What tests are done to diagnose Polymorphic Light Eruption (PMLE)?

Diagnostic Tests for Polymorphic Light Eruption (PMLE)

The diagnosis of polymorphic light eruption primarily relies on a detailed history of sun exposure-related skin eruptions, characteristic morphology of lesions, and phototesting to confirm the diagnosis.

Essential Diagnostic Approach

Clinical History Assessment

• Timing of eruptions in relation to sun exposure (typically hours to days after exposure)
• Pattern of recurrent papulonodular lesions that spontaneously regress within weeks
• Distribution limited to sun-exposed areas (V-area of chest, arms, forearms, legs, upper back)
• Seasonal variation (typically spring/early summer)
• Morphological characteristics of lesions (papules, vesicles, plaques, or erythema)
• Symptoms (itching, burning sensation)

Physical Examination

• Examination of lesion morphology (papules, vesicles, plaques, erythema multiforme-like lesions)
• Distribution pattern (limited to sun-exposed areas)
• Exclusion of other photodermatoses

Laboratory Tests

1. Phototesting:

   • Polychromatic phototest or monochromatic phototesting with UVA and UVB
   • Provocative phototests reproduce spontaneous lesions in approximately 50% of patients 1
   • Repetitive exposures to UVA or UVB to induce characteristic lesions
   • Determination of minimal erythema dose (MED) for both UVA and UVB

2. Antinuclear Antibody (ANA) Testing:

   • Important to exclude lupus erythematosus, a key differential diagnosis
   • Testing for anti-Ro and anti-La antibodies may be necessary 2

3. Skin Biopsy:

   • May be performed in atypical cases
   • Helps exclude other photodermatoses

Specialized Testing

Photopatch Testing

• Useful when allergic photocontact dermatitis is suspected
• Can identify photosensitizing agents in sunscreens or topical products

Immunological Studies

• Not routinely performed but may be considered in research settings
• May help understand the delayed hypersensitivity mechanism

Prognostic Value of Testing

Research suggests that phototesting results may have prognostic value:

• Patients with negative phototests may have a more benign form of PMLE with higher likelihood of remission
• Patients with positive phototests tend to have more severe and chronic disease 3

Important Considerations

• Approximately 20% of PMLE patients have negative phototests despite having the condition 3
• The PLE-Severity Assessment Score (PLE-SAS) based on patient history alone is not reliable enough to replace phototesting for determining disease severity 4
• All patients with suspected PMLE should be screened for lupus erythematosus, as approximately 10% may have definite or possible LE 2

Diagnostic Algorithm

1. Establish clinical suspicion based on history and physical examination
2. Perform phototesting with UVA and UVB to confirm diagnosis
3. Consider ANA, anti-Ro, and anti-La antibody testing to exclude lupus erythematosus
4. Perform skin biopsy in atypical cases or when diagnosis remains uncertain
5. Consider photopatch testing if photoallergic contact dermatitis is suspected

Remember that while phototests are positive in only about 50% of PMLE patients, a negative phototest does not exclude the diagnosis when clinical history is strongly suggestive.