What is the role of phototesting in the diagnosis of polymorphous light eruption (PMLE)?

Updated: Jan 22, 2020
  • Author: Saud A Alobaida, MBBS, FRCPC; Chief Editor: Dirk M Elston, MD  more...
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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.

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