Differential Diagnoses

Numerous conditions can mimic cellulitis:

Venous insufficiency

Venous insufficiency (lipodermatosclerosis) is also called:

• Venous eczema
• Stasis eczema
• Gravitational eczema

This condition develops secondary to poor venous drainage in the leg.

Episodes of cellulitis or deep vein thrombosis damage the valves, back pressure develops and fluid collects in the tissues, resulting in chronic skin inflammation and discolouration – see image. The affected leg is usually swollen, due to both inflammation and lymphoedema. Dermatitis is usually bilateral, with yellow crusting, scaling, and marked itching.

It may become secondarily infected (as in the left leg shown in the image) in which case discomfort and erythema increase, differentiating acute cellulitis from the chronic appearance.

Click on the image to enlarge.

Post-phlebitic limb

Thrombophlebitis results in chronic skin discoloration and scarring, sometimes with oedema – see image.

However, there is no erythema and generally no discomfort unless there is associated eczema.

Click on the image to enlarge.

Thrombophlebitis

This is an inflammation of the superficial leg veins, resulting in a tender red or purple subcutaneous ‘cord’ and associated swelling along the track on the vein – see image.

It may be secondary to deep vein insufficiency, cannulation or injection (e.g. in intravenous (IV) drug abusers), or malignancy (causing migratory thrombophlebitis known as Trousseau’s Syndrome).

The image is reproduced with permission from Wellcome Images – click on the image to enlarge.

Deep vein thrombosis (DVT)

DVT does not usually cause significant erythema or blistering, though there may be associated superficial thrombophlebitis.

Similarly it is not generally tender, although there may be tenderness along the deep veins.

Pitting oedema may be present, and swelling may be marked compared to the unaffected side – more than 3 cm – see image.

The image is reproduced with permission from Wellcome Images – click on the image to enlarge.

Other types of eczema (e.g. atopic or contact)

Various forms of eczema can affect the leg.

Discoid eczema can arise 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.

Exudative or ‘wet’ discoid eczema is often triggered by an injury to the skin (insect bite, burn or cut), 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.

Atopic eczema is seen mainly in children. In older children and adults it is commonly found behind the knees, but can affect large areas during flares or if secondarily infected. Characteristically, atopic dermatitis is very itchy.

The image is reproduced with permission from Wellcome Images – click on the image to enlarge.

Panniculitis (vasculitis such as erythema nodosum)

Panniculitis refers to a group of conditions that involve inflammation of the fat under the skin, with or without associated vasculitis.

Despite having very diverse causes, most forms of panniculitis have the same clinical appearance.

Affected skin feels thickened and woody to touch. There may be discolouration of the overlying skin, either reddening or darker, brownish pigmentation. The area is often tender. Most often, the affected areas appear as raised nodules or lumps under the skin, but may be a plaque or large flat area of thickened skin.

Purpuric discoloration may be present and there may also be bullae and erosions. Causes include the collagen vascular disorders such as rheumatoid arthritis, sarcoidosis, polyarteritis nodosum, also Crohn’s disease, steroid therapy, necrobiosis lipoidica and erythema nodosum.

The image is reproduced with permission from Wellcome Images – click on the image to enlarge.