Cellulitis is an inflammatory condition affecting the subcutaneous layer of the skin, often involving the lower dermis as well. It is almost always due to an infection. Cellulitis is often confused with erysipelas which is a more superficial infection of the skin, affecting the dermis but also frequently extending to the upper parts of the subcutaneous tissue. It is often difficult to differentiate between cellulitis and erysipelas.
The infection is mainly caused by gram positive bacteria, particularly group A streptococci, invading the skin and subcutaneous tissues. Staphylococcus aureus infection is another common cause of cellulitis and treatment may be more aggressive with MRSA (methicillin-resistant Staphylococcus aureus).
The onset of cellulitis is often preceded by an injury to the skin but in some cases like following surgery, previous history of cellulitis and leg edema (swelling), there may be not any apparent injury to the skin at the affected site. Dry skin can lead to microabrasions of the skin which may also serve as an entry point to invading microorganisms.
Some of the bacteria involved in cellulitis are otherwise harmless strains normally present on the skin, in the mouth or nasal cavity. Others are transferred from the environment, water and inanimate objects (fomites). Cellulitis can affect any part of the body but is more common on the arms and legs.
The subcutaneous layer lies under the dermis, which is the deeper layer of the skin. It is a connective tissue layer abundant in fat, nerves and blood vessels. Once the bacteria reaches the affected site, it multiplies and irritates the surrounding tissue as explained under the inflammation process. The action of the immune cells and chemicals (mediators) further propagates the inflammation.
Cellulitis of the leg (Picture from Wikimedia Commons)
The cardinal features of inflammation are present in cellulitis – pain, heat, swelling (edema) and redness of the skin (erythema). With erysipelas, the erythema is clearly demarcated with raised red borders, while it is flat in cellulitis.
Blisters on the skin surface may be apparent and an abscess, which is an accumulation of pus, may develop at the site. Other features that may be noted is swelling around the area, enlarged lymph nodes, fevers and chills. Long red streaks may be present which is due to involvement of the lymphatic system.
Cellulitis of the arm (Picture from Wikimedia Commons)