Acute skin reaction in the form of a rash, blisters and crusts, erythema and edema may be due to drug photosensitivity in a patient - the intake of certain drugs that, in combination with ultraviolet radiation, cause serious dermatological manifestations. Every year new drugs are added to the list of drugs that can cause skin reactions, so general practitioners need a thorough knowledge of potentially photosensitizing drugs.
Drug photosensitivity is an adverse skin reaction to the interaction in the skin of a chemical substance (drug) and non-ionizing electromagnetic radiation (light). A drug sensitizer can reach the skin after topical application or systemic ingestion through the digestive system. The term "drug" in this case refers not only to chemicals used for therapeutic purposes, but also to cosmetics, food preservatives, agricultural and industrial substances.
Drug photosensitivity reactions are divided into two types - photoallergic and phototoxic.
Drug photoallergy involves an immunological reaction. The skin reaction is caused by light, and its pathogenesis resembles that of allergic contact dermatitis. The drug in the skin absorbs photons and then turns into a stable or unstable photoproduct that interacts with the protein to form a complete antigen.
A phototoxic reaction resembles an irritant reaction. It occurs in 100% of the population with a high degree of probability if sufficient doses of drugs are prescribed and if light of the appropriate wavelength is available. Since any drug can be activated by light with a specific wavelength of the absorption spectrum, the photosensitivity reaction is characterized by a specific spectrum of action. The action spectrum of the photosensitive reaction is most often in the ultraviolet A (UVA) range.
Clinical characteristics and course
Photoallergic drug reactions
Such reactions are much less common than phototoxic ones. Photoallergy occurs only in previously sensitized individuals. Skin rashes usually appear 48 hours after the interaction between chemicals and light. Typically, photoallergic drug reactions are clinically characterized by an eczematous response that resembles normal allergic contact dermatitis. The primary lesions are vesicles and blisters, followed by crusts and desquamation. Skin rashes are usually localized on open areas of the body and sometimes capture adjacent areas that are closed from exposure to light.
The clinical course depends on how long the photosensitizer has been in the skin, as well as on possible cross-interactions with other photosensitizers. Usually, skin lesions last for 1 week after the medication is stopped. However, sometimes the skin reaction persists and becomes chronic (persistent light reaction).
Phototoxic drug reactions
Clinical signs are limited exclusively to the area exposed to light (face, ears, décolleté, hands, extensor surfaces of the forearms). The reaction appears immediately after exposure to UVA and is characterized by burning and soreness, erythema, edema, or vesiculation (as occurs with some dyes). Delayed reactions may sometimes occur 8-24 hours after exposure to light.
Reactions to some phototoxic drugs (eg psoralens or bergamot oil) are characterized by severe hyperpigmentation.
Diagnosis and general principles of treatment of drug photosensitivity
To clearly assess the etiological role of sunlight, a careful assessment of the appearance of the rash and its relationship to light exposure should be made. The diagnosis, together with the action spectrum, can in some cases be formulated only on the basis of an anamnesis entry.
Anamnesis also helps to know:
- latent period between exposure to light and skin reaction;
- a potentially photosensitizing drug has been applied systemically or topically;
- presence of family members with photosensitivity diseases.
Some drugs can cause photoallergic and/or phototoxic reactions, and most systemic drugs that cause photoallergy also cause phototoxicity (these different reactions cannot always be clinically distinguished). It is likely that the mechanism of action in this case depends on the drug and UV doses, and whether the drug was taken orally or applied topically (for example, systemic photoallergic reactions are much less common than local ones). Moreover, given the same potential for sensitization, drugs that are prescribed more frequently exhibit their sensitizing properties more often than drugs that are rarely prescribed.
General principles for the treatment of drug photosensitivity
As a first major step in the treatment of patients with drug photosensitivity, exposure to either the sensitizer or light, or both, must be discontinued. Since the light is more difficult to control, the chemical is usually eliminated.
There are many sensitizing drugs. These include antibiotics, nonsteroidal anti-inflammatory drugs, diuretics, antimitotic drugs, psychiatric drugs, amiodarone, and fibric acid derivatives. In addition, some fragrances (eg, musk, 6-methylcoumarin, oakmoss) and dyes (eosin, fluorescein dye, methylene blue) have a local photosensitizing effect. New drugs are added to this list each year, so general practitioners need a thorough knowledge of potentially photosensitizing drugs.
Some patients simply need to be reassured and reassured, others may require topical treatments such as other inflammatory dermatoses: water compresses, emollient lotions, and topical corticosteroids. In some cases, high doses of systemic corticosteroids or hospitalization are required.
However, even after drug withdrawal, exogenous substances may remain in the skin for several weeks. Patients with chronic photodermatitis often complain more of itching and loss of sleep than pain. These patients are usually helped by sedatives. In addition, patients should be advised on how often and how to apply sunscreen.
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