Polymorphic Light Eruption (PMLE) is the most common skin condition sensitive to light. It is a delayed hypersensitivity reaction of the immune system to endogenous antigens formed after the skin is exposed to ultraviolet (UV) rays.
Epidemiology
Polymorphic light eruption is also known as sun allergy, sun poisoning, sun eczema, or summer pruritus rash/swellings.
- Age: Common in young people aged 20 – 40 years.
- Gender: Women have a 4 times higher incidence rate than men.
- Geographical region: Common in temperate climates, areas with less sunlight, or high-altitude regions.
- Skin type: Occurs in all races but is most common in individuals with Fitzpatrick skin type I.
Causes and pathogenesis
The core mechanism of PMLE is a delayed hypersensitivity reaction. Some major contributing factors include:
- Ultraviolet rays: UVA is the main agent (accounting for 75 – 90%) and can penetrate through window glass. UVB or a combination of both can also cause the condition.
- Immune system disorder: In normal individuals, UV rays locally suppress the immune system to protect the skin. In PMLE patients, this mechanism is impaired, leading to increased inflammation with the presence of T lymphocytes (CD4 and CD8).
- Endocrine factors: Estradiol in women is believed to block the natural immunosuppressive effect of UV rays, explaining why women are more prone to the condition.
- Genetics: About 15 – 46% of cases have a family history.
Clinical manifestations
The condition usually flares up in spring and early summer, appearing after sun exposure from a few hours to 2 days and subsiding or disappearing completely in winter. However, it can also flare up in winter when the patient is exposed to strong sunlight such as sunbathing or travelling to areas with higher light intensity.
Lesion morphology: Although termed “polymorphic,” in a specific patient, lesion morphology remains consistent across episodes, including:
- Papules, blisters, or eczema-like red patches.
- Urticarial or target-like lesions (similar to erythema multiforme).
- Lesions will continue to worsen if the area is still exposed to sunlight.
- Papules and skin lesions can heal completely without scarring.
Location: Symmetrical on exposed skin areas (arms, legs, V-neck, chest). Less common on the face and back of the hands due to the “skin hardening” effect.
Symptoms: Intense itching is the main symptom. Sometimes accompanied by fever, headache, or nausea.
Skin hardening effect: With prolonged sun exposure, the skin adapts by increasing melanin and thickening the stratum corneum, helping symptoms gradually decrease by the end of summer.
Diagnosis
Diagnosis is primarily based on medical history and clinical assessment.
Laboratory tests: Skin biopsy (nonspecific, mainly showing spongiosis and lymphocytic infiltration). Severe cases may show dermal papillary edema. Antinuclear antibody (ANA) test to rule out Lupus.
Phototesting: Light stimulation test can be performed to confirm the diagnosis (positive in about 60% of patients).
Differential diagnosis: Needs to be differentiated from Lupus erythematosus, Porphyria, solar urticaria, and allergic contact dermatitis to light.
Treatment
Preventive measures
- Use broad-spectrum sunscreen (SPF 50+, against both UVA/UVB).
- Wear protective clothing, avoid going out during peak sunlight hours.
- Choose areas with soft light and shade if needing to be outdoors.
- Limit sitting near windows, especially glass windows.
Specific treatments
- Topical: Apply corticosteroids or Calcipotriol.
- Systemic: Short-term oral corticosteroids (1-2 weeks) during acute episodes. Hydroxychloroquine or Nicotinamide (used before flare-up season).
- Phototherapy: Low-dose UVB or UVA exposure at the start of spring to gradually acclimate the skin (causing desensitization).
- In severe cases: Consider Azathioprine or Cyclosporine.
About 60% of patients improve or recover after 15 years, 75% see reduction or recovery after 30 years, although PMLE can last a lifetime.
PMLE though not life-threatening and with a benign prognosis, significantly affects the quality of life and psychology of patients due to its persistent recurrence. By combining physical protection measures, “skin hardening” therapy through phototherapy, and appropriate medication use, patients can effectively manage symptoms. Over time, along with the natural adaptation of the immune system, the effects of PMLE will gradually diminish, providing confidence to patients in outdoor activities.
