Polymorphic Light Eruption (PMLE) is the most common skin sensitivity to light condition. It is a delayed hypersensitivity reaction of the immune system to endogenous antigens formed after skin exposure to ultraviolet (UV) light.
Epidemiology
Polymorphic light eruption is also known as sun allergy, sun poisoning, sunlight eczema, or summertime itch/rash.
- Age: Commonly found in young people aged 20-40.
- Gender: Women have a four times higher prevalence than men.
- Geography: Common in temperate climates, areas with little sunshine, or regions at high altitudes compared to sea level.
- 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. Key influencing factors include:
- Ultraviolet rays: UVA rays are the main cause (accounting for 75-90%), capable of penetrating window glass. UVB rays or a combination of both can also cause the condition.
- Immune disorders: In normal individuals, UV rays suppress local immunity to protect the skin. In PMLE patients, this mechanism is impaired, leading to increased inflammation with the presence of T lymphocytes (CD4 and CD8).
- Hormonal factors: Estradiol in women is believed to inhibit the natural UV-induced immune suppression, explaining the higher susceptibility in females.
- Genetics: About 15-46% of cases have a family history.
Clinical Manifestations
The condition often flares in spring and early summer, appearing after sun exposure from a few hours up to 2 days, and subsides or completely disappears in winter. However, there are cases of flaring in winter when patients are exposed to intense sunlight such as sunbathing or traveling to geographic areas with higher light intensity.
Lesion morphology: Although termed “polymorphic,” in a specific patient, the lesion morphology tends to be consistent across episodes, including:
- Papules, vesicles, or eczema-like red patches.
- Wheal-like or target-like lesions (resembling erythema multiforme).
- Lesions will worsen if the skin area continues to be exposed to sunlight.
- Papules and skin lesions can completely heal without scarring.
Location: Symmetrical on exposed skin areas (arms, legs, neck, chest). Less common on the face and the backs of hands due to the “skin hardening” effect.
Symptoms: Intense itching is a main manifestation. 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 lessen by the end of summer.
Diagnosis
Diagnosis primarily relies on history taking and clinical examination.
Paraclinical: Skin biopsy (non-specific, mainly showing spongiosis and lympho infiltrates). Severe cases may exhibit papillary edema. Antinuclear antibody (ANA) testing to exclude Lupus.
Phototesting: Light stimulation tests can be performed to confirm the diagnosis (positive in about 60% of patients).
Differential diagnosis: Should be differentiated from Lupus erythematosus, Porphyria, solar urticaria, and contact photodermatitis.
Treatment
Preventive measures
- Use broad-spectrum sunscreen (SPF 50+, against both UVA/UVB).
- Wear protective clothing, avoid going out in peak sunlight hours.
- Choose areas with soft light, shade if having to be outdoors.
- Limit sitting near windows, especially glass windows, when indoors.
Specific treatment
- Topical: Topical corticosteroids or Calcipotriol.
- Systemic: Short-term oral corticosteroids (1-2 weeks) for acute flare-ups. Hydroxychloroquine or Nicotinamide (used before flare-up seasons).
- Phototherapy: Low-dose UVB or UVA exposure at the beginning of spring for gradual skin adaptation (causing tolerance).
- Severe cases: Azathioprine or Ciclosporin can be considered.
About 60% of patients improve or recover after 15 years, 75% improve or recover after 30 years, but PMLE can persist for life.
While PMLE does not pose a life-threatening risk and has a benign prognosis, it significantly impacts the quality of life and psychological well-being of patients due to its persistent, recurrent nature. By combining physical protective measures, “skin hardening” therapy through phototherapy, and reasonable medication use, patients can effectively manage symptoms. Over time, with the natural adaptation of the immune system, the impact of PMLE will gradually subside, restoring confidence in outdoor activities.
