What condition appears on the posterior shoulder and upper arms?

Polymorphous Light Eruption (PMLE)

The condition that characteristically appears on the posterior shoulder and upper arms is polymorphous light eruption (PMLE), the most common photodermatosis that manifests as pruritic papules, plaques, or vesicles on sun-exposed areas. 1, 2

Clinical Presentation

PMLE typically presents with the following features:

• Pruritic, erythematous papules (3-6 mm) or pinpoint papules (1-2 mm) that develop on sun-exposed skin, particularly the posterior shoulders and upper arms 1, 2
• Onset occurs 30 minutes to several hours after sun exposure, with lesions persisting from a few hours up to 2 weeks 2
• Sparing of the face and flexural surfaces is characteristic, especially in the pinpoint papular variant seen in darker-skinned individuals 1
• The eruption is nonscarring and predominantly affects previously sun-exposed areas 2

Epidemiology and Risk Factors

• Females are affected 2-3 times more often than males 2
• Most commonly affects fair-skinned individuals with Fitzpatrick skin types I-IV, though it occurs in all races 2
• In African American patients, a pinpoint papular variant (1-2 mm papules) is recognized, affecting women with skin types IV-VI 1
• Overall prevalence is 10-20% in the general population 3

Pathophysiology

PMLE appears to be an immune-mediated delayed-type hypersensitivity reaction triggered primarily by UVA radiation (320-400 nm) and to a lesser degree by UVB (280-320 nm) 2, 3. The pathogenesis involves UVA-induced oxidative stress and subsequent deregulation of antioxidative immune responses 4.

Diagnostic Confirmation

Phototesting

• UVA provocation testing with 50-100 J/cm² administered to previously involved skin can reproduce PLE lesions in approximately 90% of patients 5
• Testing should be performed on large sites of previously involved skin, sometimes requiring up to four repeated exposures 5
• This represents the gold standard for confirming the diagnosis, which previously relied on exclusion of other dermatoses 5

Histopathology

Two characteristic patterns are observed 1:

• Focal lichenoid and perivascular lymphohistiocytic infiltrate with red blood cell extravasation
• Superficial and deep interstitial lymphocytic infiltrate with papillary dermal edema

Treatment and Prevention

First-Line Prevention

Combining a potent antioxidant with broad-spectrum, highly UVA-protective sunscreen is significantly more effective than sunscreen alone 4, 3:

• A formulation containing 0.25% alpha-glucosylrutin, 1% tocopheryl acetate (vitamin E), and broad-spectrum SPF 15 with high UVA protection prevented PLE in 96.6% of treated areas versus only 58.7% with sunscreen alone 4
• Sunscreens must contain effective UVA filters (such as Tinosorb M, Tinosorb S, and butyl methoxydibenzoylmethane) to prevent PLE, as most standard sunscreens protect predominantly against UVB and therefore fail 3

Acute Management

All patients respond to a combination of 1:

• Topical corticosteroids for active lesions
• Broad-spectrum sunscreens with high UVA protection
• Oral antihistamines for pruritus control

Alternative Prophylaxis

• PUVA therapy (photochemotherapy) offers effective "hardening" or desensitization through repeated irradiation, though it is time-consuming and may have side-effects 2, 5
• Oral carotenoids and antimalarials are additional therapeutic options for refractory cases 2

Critical Clinical Pitfall

Do not rely on standard sunscreens for PLE prevention—most commercial sunscreens protect predominantly against UVB radiation and will fail to prevent UVA-triggered PMLE 3. Only sunscreens specifically formulated with potent UVA-absorbing filters combined with antioxidants provide adequate protection 4, 3.