxygen species, or free radicals, can then damage cellular macromolecules and cause cell death. Frequently the cell membrane becomes damaged in photo toxic reactions however it is possible that DNA becomes damaged. The body responds by activating signal transduction pathways which produce immune mediators such as pro inflammatory cytokines. These activities manifest clinically as the skin conditions such as exaggerated sunburn that are associated with this pathology.

This image illustrates the uptake of dopamine into cells and the subsequent formation of a reactive oxygen intermediate which is phototoxic in nature
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Photoallergy


Photoallergy is a type IV immune response to a photo activated compound with delayed hypersensitivity. Photoallergy is a form of allergic contact dermatitis which is the most common form of immunotoxicity in humans. While phototoxicity and photoallergy manifest clinically with a very large degree of similarity, with shared symptoms of redness, lesions, localised swelling and itchiness, the latter is much less common concurring with an estimated 10- 20% of photosensitivity cases. It also differs in that symptoms tend to have a delayed onset which differs to the immediacy of those in phototoxicity.The immunological reaction to exogenous substances is typically by light of the U.V. A type. Administration of the offensive substance may be topical or systemic. Associated allergens include cobalt chloride, neomycin, gold, topical anaesthetics, St Johns wort (plant) and steroid creams.

This image shows some of the cutaneous effects of photoallergy:
CASPD_photoallergic_reaction.jpg

Allergens in photoallergy exist as soluble haptens (small molecules that bring about an immune response when attached to carrier molecules), which enables them to pass through the skins stratum corneum due to a low molecular weight. The condition arises out of interactions between immunoregulatory cytokines and select T- cells.
Light creates a complete antigen as the allergen binds directly to an epidermal protein or is conjugated to an epidermal protein. Langheran cells (dendritic cells of the skin) recognise the complex as a foreign protein and take the photoactivated compound or antigen to lymph nodes. In lymph nodes, langheran cells diffrentiate to become dendritic cells which have the capacity to stimulate the immune system. These cells then present the antigen to T- lymphocytes which become activated and migrate to the site of deposition which begins an inflammatory response. Clonal multiplication of T-lymphocytes allows for a faster immune response with subsequent exposures to the allergen. Photoallergy is different to phototoxicity in that a dose of a substance too low to elicit a toxic response in phototoxicity may cause a photoallergy. Photoallergy may arise out of overexposure to sunlight alone or allergens present in cosmetic products and medicines. Halogenated salicylanilides which were commonly found in soap have been cited as a major source of allergens in the 1960s for example. Today, substances present in sunscreens are attributed as a common cause of this disease.

Spongiosis (inter-cellular edema) manifests clinically as lesions
photoallergic-dermatitis2.jpg

Diagnosis


It's close resemblance to sunburn means the condition is often misdiagnosed. The condition is accurately detected using photo patch testing. In photo patch tests a number of allergens are placed in contact with healthy skin in the presence of light. The skin is then monitored for photosensitive reactions in order to identify what compounds are causing the skin condition. Early diagnosis is preferential as if the photosensitiser goes undetected and is not avoided there is a risk of developing a chronic form of the disease in question.

Treatment


Mortality is rare with photosensitivity but there exists evidence of a higher incidence of cancer among sufferers. Prognosis is in most cases highly favourable. Treatment centres on establishing and then avoiding the chemical responsible while adopting protective measures against the sun. The symptoms of the disease can also themselves be treated. Antihistamines are typically used to combat photoallergy. In acute cases of photosensitivity systemic corticosteroids and oral immune suppression might be used. Photo sensitivity conditions usually clear up quickly once the photosensitiser has been identified and it's use is discontinued. With photoallergy, symptoms can in rare cases persist for up to one month after the offending substance's use has been discontinued.

Photosensitivity MCQ


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MCQ answers
https://docs.google.com/spreadsheet/ccc?key=0AlPeQr-cV6KudFRGWjhySTltNlpaTHNSNk80WVZjTUE#gid=0

References:


Photosenitivity


Information
Casarett and Doull's Toxicology: The Basic Science of Poisons, 7th edition, C.D. Klassen
Drug induced photosensitivity. Alexandra Y Zhang, Expert Opinion on Drug Safety 2007
Delivery of photosensitizers for photodynamic therapy. Journal of Photochemistry and Photobiology 2001
Drug-induced cutaneous photosensitivity: incidence, mechanism, prevention and management. Moore D.E. Drug Safety 2002.

Image
http://www.beltina.org/health-dictionary/photosensitivity-rash-treatment.html

Phototoxicity


Information
Casarett and Doull's Toxicology: The Basic Science of Poisons, 7th edition, C.D. Klassen
Toxicology, Gary D. Osweiler
Drug induced photosensitivity. Alexandra Y Zhang, Expert Opinion on Drug Safety 2007
Phototoxicity and photoallergy. Epstein JH, Seminars in Cutaneous Medicine and Surgery, December 1999
Drug-induced photoallergic and phototoxic reactions. Stein KR, Scheinfeld NS, Expert opinion on Drug Safety 2007

Image
http://freakdeluxe.co.uk/sun-smart-year-round/
http://www.nature.com/nmat/journal/v5/n5/full/nmat1642.html

Photoallergy


Information
Casarett and Doull's Toxicology: The Basic Science of Poisons, 7th edition, C.D. Klassen
Phototoxicity and photoallergy. Epstein JH. Seminars in Cutaneous Medicine and Surgery, December 1999
Drug-induced photoallergic and phototoxic reactions. Stein KR, Scheinfeld NS. Expert Opinion on Drug Safety 2007

Image
http://dermaamin.com/site/histopathology-of-the-skin/68-p/2029-photoallergic-dermatitis-.html
http://www.medicinenet.com/script/main/art.asp?articlekey=113592

Diagnosis

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Treatment

http://www.vashonorganics.com/WSWrapper.jsp?mypage=treatment_symptoms_photosensitivity_reactions.htm