Photosensitive Atopic Dermatitis Exacerbated by UVB Exposure

Comment

Classification and Clinical Presentation
The literature on photosensitive AD is scant, and this disease entity is rare. Alternative names include photoaggravated AD, photosensitive eczema, and light-exacerbated eczema.⁵ Two main studies have been conducted in recent years that were intended to characterize photosensitive AD. ten Berge et al⁵ conducted a retrospective study of 145 patients with AD that were phototested in 2009. They found that 3% of their total AD patient population had photosensitive AD.⁵ In 2016, Ellenbogen et al² performed a similar single-center retrospective analysis of 17 patients with long-standing AD who suddenly developed photosensitivity.

Patients with photosensitive AD typically present with lesions on sun-exposed skin with coexisting eczematous lesions in sites with a predilection for AD.² In the study conducted by ten Berge et al,⁵ 2 main reaction patterns were observed: erythematous papules with pruritus and an eczematous reaction. The authors suggested one subset of patients demonstrated polymorphous light eruption (PMLE), a common photoinduced eruption thought to represent a delayed-type hypersensitivity, coexisting with AD while the other subset had true photosensitive AD.^5,6 Ellenbogen et al² also found 2 reaction patterns, which they labeled papular (PMLE type) and eczematous (photosensitive AD type). The authors contested the theory of coexisting PMLE in AD because PMLE gets better in the summer with UV radiation hardening.² On the contrary, photosensitive AD worsens with uncontrolled exposure to sunlight. Only with controlled exposure to small doses of UV radiation at a time does this condition improve. Ellenbogen et al² believe both reaction patterns are consistent with photosensitive AD and the PMLE type should be termed papular photosensitive AD type.

Histopathology
The histopathologic findings of photosensitive AD are nonspecific but are characterized by spongiotic dermatitis with a perivascular lymphohistiocytic infiltrate.²

Diagnosis With Phototesting
Phototesting of patients with AD should be considered if there is a suspicion for photosensitivity based on persistent disease despite use of photoprotection and local treatment.^5-7 Patients may not notice a correlation of skin exacerbations with UV exposure, especially if they are only sensitive to UVA, as it is still present on cloudy days and can penetrate glass windows.⁸ Phototesting evaluates the degree of sensitivity to UV light and the specific wavelength eliciting the cutaneous response. Phototesting consists of determining the MED to UVA and UVB, the minimal phototoxic dose for PUVA, and visible light exposure. Further evaluation may include photoprovocation testing or photopatch testing, as these patients can have coexisting photocontact allergies.

The MED is defined as the minimal dose of UV light needed to induce perceptible erythema in exposed skin.⁵ It is dependent on the light source and patient’s skin type, and individual units may vary. To determine the MED to UVA or UVB, 2×2-cm skin fields are irradiated with increasing cumulative UVA/UVB. The dose varies by skin type and it is then read at 24 hours. The majority of patients with photosensitive AD are reported to have a normal MED; however, some studies have reported the MED to be decreased.^5,7-9 ten Berge et al⁵ found 7% of their study participants exhibited a lower MED, as seen in our patient.

The minimal phototoxic dose for PUVA is defined as the least exposure dose of UVA 1 hour after ingestion of 0.4 mg/kg of methoxsalen that produces pink erythema with 4 distinct borders at 48, 72, or 96 hours after ingestion.¹⁰ Visible light exposure is tested using a slide projector as the light source to an approximately 10×5-cm area of skin for 45 minutes. Any immediate or delayed reaction is abnormal and considered positive.¹⁰

Photoprovocation testing has been performed in several studies.^2,5 It consists of exposing an 8-cm area of skin to 80 J/cm² UVA and 10 mJ/cm² UVB, which is read at 24, 48, or 72 hours. A papular or eczematous reaction is considered positive.^2,11

The results of phototesting have varied between studies. ten Berge et al⁵ phototested 107 patients with AD and photosensitivity and 17% were found to be solely sensitive to UVA whereas 67% were found to be sensitive to UVA and UVB. In contrast, Ellenbogen et al² only tested 17 patients with AD and photosensitivity and they found that 56% (9/16) were sensitive to UVA alone while only 44% (7/16) were sensitive to UVA and UVB.

Photopatch testing can help to rule out photosensitivity due to a substance in the presence of UV light. In studies of patients with photosensitive AD (N=125), photocontact reactions occurred in 23% and were predominantly associated with sunscreens, skin care products, and fragrances.^5,12 Photopatch testing is done by placing duplicate sets of patches on nonlesional skin using the Finn Chamber technique. A published list of allergens, which were agreed upon by the European Society of Contact Dermatitis and the European Society for Photodermatology in 2000 are seen in Table 1.¹³ The list contains mainly UV filters and drugs. The patients’ own products also should be tested in addition to the published list of allergens, but a maximum of 30 patches should be placed at one time. The patches are removed at either 24 or 48 hours; some researchers have found greater sensitivity with the 48-hour time period, while others have not found a significant difference.¹⁰ One set of skin fields then is covered with an impermeable occlusive dressing as a control while the other is irradiated with 5 J/cm² of a broad-spectrum UVA light source. UVA fluorescent lamps are the light source of choice because of their widespread availability, reproducible broad spectrum, and beam uniformity.¹⁰ In the study conducted by ten Berge et al,⁵ photopatch testing was performed on 125 patients, and 29 patients were found to be positive to one or more substances. Ellenbogen et al² photopatch tested 5 patients with photosensitive AD and a clinical suspicion of photoallergy; however, all 5 were negative. Our patient underwent traditional patch testing due to clinical suspicion of a coexisting contact allergy, which was negative.

Differential Diagnosis
The differential diagnosis for photosensitive AD includes PMLE with coexisting AD, chronic AD, and photoallergic contact dermatitis. Photosensitive AD worsens with increasing exposure to uncontrolled sunlight, in contrast to patients with PMLE who experience UV radiation (UVR) hardening with increasing UV exposure during the summer months, resulting in improvement of skin lesions. Patients with chronic AD generally report a history of chronic ambient sun exposure and exhibit well-demarcated eczematous lesions in a photodistributed pattern with sparing of sun-protected skin.² In contrast, photosensitive AD involves both sun-exposed and covered areas of the body. Chronic AD will have a positive photoprovocation test with a decreased MED (Table 2). Photoallergic contact dermatitis also will have photodistributed eczematous lesions with relative sparing of non–sun-exposed skin; however, these patients generally have negative photoprovocation testing with a normal MED.² These patients may or may not have a history of reaction to a known allergen, but they likely will have a positive photopatch test.

Treatment
The treatment of photosensitive AD is based on the severity of the photosensitivity. Treatment for mild disease is limited to sun protection in addition to topical corticosteroids or topical calcineurin inhibitors. For moderate disease and unsatisfactory relief with proper sun protection, UVR hardening is recommended. If severe disease is present, immunosuppression with medications such as corticosteroids, cyclosporine, and mycophenolate mofetil is suggested to prevent flaring of disease during UVR hardening.^2,5,8,14