Understanding Skin Plaque Causes

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Psoriasis plaque on woman's knee
What conditions cause skin plaques?. PositiveFocus/iStock

A plaque is an elevated, solid, superficial lesion that is typically more than one centimeter in diameter (a little more than half an inch) and associated with a number of skin conditions, most commonly psoriasis. The word plaque is French for "plate" which is fitting because the lesions often look like miniature (dirty) plates.

Overview

A plaque is a flat, primary lesion. Some plaques are flat; others appear as a thickened area of the skin that isn't visibly elevated above the surface of the skin.

Plaques can have defined borders or not, and they can take on many different shapes, including:

  • Annular (ring-shaped)
  • Arcuate (half-moon-shaped)
  • Polygonal (varied and not geometric)
  • Polymorphic (varied shapes)
  • Serpiginous (snake-shaped)
  • Poikilodermatous (variegated)

Plaques can appear anywhere on the body, but they appear most often on the elbows, knees, scalp and lower back. The amount of skin affected by plaques varies. Sometimes plaques are a few small spots that look similar to dandruff; other times they are huge eruptions that cover larger parts of the body, like the forearms.

Symptoms

The symptoms associated with plaques vary depending upon the skin condition present (see below.) Generally, however, there are a few symptoms associated with plaques related to any condition:

  • Itching and burning
  • Soreness (often a deep, nonspecific achy feeling)
  • Swelling and stiffness on joints where plaques are located

    Causes

    There are likely several different mechanisms behind the formation of plaques, though plaque psoriasis has been studied most extensively.

    Although the exact cause of psoriasis is unknown, it's thought to be associated with the immune system and a type of white blood cell: the T lymphocyte, or "T cell." T cells are constantly working to fend off viruses and bacteria, but for those with psoriasis, T cells are overactive and accidentally fight off healthy skin cells.

    In turn, there is an over production of healthy skin cells and more T cells and white blood cells, which disrupts the skin-shedding cycle.

    New skin cells reach the outermost layer of the skin too quickly: in days when it typically takes weeks. Because the dead skin and white blood cells can't shed fast enough, they build up and create thick, scaly plaques on the surface of the skin.

    Associated Conditions

    There are several different types of skin rashes and conditions in which plaques may be present. These include:

    • Eczema - Also called atopic dermatitis, or allergic dermatitis, rashes (and plaques) from eczema are related to an allergy to something which comes into contact with the skin.
    • Psoriasis - A condition which affects roughly two percent of the population, the rashes of psoriasis are related to a number of factors including immune system activity, genetic predisposition, and environmental components. There are several different types of psoriasis, but plaque psoriasis, also known as psoriasis vulgaris, is the most common form.
    • Pityriasis rosea - As a fairly short term rash, pityriasis occurs most commonly in older children and young adults. The cause is not currently known, but it may be distinguished from some other skin conditions which cause skin plaques by the presence of a herald patch, a large and itchy patch that can be an inch to five inches in diameter which occurs first.
    • Seborrheic dermatitis - An inflammatory skin disorder which affects skin which has sebaceous glands, seborrheic is best known as being the condition which causes dandruff.
    • Tinea versicolor - Also called pityriasis versicolor, tinea versicolor is commonly confused with other rashes. It is a fungal infection caused by the yeasts Pityrosporum orbiculare and Pityrosporum ovale that are found on the surface of the skin and in hair follicles. It becomes a problem only when there is an overgrowth of the fungus. Since the culprit yeast turns off the production of melanin by melanocytes (the chemical which causes a tan), the rash is often hypopigmented, being lighter than surrounding skin. There are many conditions which increase the risk of this condition, most notably pregnancy. It can be diagnosed in a method similar to other fungal infections, but may also be visualized in the clinic with a Woods lamp. (Under a black light the rash glows a characteristic yellow.)
    • Ringworm - Ringworm, also known as tinea corporis, is another fungal infection which may be caused by several different fungi known as dermatophytes. The fungi live on the dead cells on the surface of the skin, and grow most commonly between the fingers and toes, on the scalp, The fungus thrives in a moist environment and occurs often in wrestlers, due to close contact and sweating. Poor hygiene can also increase risk.

    Diagnosis

    Plaques are just one of the several types of primary lesions that indicate a skin disease. Plaques can be a bit of a challenge to diagnose because there are several skin conditions which can cause plaques, but there are some unique findings which can help in telling these rashes apart. For example, seborrheic dermatitis plaques are scaly, itchy, red and typically found on oily parts of the body, including the face and chest. Pityriasis rosea plaques begin with one herald patch and spread. The plaques also resemble drooping tree branches.

    A Wood's light may be used to diagnose tinea versicolor, which may also appear different due to hypopigmentation. A KOH test may be positive with some of these conditions. Sometimes, a definitive diagnosis cannot be made visually or with the use of lab tests, and a skin biopsy will be needed to diagnose the skin disorder.

    Treatment

    The treatment of plaques depends upon treatment of the underlying skin disease. That said, regardless of the skin disease with which they are associated, plaques often respond to treatment with a topical cream or ointment, like a corticosteroid or retinoid. Oral medication, like an antihistamine, may also be prescribed to help control the itching (scratching can cause a vicious cycle with some of these conditions.) Plaque psoriasis can be treated using light therapy.

    Proper hygiene, regular use of a moisturizer, oatmeal baths and sun exposure (while wearing SPF, of course) are all said to ease discomfort and keep skin healthy, although they can't completely prevent plaques from occurring.

    Sources:

    Errichetti, E., and G. Stinco. Dermoscopy in General Dermatology: A Practical Overview. Dermatology and Therapy. 2016. 6(4):71-507.

    Kasper, Dennis L.., Anthony S. Fauci, and Stephen L.. Hauser. Harrison's Principles of Internal Medicine. New York: Mc Graw Hill education, 2015. Print.

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