Phytophotodermatitis is a specific form of phototoxic or photoirritant contact dermatitis. The offending agent is a plant species from one of a few specific families. This form of dermatitis has an insidious onset and is typically preceded by little to no inflammation. This can make the diagnosis difficult for the clinician. Recognition of the key clinical features and the species of plant involved help make the diagnosis.
Clinical Findings: Phytophotodermatitis is caused by certain species of plants that come into contact with the skin. Lone contact with skin is not enough to cause the inflammatory reaction and subsequent postinflammatory hyperpigmentation: After exposure to the plant material, there is a time frame during which the exposed area must be introduced to ultraviolet radiation. It is the plant oils and resins in combination with the correct ultraviolet source that leads to the characteristic rash.
The most typical clinical scenario encountered is one in which the patient comes into contact with a plant that contains a psoralen compound. One of the most frequently reported causes is the juice of a lime (Citrus aurantifolia). This plant is categorized within the Rutaceae family. The Rutaceae family is the most wide spread family of plants that have been described to cause these types of reactions, with the lime being by far the most common offender.
Patients often describe the use of a lime in a mixed drink while vacationing on the beach. The lime juice contacts the skin, and when the skin is exposed to a specific threshold of ultraviolet light, the reaction develops. Most often, patients do not complain of any acute symptoms. If the reaction is severe, burning occurs acutely and the diagnosis is relatively straight forward. However, most reactions are subtle and do not appear for a few days to weeks. Patients typically return home from vacation and notice a subtle hyperpigmentation around the mouth or scattered on the body where they have splashed or consciously applied the juice from a lime during sun bathing. The hyperpigmentation may last for months to years. On rare occasions, a severe acute reaction occurs with red plaque and vesicle formation.
The many families of plants capable of initiating this type of reaction all contain the chemical psoralen. Psoralen is a potent photosensitizer that is used clinically. Once purified, it can be given orally in the form of psoralen + ultraviolet A light (PUVA) therapy or painted on for topical PUVA therapy. It is especially helpful for treating refractory hand and foot dermatoses.
The areas of involvement are typically asymptomatic and do not show any overt inflammatory features. They appear as hyperpigmented, irregularly shaped macules on the skin. These spontaneously remit over a few months. Many plants are capable of producing the reaction.
Pathogenesis: Almost all of the plants responsible for phytophotodermatitis come from four specific families: Umbelliferae, Rutaceae, Moraceae, and Leguminosae. These plants all contain potent photosensitizers in varying concentrations. The chemicals responsible for photosensitization are the furocoumarins; more specifically, the psoralens are by far the most important of the photosensitizer chemicals. On contact, the psoralen penetrates the skin. Subsequent exposure to ultraviolet A light in the spectrum of 320 to 400 nm causes pyrimidine dimers to form within the DNA strands, which act to interrupt DNA synthesis. The psoralen and ultraviolet light also can cause hyperpigmentation (tanning).
Histology: The pathological features are dependent on the timing of the biopsy. An acutely inflamed lesion shows a superficial perivascular lymphocytic infiltrate and dermal edema with apoptotic keratinocytes within the epidermis. Late lesions show melanophages within the dermis.
Treatment: Acute areas of involvement can be treated with topical corticosteroid creams. The main issue in management is dealing with the prolonged postinflammatory hyperpigmentation. No therapy has been shown to be helpful, but almost all reactions resolve slowly over time. Care should be taken not to perform a treatment that might lead to a worse cosmetic outcome.