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Rashes affecting the lower legs

Most often, a rash affecting the lower legs is a type of dermatitis. The terms ‘dermatitis’ and ‘eczema’ are often used interchangeably. Acute dermatitis presents as erythematous, oedematous and blistered plaques. Chronic dermatitis accompanied by rubbing and scratching results in darkened (hyperpigmented), and thickened (lichenified) plaques.

There are several different types of lower leg dermatitis, which is common at all ages.

Atopic dermatitis

Atopic dermatitis is particularly prevalent in children; in infants it frequently affects the feet and ankles, and in older children and adults it is often prominent behind both knees (the popliteal fossae). Characteristically, atopic dermatitis is very itchy and complicated by acute flare-ups. Sometimes the dermatitis is secondarily infected.

Discoid eczema

Discoid eczema, also called nummular dermatitis, arises at any age and results in round or oval plaques. Dry discoid eczema is often relatively non-itchy, and is often due to over-dry skin, when it is known as ‘asteatotic’ eczema. Exudative or ‘wet’ discoid eczema is much more resistant to treatment. It is often triggered by an injury to the skin (insect bite, thermal burn, cut, surgical wound), and colonised or infected by Staphylococcus aureus. Discoid eczema starts with a single patch on one leg, but soon multiple lesions appear on both lower legs and may later affect the trunk and arms. In severe cases, a diffuse generalised rash due to autosensitisation dermatitis may occur.

Varicose eczema

Varicose eczema refers to a discoid eczema that follows the line of varicose veins on one or both legs. Removal of the veins may be curative.

Gravitational dermatitis

Gravitational dermatitis, also called venous eczema, is due to valvular damage in leg veins and is due to past deep venous thrombosis (blood clot) or cellulitis (deep skin infection). The result is swelling, particularly after prolonged standing and during hot weather. Dermatitis can arise as discrete patches or affect the leg all the way around the ankle. The affected skin is red and scaly, and may ooze, crust and crack. It is frequently itchy. Surrounding skin may be irregularly discoloured from red blood cells that have leaked from the blood vessels because of the high venous pressure. Atrophie blanche is the term used for white scarred areas surrounded by tiny red spots (enlarged capillaries). Complications of gravitational dermatitis include secondary infection (impetigo and cellulitis), lipodermatosclerosis (hard deep tissues), autosensitisation (generalised rash) and ulceration.

Contact dermatitis

Contact dermatitis can be due to irritants, such as excessive bathing, soaps and tight socks, or to a specific allergic reaction. For unknown reasons, certain medicaments and cosmetics containing topical antibiotics, fragrances and preservatives are more likely to sensitise when applied to the lower leg than on other sites. The result is a blistering eruption precisely corresponding to the site of application of the topical agent. If the cause is uncertain, a dermatologist may be able to identify the specific allergens by patch testing.

Lichen simplex

Lichen simplex is a solitary well-demarcated plaque of chronic eczema and most often arises on the lateral lower leg, due to a long-standing habit of rubbing. The more it's rubbed, the itchier it becomes.

Prurigo nodularis

Prurigo nodularis refers to extremely itchy hard nodules 1-3 cm in diameter. They are very resistant to treatment. They most often appear on the lower legs of atopic subjects and can be extremely debilitating.

Other common conditions affecting the lower legs include:

Psoriasis

Psoriasis presents with symmetrical, red, scaly plaques, most marked on knees and shins.

Tinea corporis

Tinea corporis (fungal infection) results in irregular plaque(s) usually affecting only one leg, with spreading and more prominent edge, often originating from tinea pedis (look for a scaly foot) or tinea unguium (discoloured and crumbling toenails). Diagnosis should be confirmed by mycology of skin scrapings prior to use of topical or oral antifungal agents.

Folliculitis

Folliculitis are itchy or non-itchy pustules affecting hair follicles, often resulting from shaving or waxing.

Skin cancers

Skin cancers are relatively asymptomatic slowly growing red dry patches that tend to bleed (basal cell carcinoma, Bowen disease), or faster-growing scaly lumps (invasive squamous cell carcinoma). Surgical removal is usual, but options for treatment may include cryotherapy, photodynamic therapy or imiquimod cream.

Less common conditions favouring the lower legs include:

Pretibial myxoedema

Associated with an overactive thyroid gland.

Necrobiosis lipoidica

Associated with insulin dependent diabetes mellitus.

Non-venous leg ulcers

Many possible underlying causes.

Vasculitis

Small and/or large vessels may be affected

Capillaritis:

Brownish non-blanching spots.

Panniculitis:

Inflamed lumps under the skin.

Management

Management depends on identifying the particular pattern of dermatitis or other skin condition. General advice should include:

• Avoid and treat dry skin, using non-soap cleansers and thick simple emollients. Avoid exposing the legs to direct heat or the hot air of the heater in a car.
• Minimise swelling – avoid standing for prolonged periods, take regular walks, elevate the feet when sitting or overnight and wear special graduated compression stockings long term.
• Exudative eczema can be dried up using Condy's solution (1:10,000 potassium permanganate in water) or dilute vinegar on gauze as compresses.
• Creams/ointments containing coal tar may be helpful.
• Prescription treatments may include oral antibiotics for secondary infection and topical steroids of varying potency – weak products can be used long term if necessary but potent topical steroids should be used once or twice daily for short courses of one to four weeks. Severe dermatitis may require oral corticosteroids or immunosuppressive agents.
• Surgical or laser treatment of varicose veins may be appropriate.

Non-sedating oral antihistamines are not helpful, but promethazine can allow a more restful sleep.

Avoid topical antibiotics, topical antihistamines and multi-ingredient fragranced or herb-containing emollients because of the risk of provoking contact allergy.