Polymorphous light eruption (PMLE) is an acquired disease and is the most common of the idiopathic photodermatoses. PMLE is characterized by recurrent, abnormal, delayed reactions to sunlight, ranging from erythematous papules, papulovesicles, and plaques to erythema multiforme –like lesions on sunlight-exposed surfaces. Within any 1 patient, only 1 clinical form is consistently manifested.
The possibility that a subset of PMLE called benign summer light eruption or BSLE which might be milder and might be more UVA driven has been suggested by an Italian group. 
Richards et al found that PMLE engenders a substantial psychosocial impact on patients who have the condition.  Based on results from the Illness Perception Questionnaire sent to 302 patients and returned by 150 patients, 40% experienced emotional distress linked to PMLE. The psychological impact was related to the predicted consequences of PMLE, whereas health-related variables played a lesser role. Women associated more severe consequences linked to PMLE and were more emotionally distressed than men.
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The etiology of polymorphous light eruption (PMLE) is not fully known, and it is likely to be multifactorial. The immunologic pathogenesis of PMLE is supported by the study of timed biopsy samples of PMLE lesions. The CD4 subtype of T cells seen very early after exposure is replaced by CD8 lymphocytes 72 hours after irradiation. In general, findings conform to type IV delayed-type hypersensitivity mechanism.
Some have suggested that glutathione S-transferases (GSTs) act to protect against PMLE, but a study of the isoenzymes of the GST genes GSTM1, GSTT1, and GSTP1 found no protective relationship of these isoenzymes to PMLE. 
It is possible that the use of tobacco makes PMLE worse. 
In some PMLE lesions induced by UV-A, keratinocytes were found to express intercellular adhesion molecule 1 (ICAM-1). [5, 6] ICAM-1 is absent from normal keratinocytes, but it is known to be strongly induced by interferon gamma. The induction of ICAM-1 on keratinocytes results either from direct effects of UV on the promoter region of the ICAM-1 gene or from indirect effects of interferon gamma produced by activated lymphocytes aggregating in an underlying PMLE.
Intravascular and focal perivascular deposits of fibrin were detected in biopsy samples of PMLE papules. Vascular deposits of C3 and immunoglobulin M (IgM) were noted in a few patients. These findings may suggest that vascular injury with activation of a clotting cascade may play a role in the pathogenesis of PMLE. Repair of ultraviolet-damaged DNA is normal.
The demonstration that the female hormone 17beta-estradiol prevents UV radiation–induced suppression of the contact hypersensitivity response caused by the release of immunosuppressive cytokines (interleukin [IL]–10) from keratinocytes might thus explain why the risk of PMLE is higher in females than in males and why the risk decreases in women after menopause. 
Neutrophils may play a role in the development of PMLE. Immunohistochemical analysis by Schornagel et al in 2004 showed a significant decreased neutrophil infiltration in PMLE skin after UV-B irradiation compared with healthy case control subjects (P < .05).  ICAM-1 and E-selectin expression on endothelial cells increased in both healthy controls and in the PMLE patients after UV-B irradiation. Chemotactic response towards IL-8 and C5a was not different between PMLE patients and healthy controls. The authors concluded that PMLE is marked by an altered immune response resulting in decreased skin infiltration of neutrophils after UV-B irradiation.
Kölgen et al noted that the reduced expression of tumor necrosis factor-alpha, IL-4, and IL-10 in the UV-B–irradiated skin of patients with PMLE.  The reduction of these cytokines seems linked to a relative neutropenia and is a manifestation of decreased Langerhans cell migration and reduced TH2 skewing. An impairment of these mechanisms underlying UV-B–induced immunosuppression may be important in the pathogenesis of PMLE. 
A study by Koulu et al assessed a total of 48 subjects (24 patients with PMLE and 24 healthy sex-matched and age-matched controls).  The study found similar immunosuppression of contact sensitization to diphenylcyclopropenone due to earlier exposure to solar-simulating UV radiation between both groups. However, among patients with PMLE who were immunosuppressed by UV radiation, only one exhibited immunotolerance to the same allergen 10–24 months later (P=0.023). The study concluded that impaired propensity to UV-induced, allergen-specific immunotolerance may promote recurrent PMLE.
It has recently been shown that UV-C can cause PMLE. 
Polymorphous light eruption (PMLE) affects about 10% of the US population. This figure is likely to be an underestimate because many patients do not seek medical attention. Many of the photodermatoses were lumped together before their individual pathogeneses were identified. PMLE is now a distinct clinical entity, as are many of the other photodermatoses (eg, solar urticaria, photoallergic dermatitis, hydroa vacciniforme, chronic actinic dermatitis, erythropoietic porphyria, lupus erythematosus).
Reported in 2007, Kerr and Lim identified 280 patients with photodermatoses.  One hundred thirty-five (48%) were African Americans, 110 (40%) were white, and 35 (12%) were patients of other races. They noted a statistically significantly higher proportion of African Americans with PMLE compared with whites.
Deng et al used a questionnaire to survey 4899 residents (49% men and 51% women) of random Chinese villages in Yuan Jiang county (Dai and Hani minorities), Kunming city (Han people and Yi minority), Lijiang county (Naxi minority), and Shangri-La county (Zang minority).  The altitudes of these regions were 380 meters, 1870 meters, 2410 meters, and 3280 meters, respectively. The prevalence of PMLE was 32 (0.65%) in 4899 residents and was 3.8 times higher in women compared with men. At higher elevations, the prevalence of PMLE increased. The mean time of sun exposure for PMLE was 6 h/d. The mean duration of PMLE was 5.8 years.
PMLE affects 21% of the population in Sweden.
A report from India in 2013  notes that PMLE is the most common photodermatitis after chronic actinic dermatitis, while hydroa vacciniforme and solar urticaria are uncommon. Furthermore, this report stated lichenoid PMLE and pin-point papular PMLE are the most common types found in the subcontinent.
Polymorphous light eruption (PMLE) affects all racial skin types. Overall, family history is positive for PMLE in about 15% of the patients. However, Native Americans have a hereditary form of PMLE with apparent autosomal dominant inheritance; 75% reveal disease in a family member.  The popular variant can involve the face and seems most common in patients with Fitzpatrick type skin III-VI. [19, 20]
One study of a cohort of 229 patients with photodermatoses reported that 63 (42.2%) were white and 138 (46.6%) were African American and PMLE was present in 54% and 86.2%, respectively, (P < 0.0001); thus, the researchers suggested that PMLE occurs more frequently in African Americans. 
Polymorphous light eruption (PMLE) affects females 2-3 times more often than males. However, these data may be skewed because women are more likely to seek medical attention for cosmetic problems than males.
Polymorphous light eruption (PMLE) usually has an onset in the first 3 decades of life. Men seem to have later onset of the disease than women.
Naleway et al reviewed records of 124 patients diagnosed with PMLE and found most were women and that the mean age of PMLE onset was 37.8 years.  They noted only 4 required phototherapy treatment.
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