What is photosensitivity?
Photosensitivity (or sun sensitivity) is inflammation of the skin induced by the combination of sunlight and certain medications or substances. This causes redness (erythema) of the skin and may look similar tosunburn. Both the photosensitizing medication or chemical and light source have to be present in order for a photosensitivity reaction to occur.
Generally, these reactions can be divided into two mechanisms, 1) phototoxic reactions and 2) photoallergic reactions. Phototoxic drugs are much more common than photoallergic drugs.
What is the difference between a photoallergic and a phototoxic reaction?
Phototoxic reactions
In phototoxic reactions, the drug may become activated by exposure to sunlight and cause damage to the skin. The skin’s appearance resembles sunburn, and the process is generally acute (has a fast onset). Ultraviolet A (UVA) radiation is most commonly associated with phototoxicity, but ultraviolet B (UVB) and visible light may also contribute to this reaction.
Rash from a phototoxic reaction is mainly confined to the sun-exposed area of the skin. A phototoxic reaction typically clears up once the drug is discontinued and has been cleared from the body, even after re-exposure to light.
Photoallergic reactions
In photoallergic reactions, the ultraviolet exposure changes the structure of the drug so that is seen by the body’s immune system as an invader (antigen). The immune system initiates an allergic response and cause inflammation of the skin in the sun-exposed areas. These usually resemble eczema and are generally chronic (long-lasting). Many drugs in this family are topical drugs.
This type of photosensitivity may recur after sun exposure even after the drug has cleared from the system and can sometimes spread to areas of the skin unexposed to the sun.
What are the symptoms of sun sensitivity (photosensitivity)?
Symptoms of phototoxic reaction
Individuals with phototoxic reactions may initially complain of a burning and stinging sensation. Then the redness typically occurs within 24 hours of the exposure to sun in the exposed areas of the body such as the forehead, nose, hands, arms, and lips. In severe cases, the sun protected areas of skin may be also be involved.
The range of skin damage may vary from mild redness to swelling to blister formation (bullae) in more severe cases. The rash from this photosensitivity reaction usually resolves with sloughing off (desquamation) of the affected area within several days.
Symptoms of photoallergic reactions
Individuals with photoallergic reactions may initially complain of itching (pruritus). This is then followed by redness and possibly swelling and eruption of the involved area. Because this is considered an allergic reaction, there may be no symptoms for many days when the drug is taken for the first time. Subsequent exposure to the drug and the sun may cause a more rapid response in 1-2 days.
Hyperpigmentation after reaction
Hyperpigmentation (darkening) of the affected area of the skin may develop after the resolution of a phototoxicity reaction, but it is rare in a photoallergic reaction. In phototoxic reactions, high doses of the drug and long exposures to light may be required to cause the reaction.
