Polymorphous light eruption (PMLE) is a common photodermatosis that occurs more commonly in patients who reside farther from the equator. In Canada, the incidence may approach 20%, whereas in areas near the equator it may be less than 1%. Women tend to be affected more than men and it’s more common in teenagers and people in their 20s. There’s often improvement over time. There isn’t a strong genetic predisposition but a familial tendency has been reported.
The typical patient with PMLE will develop outbreaks following sun exposure, predominantly in the spring and early summer, or when travelling south. The symptoms tend to be less severe over the summer as “hardening” occurs.
When UV radiation interacts with the skin, it induces cellular changes that can potentially induce an immune response. In unaffected individuals, there’s local immunosuppression induced by the UV radiation, preventing any unwanted reaction to sunlight. In patients with PMLE, this local immune downregulation is minimal, and there’s a delayed-type hypersensitivity reaction. Nearly half of all cases result from UVA exposure, while a quarter of cases occur following UVB exposure or a combination of UVA and UVB.
The eruption starts within minutes to hours of sun exposure. It can last for up to two weeks. The most commonly involved areas are sun-exposed regions on the neck, forearms and legs. The hands and face tend to be spared as they’re exposed to sunlight year round. The eruption is usually symmetric with itchy papules that coalesce into plaques, and confluent edema. Sometimes there can be vesicles and bullae. If the sun penetrates through clothing, then those normally “covered” areas can be affected as well. Systemic symptoms are rare. PMLE sine eruption is a variant where there are no skin lesions, only pruritus.
When considering an eruption triggered by sun or light exposure, it’s important to consider other photodermatoses. These include hydroa vacciniforme, actinic prurigo, chronic actinic dermatitis and solar urticaria. Porphyrias often produce skin findings after exposure to visible light rather than UV radiation. Also consider that many primary skin diseases can be exacerbated by sun exposure. These include lupus, dermatomyositis, lichen planus, acne, rosacea, and viral infections such as herpes labialis.
A biopsy can be helpful to rule out other causes, especially photoaggravated dermatoses such as lupus, which is primarily a clinical diagnosis. Most mimickers can be ruled out after a thorough history and physical. If in doubt, blood work to rule out lupus and porphyrin analysis may be helpful.
Once PMLE is identified, patient counselling is a key component of management. Inform all patients with PMLE to avoid the sun at peak hours — 11am to 4pm, wear protective clothing including a hat and sunglasses, and apply a broad-spectrum sunscreen with an SPF of 50 or higher to uncovered skin.
Most patients can be treated with topical steroids when lesions occur. The potency of the steroid should be selected based on the body site — mild for face, moderate for body, and potent for hands. In more extensive cases, a short course of oral prednisone can be initiated at the first sign of pruritus and continued until all lesions have resolved. Antimalarials such as hydroxychloroquine initiated before sun exposure can also control and prevent flares. Hydroxychloroquine works as a systemic sunscreen, helping to minimize radiation-induced DNA damage. Consider starting this before a southern vacation and continue during the summer months. If this isn’t effective, steroid-sparing agents such as azathioprine or cyclosporine are worth a try.
Hardening refers to the condition tending to improve after repeated exposure to UV radiation, such as over the summer season. This can be artificially induced by repeated exposure to UVA or narrow-band UVB phototherapy. A typical regimen involves treatment 2-3 times/week for four to six weeks before significant sun exposure. The key to prevent flares with this method is ongoing exposure or tolerance may be lost.
John Kraft, MD, FRCPC is a dermatologist in private practice in Markham and Thunder Bay, ON.
Charles Lynde, MD, FRCPC is an assistant professor of dermatology at the University of Toronto.