Although PPT is necessary for the accurate diagnosis of PACD, it is infrequently used. Two surveys of 112 and 117 American Contact Dermatitis Society members, respectively, have revealed that only around half performed PPT, most of them testing fewer than 20 times per year.16,17 Additionally, there was variability in the test methodology and allergens employed. Nevertheless, most respondents tested sunscreens, nonsteroidal anti-inflammatory drugs (NSAIDs), fragrances, and their patients’ own products.16,17 The most common reasons for not performing PPT were lack of equipment, insufficient skills, rare clinical suspicion, and cost. Dermatologists at academic centers performed more PPT than those in other practice settings, including multispecialty group practices and private offices.16 These findings highlight multiple factors that may contribute to reduced patient access to PPT and thus potential underdiagnosis of PACD.
Common Photoallergens
The most common photoallergens change over time in response to market trends; for example, fragrance was once a top photoallergen in the United States in the 1970s and 1980s but declined in prominence after musk ambrette—the primary allergen associated with PACD at the time—was removed as an ingredient in fragrances.18
In the largest and most recent PPT series from North America (1999-2009),2 sunscreens comprised 7 of the top 10 most common photoallergens, which is consistent with other studies showing sunscreens to be the most common North American photoallergens.19-22 The frequency of PACD due to sunscreens likely relates to their increasing use worldwide as awareness of photocarcinogenesis and photoaging grows, as well as the common use of UV filters in nonsunscreen personal care products, ranging from lip balms to perfumes and bodywashes. Chemical (organic) UV filters—in particular oxybenzone (benzophenone-3) and avobenzone (butyl methoxydibenzoylmethane)—are the most common sunscreen photoallergens.2,23 Para-aminobenzoic acid was once a common photoallergen, but it is no longer used in US sunscreens due to safety concerns.19,20 The physical (inorganic) UV filters zinc oxide and titanium dioxide are not known photosensitizers.
Methylisothiazolinone (MI) is a highly allergenic preservative commonly used in a wide array of personal care products, including sunscreens.24 In the most recent NACDG patch test data, MI was the second most common contact allergen.25 Allergic contact dermatitis caused by MI in sunscreen can mimic PACD.26 In addition, MI can cause photoaggravated contact dermatitis, with some affected patients experiencing ongoing photosensitivity even after avoiding this allergen.26-30 The European Union and Canada have introduced restrictions on the use of MI in personal care products, but no such regulatory measures have been taken in the United States to date.25,31,32
After sunscreens, another common cause of PACD are topical NSAIDs, which are frequently used for musculoskeletal pain relief. These are of particular concern in Europe, where a variety of formulations are widely available OTC.33 Ketoprofen and etofenamate are responsible for the largest number of PACD reactions in Europe.2,34,35 Meanwhile, the only OTC topical NSAID available in the United States is diclofenac gel, which was approved in 2020. Cases of PACD due to use of diclofenac gel have been reported in the literature, but testing in larger populations is needed.36-39
Notably, ketoprofen may co- or cross-react with certain UV filters—oxybenzone and octocrylene—and the lipid-lowering agent fenofibrate due to chemical similarities.40-43 Despite the relatively high number of photoallergic reactions to ketoprofen in the NACDG photopatch series, only 25% (5/20) were considered clinically relevant (ie, the allergen could not be verified as present in the known skin contactants of the patient, and the patient was not exposed to circumstances in which contact with materials known to contain the allergen would likely occur), which suggests that they likely represented cross-reactions in patients sensitized to sunscreens.2
Other agents that may cause PACD include antimicrobials, plants and plant derivatives, and pesticides.2,4,18 The antimicrobial fentichlor is a common cause of positive PPT reactions, but it rarely is clinically relevant.44
Treatment
The primary management of PACD centers on identification of the causative photoallergen to avoid future exposure. Patients should be educated on the various names by which the causative allergen can be identified on product labels and should be given a list of safe products that are free from relevant allergens and cross-reacting chemicals.45 Additionally, sun protection education should be provided. Exposure to UVA radiation can occur through windows, making the use of broad-spectrum sunscreens and protective clothing crucial. In cases of sunscreen-induced PACD, the responsible chemical UV filter(s) should be avoided, or alternatively, patients may use physical sunscreens containing only zinc oxide and/or titanium dioxide as active ingredients, as these are not known to cause PACD.4
When avoidance alone is insufficient, topical corticosteroids are the usual first-line treatment for localized PACD. When steroid-sparing treatments are preferred, topical calcineurin inhibitors such as tacrolimus and pimecrolimus may be used. If PACD is more widespread and severe, systemic therapy using steroids or steroid-sparing agents may be necessary to provide symptomatic relief.4
Final Interpretation
Photoallergic contact dermatitis is not uncommon, particularly among photosensitive patients. Most cases are due to sunscreens or topical NSAIDs. Consideration of PPT should be given in any patient with a chronic photodistributed dermatitis to evaluate for the possibility of PACD.