Cellulitis (more specifically referred to as superficial cellulitis) 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 occurs following disruption of the skin and invasion by microorganisms that may be indigenous flora, such as Staphylococcus aureus, or exogenous bacteria. Patients with peripheral vascular disease or diabetes mellitus are particularly susceptible to this type of infection, 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), with a narrow, relatively hyperechoic skin (epidermal-dermal) layer immediately superficial and the fascia overlying muscle lying deep.
- sonographic hallmarks include abnormal echogenicity and increased thickness of the skin with indistinct "haziness" and increased echogenicity of the subcutaneous tissue
- helpful to compare sonographic appearance to analogous contralateral structures
- 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 cellulitic process
- linear anechoic bands of fluid deep to the subcutanous layer favor lymphedema
CT is used to accurately differentiate between superficial cellulitis and deep cellulitis (cellulitis associated with deep-seated infection).
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, deep cellulitis, soft-tissue abscess, infectious myositis, necrotizing fasciitis, and osteomyelitis can all be detected with CT.
Uncomplicated cellulitis is a clinical diagnosis and is treated conservatively with antibiotics and locally supportive measures. If the infection spreads to deeper tissues complications and associated pathology can occur, e.g. deep cellulitis, soft-tissue abscess, infectious myositis, necrotizing fasciitis, or osteomyelitis. Special consideration should be given to geriatric patients, in whom cellulitis of the lower extremities is more likely to develop into thrombophlebitis.