What Is the Cause of This Irregular Rash?

A 42-year-old woman presented with a rash, present for 1 week, which developed after a vacation to Mexico. The rash started on her right leg and then spread to her face and arms (Figure 1). It was associated with pruritus and burning. Physical exam revealed well-demarcated, irregularly shaped erythematous, edematous patches and plaques. She noted some weeping initially, but no vesicles or pustules.

While on vacation, the patient consulted her primary care physician about the rash and was started on cephalexin and miconazole cream. This resulted in little improvement after several days. Upon returning home, she saw another provider who noted that the lesions were erythematous and intact without warmth, weeping, or blisters. No potassium hydroxide test or biopsy was obtained. This provider placed the patient on ciclopirox cream.

Since the patient did not experience great improvement with these treatments, she sought a dermatologist, where it was elucidated that the patient had spent most of her vacation outdoors while drinking various tropical beverages with citrus garnishes.

What Is The Diagnosis?

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Phytophotodermatitis occurs when furocoumarins contact the skin with subsequent absorption of UV-A light (320-400 nm) and a resultant phototoxic reaction. Furocoumarins are psoralen isomers contained in the plants of families Umbelliferae, Rutaceae, and Moraceae.¹ Common foods and plants from these groups include celery, parsnips, carrots, fennel, dill, and citrus fruit.¹ The Table lists common genera and names of plants that are implicated in causing phytophotodermatitis.

Phytophotodermatitis clinically manifests as well-defined, angulated, or irregularly outlined, erythematous, and edematous patches or plaques that are occasionally associated with vesicles or bullae (Figure 1). Postinflammatory changes such as hyperpigmentation are common and may take 6 to 12 months to fade.²

The eruption can develop hours to days after sunlight exposure. The observed pattern of lesions corresponds to where the photosensitizer contacted the skin, for example, juices from fruit or beverages, handprints from contact, or brushing against plants. A key to the diagnosis is geometric or sharply angulated borders and unusual shapes to the plaques, in the setting of recent sun exposure. It is important to obtain a thorough history to correctly diagnose phytophotodermatitis. This dermatitis can also be mistaken for several other commonly encountered conditions, including allergic contact dermatitis, infectious lymphangitis, fungal infection, impetigo, and cellulitis.³ Phytophotodermatitis has also been misdiagnosed as child abuse⁴ and nonaccidental injury or self-harm.⁵

Treatment is largely symptomatic, with topical steroids and the application of cool compresses to alleviate pain, itching, and to reduce duration of symptoms.⁶ The use of topical steroids is useful  only in the early phase of erythema and vesiculation; if pigmentation has already occurred, this is an indication that active inflammation is already over. Nonsteroidal anti-inflammatory drugs (NSAIDs) for pain and antihistamines for itch can be used. Oral steroids should be reserved for severe cases.

In this case, the patient was instructed to apply triamcinolone 0.05% ointment twice daily and cool compresses to the areas 15 to 20 minutes several times a day as tolerated. She was advised to take oral NSAIDS for pain relief as needed. She was also instructed to avoid sun and sources of heat, such as hot showers and baths, hot tubs, and steam showers—activities she frequently engaged in.

On follow-up several days later, the patient reported that the lesions had rapidly improved, and she was no longer experiencing pruritus or pain. There was still hyperpigmentation, which was slowly improving (Figure 2).       

Prevention is key, with the avoidance of sunlight after exposure to photosynthesizing agents. Patients should be advised to wash off any photosensitizers after exposure. Additionally, patients should be counseled on applying broad-spectrum sunscreen regularly, as it is UV-A light specifically that causes this dermatitis.⁶ Postinflammatory hyperpigmentation responds rather poorly to topical bleaching creams and should also be protected from further sun exposure.⁶

Phytophotodermatitis is a cutaneous phototoxic reaction elicited by contact with photosensitizing chemicals in some plants, followed by exposure to UV-A light. A key feature of the rash is its geometric, angulated shape, which is a clue to an exogenous trigger. Patients and other providers should be educated about this condition to increase awareness and allow for the correct diagnosis, as it may be misdiagnosed as a variety of other skin conditions.