Actinic Prurigo (Sunlight Induced Itchy Skin Rash)

What is actinic prurigo?

Actinic prurigo is an itchy skin condition that arises with sunlight exposure. Small and extremely itchy pimples (papules) appear on the areas exposed to the sun usually without a few days after intense sunlight exposure. Sunlight is the trigger but not the cause of the disease. The exact cause of this hypersensitivity to sunlight is unknown. Genetic factors that lead to abnormal immune activity seems the most likely contributing factor to actinic prurigo. The condition not only affects the skin as a significant number of sufferers experience lip and eye symptoms as well.

Actinic Prurigo Incidence

The exact incidence of actinic prurigo is unknown. It is more commonly seen in both North, Central and South America and particularly among Latin Americans and American Indians. The condition is rarely seen in Europe and Asia. About one third of all cases occur in children and women are more affected than men among adult sufferers. Unlike many skin conditions associated with sunlight exposure., actinic prurigo is more likely to affect darker skin complexions than people with lighter skin.

Actinic Prurigo Pathophysiology

The exact disease process of actinic prurigo is not well understood. It appears to be similar to an allergic reaction (type IV hypersensitivity). These reactions are triggered by specific antigens that activates a type of immune cells known as a T cell lymphocyte. The antigen in actinic prurigo is unknown. However, it is believed that a protein of the skin epidermis is transformed by sunlight and is then the antigen that provokes the reaction.

The abnormal immune response in actinic prurigo essentially makes it an autoimmune disease. This means that inflammation is a result of immune activity directed at the body’s own tissues or proteins. This inflammation leads to the formation of papules (raised lesions like pimples) that are intensely itchy.

Actinic Prurigo Causes

Although the exact cause of actinic prurigo is unknown there is an obvious genetic factor. This explains the prevalence among certain ethnic groups like Latin Americans and Native American Indians.  While several itchy skin diseases are aggravated or even triggered within a few minutes or hours after sunlight exposure, actinic prurigo is different in that it occurs several hours or even days after exposure. However, it has to be noted that sunlight is not a cause but simply a provoking factor of an underlying abnormal immune response.

Genetics

Several genes that regulate immune function may be an underlying factor in the development of actinic prurigo. This mainly involves :

  • HLA-Cw4
  • HLA-A24
  • HLA-DR4

The identification of these genes as possible contributing factors to actinic prurigo is largely due to its prevalence among the highest risk groups.

Actinic Prurigo Symptoms

The symptoms of actinic prurigo tends to arise a few hours or days after sunlight exposure. The condition is therefore worse in the summer and spring months when the sun exposure is the greatest.

Itching

Actinic prurigo is intensely itchy, hence the term prurigo (pruritis ~ itching). The itchiness accompanies the rash on the sun exposed areas. However, itching may even occur in regions covered by clothing in severe cases of actinic prurigo since UV (ultraviolet) rays can penetrate clothing.

Rash

Actinic prurigo is a form of dermatitis and the skin will typically be red and inflamed. The rash in actinic prurigo appears as :

  • slightly raised lesions like bumps (papules) which are similar to pimples
  • red inflamed patches (plaques)
  • nodules (lumps)

Since the rash primarily appears on the sun exposed ares of the skin, it is typically seen on the face, ears, neck, arms and hands.

Lips

About 2 out of 3 patients with actinic prurigo will also experience lip inflammation (cheilitis). The lips become swollen, itchy and becomes scaly with cracks and open sores (ulcers).

Eyes

Inflammation of the conjuctiva, known as conjunctivitis, occurs in about half of all actinic prurigo cases. There is excessive tearing with light sensitivity. Externally the eyes appear red and may even yellowish brown in color.

Actinic Prurigo Pictures

Pictures sourced from Dermatology Atlas (Brazil) courtesy of Samuel Freire da Silva, M.D.

Actinis Prurigo Diagnosis

There are no specific tests that can conclusively identify actinic prurigo. The diagnosis is largely dependent on the clinical presentation, medical history and exclusion of similar skin diseases. Phototesting may help confirm the diagnosis. Here a patch of the skin is demarcated and exposed to a source that simulates sunlight. A nearby control patch that is not exposed to the light source is demarcated. The test patch typically shows redness or other types of a rash within a few hours or up to 4 to 7 days. Microscopic examination of a skin biopsy sample may reveal inflammation of the skin and at times thickening of the skin.

Actinic Prurigo Differential Diagnosis

There are several skin conditions that are triggered or exacerbated by sunlight exposure. The most notable of these that may resemble actinic prurigo is polymorphous light eruption (PLE) and solar urticaria. PLE is more commonly seen in fairer skin complexions. The onset of solar urticaria typically arises within minutes after sun exposure, unlike the hours or days with actinic prurigo. Drug-induced photosensitivity of the skin another condition that needs to be excluded in the process of diagnosing actinic prurigo.

Actinic Prurigo Treatment

Actinic prurigo is incurable. It is a chronic skin disease and patients need to adopt lifestyle changes to minimize sun exposure especially in summer and spring. The two key preventative measures includes :

  • Sunscreen applications and sunglasses although both measures offer limited protection.
  • Covering up as much as possible with clothing although in severe cases, the rash can arise on the covered areas.

Medication

  • Thalidomide is the main drug used for actinic prurigo due to its immune-modulating properties.
  • Antimalarial drugs are used for its anti-inflammatory effects.
  • Topical corticosteroids are also effective anti-inflammatory agents.
  • 2% cyclosporine A is used for the eye symptoms like conjunctivitis.

Unmedicated emollients moisturize the skin and thicker applications can also provide protection from the damage associated with scratching.

Actinic Prurigo Complications

There are no significant permanent complications associated with actinic prurigo. Acute bacterial infections primarily arise with scratching. Long term and at times excessive use of sunscreens can irritate skin that is not inflamed. This is known as irritant contact dermatitis.

References :

http://www.atlasdermatologico.com.br/

http://emedicine.medscape.com/article/1120153-overview

http://dermnetnz.org/reactions/actinic-prurigo.html

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