Phytophotodermatitis

Discussion

The differential diagnosis for phytophotodermatitis is broad (Box 1).^1–6 Evaluation for the presence of more serious conditions can generally be accomplished through a detailed history and physical examination, along with other investigations if needed.

In the case presented, the differential diagnosis included contact dermatitis, polymorphic light eruption, and an infectious exanthem. Contact dermatitis is a pruritic rash appearing after contact with an irritating substance. Polymorphic light eruption occurs in a symmetrical fashion in areas of prolonged sun exposure—typically the arms, back of the hands, neck, chest, lower legs, and feet; the face is usually spared. Infectious exanthem can have a varied presentation; however, the lengthy timeline of the rash and the companion with diarrhea and no rash made this less likely.

A PubMed literature search using the key words phytophotodermatitis, travel and dermatitis, pathophysiology of phytophotodermatitis, and lime dermatitis was completed. Search results consisted predominantly of case reports from 1980 to 2015. Most reports are published in dermatology or pediatric journals; few have been written from the perspective of family physicians.

Phytophotodermatitis is a rash occurring after contact between the skin and furanocoumarins, a class of chemicals found in many plants.^2,4 Box 2 lists plants and fruits associated with phytophotodermatitis.^1–3

The active substance within the furanocoumarins is psoralen,^2,4 which reacts with UVA light to cause an intense cutaneous reaction. Phytophotodermatitis does not involve any immune mechanism but must be differentiated from an allergic response.⁷ At a histologic level, cell damage is first detectable at 24 hours after the initial insult; clinical signs can be identified 48 hours after UVA exposure.^1,7 The acute dermatitis is self-limited and resolves over a period of days to weeks, but the ensuing hyperpigmentation, caused by psoralen-stimulated melanin hyperproduction, can last weeks to months.^1,2

The phytophotodermatitis reaction is highly variable. Rash characteristics can range from simple hyperpigmentation to erythema, vesicles, or bullae.^1,2 The variability is dependent on the quantity of photosensitizing agent, the length of exposure,⁸ the type of psoralen-containing substance,^2,5 and the method of contact.^1,2 For this reason, practitioners might believe the rash has more than one cause. One case report documented a leg rash that was consistent with the shape of the patient’s palm and fingertips.² The rash has also been confused with nonaccidental trauma.^1,4,5 Usually, the rash is nonpruritic; pruritus should prompt consideration of other diagnoses.³

Treatment is sometimes pursued; however, there is limited evidence on efficacy. Severe bullous reactions and resulting wounds should be managed with standard care to prevent secondary infection.⁸ In some cases, physicians might choose to prescribe topical steroids^1,3 or antihistamines,³ or advise the use of cold compresses¹ for relief of pain or itching. Evidence for these treatments is based on expert opinion. While the rash might be aesthetically disturbing to patients, reassurance is often all that is needed.²

Patients can be counseled on prevention. If an exposure can be identified early, the chemical stimulus can be cleansed from the skin with water. Psoralen must be absorbed into the skin before activation by UVA light, a process that can take anywhere between 30 and 120 minutes.² This knowledge might be useful for patients with occupational or travel-related exposures. Not all sunscreens are helpful in preventing the rash, as some sunscreens only protect against UVB light.⁹