Cellulitis (rare plural: cellulitides) is an acute infection of the dermis and subcutaneous tissues. It results in pain, erythema, edema, and warmth. Since the epidermis is not involved, cellulitis is not transmitted by person-to-person contact.
Cellulitis can affect any region of the body, and commonly affects a lower limb. Clinical presentations include skin erythema without a well-defined border, increased skin temperature, swelling of the affected area, and regional lymphadenopathy and lymphangitis. Additionally, systemic features such as fevers and rigors may also be present.
If the infection spreads to deeper tissues, complications can occur, such as soft-tissue abscess, infectious myositis, necrotizing fasciitis, or osteomyelitis. Occasionally sepsis may result. Special consideration should be given to geriatric patients, in whom cellulitis of the lower extremities is more likely to develop into thrombophlebitis.
Cellulitis occurs after disruption of the skin and invasion of the subcutaneous tissues by microorganisms that may be skin flora, such as Staphylococcus aureus, or other bacteria. Patients with peripheral vascular disease or diabetes mellitus are particularly susceptible to cellulitis since minor injuries to the skin or cracked skin in the feet or toes can serve as a point of entry for infection.
Ultrasound is usually the first investigation to evaluate a clinical suspicion of cellulitis. Normally the subcutaneous tissue is hypoechoic with few hyperechoic strands (representing connective tissue). Above this, there is a narrow, relatively hyperechoic epidermal-dermal layer. Muscular fascia lies deep to the subcutaneous layer.
- sonographic hallmarks of cellulitis include abnormal echogenicity and increased thickness of the dermis with indistinct "haziness" and increased echogenicity of the subcutaneous tissue
- it is often helpful to compare the area in question to the (presumably normal) contralateral side
- progressive accumulation of edema in the subcutaneous tissue appears as branching, anechoic striations which impart a lobulated ("cobble-stone" appearance)
- also present in other edematous states
- presence of thickened and abnormally echogenic overlying skin will favor cellulitis over edema
- linear anechoic bands of fluid deep to the subcutaneous layer favor lymphedema
CT is used to accurately differentiate between superficial cellulitis and deep cellulitis.
In uncomplicated cellulitis, CT demonstrates skin thickening, septation of the subcutaneous fat, and thickening of the underlying superficial fascia. If the infection spreads to deeper tissues, soft-tissue abscess, infectious myositis, necrotizing fasciitis, and osteomyelitis can all be detected with CT.
Treatment and prognosis
Uncomplicated cellulitis is usually treated conservatively with antibiotics and locally supportive measures.
Clinical differential diagnoses include:
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