Approach Considerations
The diagnosis of polymorphous light eruption (PMLE) is usually based on the clinical picture and history.
Laboratory Studies
In polymorphous light eruption (PMLE), laboratory tests can be performed to rule out other dermatoses such as erythropoietic protoporphyria or lupus erythematosus. Antinuclear antibody (ANA), anti-Ro (SS-A), and anti-La (SS-B) tests, as well as urine, stool, and blood porphyrin levels, should be obtained when clinically indicated. [43]
Normal titers of ANA, as well as normal urine, stool, and blood porphyrin levels, support the diagnosis. A false-positive ANA can be seen, as a titer of 1:80 has been reported in up to 13% of normal population.
Other Tests
Results of phototesting in polymorphous light eruption (PMLE) patients are controversial, ranging from an ability to reproduce the eruption by repeat phototesting in 60-100% of patients to an inability to do so, except in patients who are very photosensitive. [44] These differences may be explained by a lack of a standardized test procedures, variation in radiation sources used, and imprecision in diagnostic criteria for the disease. Minimal erythema doses (MEDs) are normal in PMLE and lowered or abnormal in chronic actinic dermatitis. In solar urticaria, irradiation results in reproduction of the lesion.
Photoprovocation test
Perform repetitive light testing; irradiating three times the MED to UVA on the right forearm and three times the MED to UVB on the left forearm for 3 consecutive days. Results are read immediately, at 24 hours, and at 72 hours. A delayed reading at 1 week may also be helpful. The test results are often positive in PMLE. A negative result does not exclude the diagnosis. If a lesion (eg, papule, vesicle) develops, biopsy confirmation can be performed. Histologically, a superficial and deep perivascular lymphocytic infiltrate is apparent with dermal edema. The test is best done in spring or early summer to avoid false-negative results. The area for testing should include a skin site previously involved with PMLE.
Photopatch tests
These can be used to rule out photoallergic or airborne contact. Two identical strips of standard photoallergens are placed on the back. One of the two strips is exposed to UVA radiation 24 hours later. Both the irradiated site and the unirradiated site are read at 24, 48, and 96 hours. A positive reaction at the irradiated site but not at the unirradiated patch test site is diagnostic of a photocontact allergy. Positive reactions at both the irradiated site and the unirradiated site are indicative of a contact allergy. According to Leroy et al in 2002, [45] polychromatic phototesting seems to be more sensitive than UVA phototesting to assess PMLE, and results suggest UVB is a key trigger of PMLE.
Histologic Findings
The most striking feature of the biopsy specimen from a patient with polymorphous light eruption (PMLE) is edema in the upper part of the dermis. Tight, perivascular lymphocytic infiltrate is observed in the upper and mid dermis. When eczematous epidermal changes are present clinically, spongiosis, edema, dyskeratosis, and basal cell vacuolization may be observed. Occasionally, neutrophils and eosinophils may be present in the infiltrate. The dominant cell, however, is the lymphocyte. Mucin, which is thought to distinguish PMLE from lupus, can be present in skin biopsies of PMLE patients. A study of dermal mucin from lupus samples found that it is not so much different from that found in PMLE in comparison with other dermatitides, which also had some, but less, mucin (eg, erythema multiforme, fixed drug eruption, graft versus host disease, lichen planus, photodamaged skin). [46] Erythema multiforme subtype can show evidence of a vacuolar interface with liquefactive degeneration of the dermoepidermal junction. A predominantly neutrophilic subtype also can be seen, which may be confused with Sweet syndrome. Chilblains has similar pathological features, but the history and location of the lesions can usually help make the diagnosis.
Note the images below.
Polymorphous light eruption pathology showing papillary dermal edema. Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-1.jpg).
Polymorphous light eruption pathology showing papillary dermal edema. Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-2.jpg).
Polymorphous light eruption pathology showing papillary dermal edema with lymphocytes in the epidermis (exocytosis). Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-3.jpg).
Polymorphous light eruption pathology. The infiltrate is mainly lymphocytic but there may be intermixed eosinophils, neutrophils, and histiocytes. Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-4.jpg).
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Photograph of a springtime eruption of polymorphous light eruption in a 6-year-old boy. The eruption has occurred in the spring since the patient was aged 5 years, and it resolves completely with no scarring by mid to late summer. High block sunscreens attenuate but do not prevent the eruption. No itching or pain occurs. Courtesy of Dr. Jeremy F. Harrison, FRCA, FFAEM.
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Polymorphous light eruption on the thighs and hand. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/reactions/pmle2.jpg).
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Polymorphous light eruption on the chest. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/_resampled/FitWzY0MCw0ODBd/WatermarkedWyIyNTg0MSJd/pmle-14.jpg).
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Polymorphous light eruption on the chest. Courtesy of DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/_resampled/FitWzY0MCw0ODBd/WatermarkedWyIyNTg0MCJd/pmle-15.JPG).
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Polymorphous light eruption on the arm. Courtesy of Waikato District Health Board and DermNet New Zealand (http://www.dermnetnz.org/assets/Uploads/_resampled/FitWzY0MCw0ODBd/WatermarkedWyIyNTg0MCJd/pmle-22.JPG).
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Polymorphous light eruption pathology showing papillary dermal edema. Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-1.jpg).
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Polymorphous light eruption pathology showing papillary dermal edema. Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-2.jpg).
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Polymorphous light eruption pathology showing papillary dermal edema with lymphocytes in the epidermis (exocytosis). Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-3.jpg).
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Polymorphous light eruption pathology. The infiltrate is mainly lymphocytic but there may be intermixed eosinophils, neutrophils, and histiocytes. Courtesy of DermNet New Zealand (https://www.dermnetnz.org/assets/Uploads/pathology/e/pmle-figure-4.jpg).
