WOODINVILLE, WASH. – The diagnosis of photocontact dermatitis can be challenging and often requires careful photopatch testing, according to Dr. Vincent A. DeLeo.
He reviewed the finer points of diagnosing the two main types of photocontact dermatitis – photoirritant and photoallergic – and differentiating them from other conditions with a similar clinical presentation at the annual Coastal Dermatology Symposium.
Photoirritant contact dermatitis
In photoirritant contact dermatitis, a chemical deposited on the skin absorbs radiation and then causes damage directly, without any immune reaction. The diagnosis is a clinical one, said Dr. DeLeo, chairman of the department of dermatology at St. Luke’s–Roosevelt Hospital Center and Beth Israel Medical Center in New York.
"You as a clinician know it is a photodistributed disease," he said. "You take a good history, you get a history of the patient being exposed topically to the chemical, and that’s how you make the diagnosis," he commented. "This is universal; it will happen to any of us if we get exposed to these photosensitizing chemicals in enough concentration and we get enough light to develop this kind of reaction."
Photos courtesy Dr. Vincent A. DeLeo
Figure 1: Photoirritant contact dermatitis on the forearm due to pitch.
Photoirritant contact dermatitis is almost always induced by UVA radiation; therefore, it can occur even if a patient is wearing a sunscreen that protects against UVB light.
The reaction has the appearance of a sunburn, with erythema, edema, and possibly blistering. It usually resolves with hyperpigmentation. "The interesting thing is that a lot of these reactions are subclinical, at least the erythema component that you don’t see. All you do see is the resulting hyperpigmentation," he said at the meeting, presented by the Caribbean Dermatology Symposium.
A common culprit is tar, in which case the condition is known as tar smarts. These cases usually result from occupational exposure in roofers, but can be seen as well in people who use tar shampoos and then go out in the sun.
Figure 2: Photoallergic contact dermatitis on the arm due to sunscreen.
The furocoumarins (psoralens) also can produce photoirritant contact dermatitis. The most common form is phytophotodermatitis, which involves plant compounds, especially those in the exocorp (green peel) of limes. The hallmark is tiny blisters on involved skin.
"It’s a delayed reaction. So people are squeezing limes into their gin and tonics on the weekend, and then Monday or Tuesday, when they are back at work, all of a sudden they develop these blisters, and they don’t associate it with what they did on weekend," Dr. DeLeo noted.
Phytophotodermatitis also can result from contact with celery that has been bred to have a high psoralen content so that it stays fresh longer. "You see this in grocery workers primarily, people who are exposed to a lot of celery, handling it and then going into the sun and getting the reaction," Dr. DeLeo said.
Photoallergic contact dermatitis
In photoallergic contact dermatitis, the chemical deposited on the skin – most commonly a sunscreen, fragrance, or medication – absorbs radiation and becomes an allergen. Only sensitized people have a reaction.
This dermatitis is exematous and pruritic, and shows a sun-exposed distribution, although it is usually patchy. The reaction is delayed, typically starting 1-2 days after sun exposure.
"The photodistributed eruption will usually be in all photoexposed areas. And this is important in making the distinction between this and other photosensitivities," Dr. DeLeo said. "Usually it will occur on all areas including the face ... and usually it spares upper lids and beneath the chin, although not always," he noted.
"The diagnosis of photoallergic contact dermatitis is confirmed by photopatch testing," he said. "Almost all photosensitive patients with eczematous morphology should be photopatch tested."
However, survey data suggest that only about two-thirds of dermatologists do this testing (Am. J. Contact Dermat. 2003;14:5-11), largely because this dermatitis is uncommon today as most sensitizing agents are identified before products come to market.
Photopatch testing can be done in the office; it requires at least a UVA source, photoantigens, and general patch test supplies. A UVB source is helpful for distinguishing photoallergic contact dermatitis from conditions such as chronic actinic dermatitis.
"If I were to buy four photoantigens, I would probably buy oxybenzone, musk ambrette, padimate O, and avobenzone," Dr. DeLeo said. "You can also test directly to the patient’s products, and usually it’s going to be sunscreens, so you can just have them bring in whatever they are using."
In a study by the North American Contact Dermatitis Group in which 363 consecutive patients with a history of photosensitivity were given a final diagnosis after photopatch testing, the leading diagnosis was allergic contact dermatitis (122 patients), followed by photoallergic contact dermatitis (75 patients) and polymorphous light eruption (72 patients).