Laboratory Studies
Phototoxicity testing is not carried out diagnostically, but rather for predictive purposes. It routinely is included in the safety evaluation of raw materials by the Research Institute for Fragrance Materials and several methods for identifying phototoxic compounds have been reported. Both in vitro and in vivo methods are used currently. Generally, for in vivo testing, measured amounts of fragrance material are tested, either in laboratory animals (eg, mouse, rabbit, guinea pig models), [7] or ultimately in humans, with an artificial light source. This identifies potential phototoxic substances before they are marketed.
In an attempt to decrease animal use in predictive dermatology, the European Union, in cooperation with the European Centre for the Validation of Alternative Methods (ECVAM) and the Interagency Coordinating Committee for the Validation of Alternative Methods (ICVAM), has supported the development of in vitro alternatives. [8, 9, 10, 11] Initial trials revealed reasonable sensitivity and specificity [12, 13, 14, 15] ; false-positive results and false-negative results have already been documented. [16, 17, 18] Thus far, several cosmetic products have been examined in vitro for phototoxicity. [19]
Other Tests
Clinical identification of phototoxicity largely resides in morphology and a high index of clinical suspicion. Photopatch testing may be performed if photoallergy is strongly suspected. This consists of occlusive application of the test chemical(s) to the back, followed by irradiation with an UV light source. The results are evaluated at several time intervals, according to an established score based on the skin reaction pattern. Adequate controls are imperative to differentiate phototoxicity from photoallergy. In phototoxicity, all controls will have a positive response, whereas in photoallergy, controls should be negative.
Histologic Findings
The histopathological findings in berloque dermatitis are identical to other phototoxic reactions, an irritant cutaneous response. The epidermal changes consist of keratinocyte necrosis, intercellular and intracellular edema, and intraepidermal blisters. In severe cases, these blisters may rupture, resulting in subepidermal bullae. Neutrophils enter the epidermis at an early stage. In contrast to the extensive epidermal damage, only a mild perivascular infiltrate is present. Changes associated with berloque pigmentation are an increased number and size of melanosomes, melanocyte hypertrophy with increased arborization of dendrites, increased transfer of melanosomes to keratinocytes, and increased tyrosinase activity within the proliferating melanocytes.
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Hyperpigmented streaks on the dorsa of hands of a patient with bergapten phototoxicity
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Molecular structure of 5-methoxypsoralen (bergapten)
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Bergapten-containing plants
