Including the present case, there have been 20 reports of allergic contact dermatitis to MAL (Table 2).1–6 Of these cases, 11 were women (55%) and 9 men (45%). The mean age was 59.25 years (range, 30-80 years).The indications for treatment were actinic keratosis (7 patients, 35%), basal cell carcinoma (6 patients, 30%), actinic keratosis and basal cell carcinoma (3 patients, 15%), necrobiosis lipoidica (1 patient, 5%), porokeratosis (1 patient, 5%), and segmental Darier disease (1 patient, 5%).

The mean number of PDT sessions that patients had received prior to presenting with the dermatitis was 7.55 (range, 2-21 sessions). The condition was considered occupational in only 1 patient, a clinical assistant who worked in a PDT unit.6

Some authors have reported severe reactions with dissemination that occasionally required hospitalization and treatment with systemic corticosteroids.1–3 Others reported less severe, localized reactions.6

Patch tests with Metvix as is were positive in all patients and, in the 19 patients who underwent them, patch tests with the vehicle were negative. In none of the cases reported previously had patch tests been performed with pure MAL, so the possibility of a combined sensitization had not been completely ruled out. In our case the positive test with MAL supplied by the manufacturer allows us to attribute the sensitization to the active ingredient. Patch tests were negative in 80 of the controls reported in the literature, including our 10, confirming that the reactions were allergic in nature.

ALA patch tests were positive in only 3 of the 10 cases in which they were performed, so there is no evidence of cross-reactivity between the 2 agents despite their structural similarity.

Korshøj et al.5 determined that the frequency of sensitization to MAL in a group of 20 patients treated with at least 5 sessions of PDT was 35% compared with 1.7% in the control group (P<.0001).

A skin safety test performed in the United States showed that 52% of a group of 58 healthy volunteers became sensitized after exposure to MAL cream applied to the back under occlusion.7

The use of PDT in patients with nonsegmental Darier disease has been reported in 3 cases.8–10Exadaktylou et al.8 reported 5 cases treated with ALA-PDT. All 5 developed erythema and exudation, which were interpreted as an inflammatory response to treatment. Van’t Vesteinde et al.9 described a patient treated with ALA-PDT who developed erythema, edema, and papules in areas previously unaffected by Darier disease. Avery et al.10 reported a patient treated with MAL-PDT who showed a mild, self-limiting inflammatory response.Patch testing was not performed in any of the above cases.

The risk of sensitization to MAL may have been underestimated. Intense inflammatory reactions after PDT, commonly observed in clinical practice, are usually attributed to the mechanism of action of the drug and no patch tests are performed. It is likely that many of these reactions reflect genuine contact dermatitis, so it is advisable to perform patch testing in these cases.

In conclusion, we describe the first case of contact dermatitis to Metvix in which patch tests were performed with pure MAL. For the first time the sensitization can be unequivocally attributed to the active ingredient.