Polymorphous light eruption

1. General info about the skin
2. Sun and sunlight
3. Definition
4. Aetiology
5. Clinical features
6. Investigation and diagnosis
7. Treatment and prophylaxis

Synonyme

• PMLE
• Benign Summer Eruption
• Prurigo Aestivalis
• Eczema Solare
• Summer Prurigo

Differential diagnoses

• Acne Aestivalis
• Allergic Contact Dermatitis, Acute & Chronic
• Allergic Contact Eczema Type I
• Allergic Vasculitis
• Atopic Eczema
• Bloomīs Syndrome
• Discoid Lupus Erythematosus (DLE)
• Eczema (prurigo like)
• Erythema Exsudativum Multiforme, Minor Form
• Erythropoietic Protoporphyria
• Hartnupīs Syndrome
• Hidroa Vacciniforme
• Lymphocytic Infiltration of the Skin Jessner-Kanof
• Parapsoriasis en Plaque Brocq, Large-Plaque Type
• Photoallergic Contact Dermatitis
• Photoallergic Drug Eruption
• Prurigo Actinica
• Prurigo Simplex Subacuta
• Prurigo, Acute of Childhood
• Pruritic Urticarial Papules and Plaques of Pregnancy
• Secondary Lues
• Solar Urticaria
• Subacute Cutaneous Lupus Erythematosus SCLE

1. General info: What are the Main Functions of the Skin?

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With a total surface of about 1,8 m² and a total weight of about 11 kg, our skin is the largest human organ. The skin not only gives us our appearance and shape, it also serves other important functions:

Protection
Our skin is a shield that protects us from:
- mechanical impact such as pressure and stroke
- thermal impact such as heat or cold
- environmental impact such as chemicals, the sunīs UV-radiation and bacteria

Regulation
The skin regulates our body temperature. The production of sweat, which evaporates on the skinīs surface, will cool us down.

The 5th Sense
Besides the senses of smell, taste, sight and hearing the sense of touch is one of our body's most important senses. Without it, we would not be able to feel the gentle touch of another person, the warmth of a hot cup of tea, or the wind blowing in our face. This sense is made possible by various cells and nerve endings in the skin, which send impulses to our central nervous system.

Different Parts of the Skin

Epidermis

Epidermis
This layer is seen on the surface of the skin. It is made up of cells called keratinocytes, which are stacked on top of each other, forming different sub-layers. The keratinocytes develop at the bottom and rise to the top, where they are shed from the surface as dead cells. So this layer is constantly renewing itself, the live cells changing into dead, hard, flattened cells. Melanocytes and Langerhans cells are other important cells found in the epidermis which have special functions.

• Melanocytes
  These cells produce a dark pigment called melanin which contributes to skin color and provides UV protection. They are located at the bottom of the epidermis.



• Dendritic (Langerhans) cells
  These cells are involved in the epidermal immune system. They engulf foreign material that invades the epidermis and migrate out of the skin to stimulate an immune response.



• Basal cells
  Small cells found at the bottom of the epidermis. Earlier it was believed that basal cell carcinoma is derived from these cells. As of this writing basal cell carcinoma is thought to arise from immature cells from the basal cell layer.

• Blood Vessels
  These are tiny pipes through which blood circulates. The blood vessels supply the skin with fresh blood, which contains nutrients and oxygen, and carry away waste products.



• Meissner's corpuscle
  These touch receptors are especially effective in detecting light touch and soft, fleeting movements.



• Pacinian corpuscles
  Pacinian corpuscles function as receptors for deep pressure and vibration.



• Free Nerve Endings
  Free nerve endings are sensitive to pain, temperature changes and itchiness.



• Nerve Fibers
  Nerve fibres forward information.



• Sebaceous Glands
  Sebaceous or oil glands are small, sacculated organs that secrete sebum. This oily substance is a natural moisturiser which conditions the hair and skin. Sebaceos glands are found all over the body, but they are more numerous in the scalp area and around the forehead, chin, cheeks and nose.



• Sweat Glands
  These are sweat-producing structures consisting of a single tube, a coiled body and a superficial duct. They are involved in thermoregulation, as they cool the skin by sweating.



• Hair Follicles
  Hair follicles are downward growths into the dermis of epidermal tissue and produce hair. They are found all over the body except on the palms of the hands and soles of the feet or the lips. When the body gets cold, the hair stands upright with the help of the arrector pili muscle, closing up the skin's pores and keeping the warmth in.



• Arrector pili muscle
  This small muscle is attached to the base of the follicle. When it is stimulated by cold or fright, it pulls the hair follicle up, causing it to stand upright.

2. The Sun and Sunlight

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The sun is the star at the center of our solar system that is the source of warmth and energy for our planet. The sun's energy production takes place through nuclear fusion. These reactions emit electromagnetic energy, which is what interests us. Part of the electromagnetic spectrum is light.

The light can be broken down into its various wavelengths (colors). Part of the magnetic spectrum that we can not see is UV or ultraviolet light. UV light from the sun can damage DNA which is the genetic material of our cells. Research has shown, for example, that melanoma is primarily caused by UV-damaged melanocytes in our skin. These cells then grow out of control and can spread to other parts of the body

Important facts about sunlight

There are a few facts you need to know about sunlight to better understand its impact on the human body and our environment. Without sunlight no life would have come into being on our planet. It is in many ways essential to our existence on earth. But it can also harm us.

The concept of sunlight

If we think of the rainbow, we see light can have different colors, ranging from violet to red. But there is more to light than the colors we are able to see:

Light rays form waves and carry energy. Therefore they can be compared to waves of water. On a calm day the ocean has waves that are far apart. Their wavelength (from one peak to the other) is long and they do not carry much energy. The same with light – some light has long waves and does not carry much energy, other light has short waves, very close together, that carry lots of energy which may be harmful (ultraviolet-light). Just like the ocean waves after a hurricane.

UVA and UVB
The light we are able to see has a wavelength of 400 to 760 nm. Ultraviolet light, also called UV radiation, is classified into three types by its wavelength: UVA ranges from 400 to 320 nm, UVB from 320 to 290 nm, and UVC from 290 to 200 nm. Always keep in mind the shorter the wavelength, the more harmful the energy it carries. Fortunately our earth is surrounded by a defensive shield of gases which absorbs all of the UVC radiation and some of the UVA and UVB radiation. The rest of the UVA and UVB light, which passes through no matter if it is cloudy or sunny, can have different impacts on our skin.

The impact on our skin
Through UV exposure our DNA in the cells of our skin is damaged. Either those damaged cells die or they get repaired by the cellīs own repair mechanism. But if the damage is too severe and the cells cannot regenerate, those damaged cells can turn into skin cancer.

UVB radiation
UVB is known to be responsible for the long lasting tan one possibly gets when staying in the sun. It is the main factor in causing sunburn and skin cancer. Although it does not penetrate into the skin as deeply as UVA it carries lots of energy. That is why the first sunscreens included a UVB filter.

UVA radiation
Until today, scientists thought that only UVB was responsible for causing skin cancer but they had to realize that 10 to 100 times more UVA (compared to UVB) reaches the earth. In high doses, it caused skin cancer in animal experiments. They also had to realize that although it has less energy, UVA radiation penetrates deeper into human skin than UVB. It is also the main cause of skin aging. So there are enough reasons why you should always wear a sunscreen that protects you from UVA and UVB (so-called broad spectrum sunscreens).

3. Definition Polymorphic Light Eruption

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The polymorphic light eruption (PLE) is the most common photodermatoses. Photodermatoses are skin changes, i.e. rashes, induced by ultraviolet light. PLE characterized by a recurrent, delayed cutaneous reaction appearing a few hours after exposure to ultraviolet radiation mostly the sun in susceptible individuals. The reaction is variable in morphology, hence the term polymorphic. However, in each patient, a single morphology predominates and remains constant. The intermittent eruptions consist of non-scarring, red itchy papules (small spots), plaques (larger spots) or vesicles (blisters), which appear on sun-exposed skin. The V of chest, neck, and the upper and lower extremities are most commonly involved. Symptoms are worse in spring and early summer. The disorder usually begins in young and mid-adult life.

4. Aetiology

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The prime mechanism remains unknown, evidences suggest an allergic reaction. Experimental data reveals that ultraviolet radiation induces a T-cell response with features compatible with delayed hypersensitivity. The responsible allergen has not been identified

5. Clinical features:

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The polymorphic light eruption appears more frequent in shorter latitudes. While in Australia about 5%, in Scandinavia up to 20% of the population are affected. PLE is more common in light-skinned especially north Europeans but also dark-skinned people can be affected.

The condition is more frequent in females and begins often in young adults. Also children can be affected. Most patients report about itching or burning sensations and red raised eruptions of sunlight-exposed skin. The lesions are initially pinhead-sized and can coalesce into larger areas. The reaction is variable in morphology, hence the term polymorphic. However, in each patient, a single morphology predominates and remains constant. Also consistent is the timing of onset. Some subjects experience almost immediate symptoms and red colouration. In others the lesions appear after hours or the next day. Many lesions disappear within days while others take weeks to heal. The intermittent eruptions consist of non-scarring, red itchy papules, like insect bites, larger raised spots (plaques) or blisters, which appear on sun-exposed skin. The face, neck, and the upper and lower extremities are most commonly involved. Symptoms are worse in spring and early summer. The disorder usually begins in young and mid-adult life. The intensity between individuals tends to differ and can change with time. While some patients require only minutes to induce their eruption others have only once a year a mild episode. Mostly UVA wavelengths induce PLE, but also UVB wavelengths as well as both can be responsible. UVA penetrates window glass and even light cotton clothing, therefore eruptions may occur even from sitting in window seats or while wearing clothes. Well known is a hardening process after repeated sunlight. Thus a typical clinical feature is the sparing of some frequently exposed areas like face and hands. An induction by UVA tanning beds is common.

Figure 1: Young man with many large, red, confluent papules on the upper arms

Figure 2: 20 years old man with raised irregular red plaques on the upper arms

Figure 3: 66 years old woman with red plaques and small blisters

Figure 4: 42 years old man with many small follicular papules on the upper leg

Figure 5: Erythema and papules on the upper back

6. Investigation/Diagnosis:

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PLE is characterized by a typical course and appearance. Provocations of PLE can be performed in a special unit. About 50% of patients get typical lesions after challenge with artificial UV lamps. Microscopic analysis of skin tissue is mostly not necessary, but can be helpful where there is diagnostic difficulty.

7. Treatment/Prophylaxis

The mildly affected majority of patients will prevent their PLE to significant degree by control of sunlight exposure and by using a topical high factor broad-spectrum sunscreen. For others gradual sun exposure in spring effects browning and thickening of the skin (so called hardening) which often helps to avoid PLE. The skin hardening can also be reached using UVB phototherapy or photochemotherapy (PUVA) in spring. The latter treatment needs to be supervised by a trained physician. Commercial “sunbeds” are not recommended since they are most likely to provoke the PLE rash. Once the rash is present, it may be relieved by the use of a steroid cream, which is discontinued after a few days. On occasion, steroid tablets or injection may be needed – these are not used frequently due to potential side-effects.