Laboratory Studies
Actinic prurigo is not a systemic disease, and no anomalous results occur in routine laboratory workup. Laboratory tests are performed to rule out systemic diseases with photosensitivity, such as lupus erythematosus.
Antinuclear antibody levels, anti-Ro and anti-La titers, and porphyrin levels are in the reference range.
Direct immunofluorescence study results are negative. These findings should support the diagnosis on the basis of the clinical picture.
Other Tests
In a similar way to what occurs in polymorphous light eruption (PLE), the minimal erythema dose (MED) is decreased in patients with actinic prurigo.
Great variation exists in the literature concerning protocols and results of phototesting for photodermatoses, such as PLE and actinic prurigo. Lesions can be reproduced on healthy skin when it is irradiated with 10-20 times the MED with solar simulated light in 75% of cases. A delayed reading at 4-7 days confirms the presence of induced lesions. Patients in all the series reported from the authors' institution are based on these criteria. [7, 8] This phototest protocol may be avoided for routine treatment of patients with actinic prurigo; however, this protocol may be useful for clinical research studies. See the image below.
A phototest with UV-B light shows reproduction of lesions on the inner aspect of the arm. The result from the phototest with UV-A light was negative.
Another common phototesting protocol consists of irradiating the volar aspect of the forearm with 0.75 MED for 3-5 consecutive days; the lesions show similar reproduction rates.
Photopatch testing is negative.
A negative phototesting result does not exclude the diagnosis.
Histologic Findings
Histologic examination shows acanthosis, mild spongiosis, edema of the lamina propria, a moderate-to-dense in a bandlike lymphocytic inflammatory infiltrate, and, occasionally, lymphoid follicles. This latter finding is somewhat more common in the mucosa, the conjunctivae, and the lips (15-20%). [3] Abundant eosinophils are usually present. Notably, Cedeno-Suarez et al found the toluidine blue test useful in locating lymphoid follicles in actinic prurigo biopsies. [37] The affected conjunctivae show epithelial hyperplasia alternating with atrophy. Vacuolization of the basal cell layer and dilated capillaries in the dermis are also noted. [23] When lesions are experimentally reproduced, histopathologic findings are similar to those mentioned. See the images below.
Histologic examination shows acanthosis, mild spongiosis, edema of the lamina propria, and a moderate-to-dense perivascular lymphocytic inflammatory infiltrate.
A close-up view shows edema of the lamina propria as well as a lymphocytic inflammatory infiltrate in the dermis.
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Itchy plaques mainly on photoexposed areas of the face; these plaques are characteristic of actinic prurigo.
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Photodistribution of lesions over the body. Note the hypopigmented areas of the skin, which are very common after intense scratching in children.
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Multiple itchy papules coalescing into plaques on the neck. These lesions are similar to lesions of polymorphous light eruption. Note the excoriations induced by scratching.
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One third of patients are children. The nose is frequently affected. This clinical feature is useful in distinguishing it from other entities, such as atopic dermatitis.
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One half of patients have bilateral conjunctivitis. Eye protection is needed to avoid disease progression.
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About 75% of patients have cheilitis, which can take the form of solid lesions or erosions.
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A phototest with UV-B light shows reproduction of lesions on the inner aspect of the arm. The result from the phototest with UV-A light was negative.
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Histologic examination shows acanthosis, mild spongiosis, edema of the lamina propria, and a moderate-to-dense perivascular lymphocytic inflammatory infiltrate.
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A close-up view shows edema of the lamina propria as well as a lymphocytic inflammatory infiltrate in the dermis.
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Young girl with a history of atopic dermatitis and itchy, lichenified plaques on her face for the last 3 months. Atopic dermatitis with photosensitivity is the main differential diagnosis with actinic prurigo in children.
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Actinic cheilitis resulting from actinic prurigo.
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Erythematous and very itchy plaques on solar exposure areas of the face and pseudopterygium are commonly observed in actinic prurigo.
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Lichenified plaques, excoriated nodules, and atrophic scars on the dorsal aspect of hands are frequently seen in children.
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Contact dermatitis due to sunscreen in a patient with actinic prurigo.
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Impetiginous area located on the right ear lobe due to intense scratching following an acute relapse.
