A subcutaneous abscess is a manifestation of a spectrum of soft tissue skin infection which includes cellulitis and necrotizing fasciitis. It is a form of abscess which lies within the dermis and subdermal cutaneous layers. Along with dental abscesses, subcutaneous abscesses are the most common site for abscess formation.
Patients typically present with an acute or subacute history of a focal swelling or lump in the affected skin with accompanying signs of cellulitis. If there is bacteremia patient may present with systemic signs of sepsis such as fever, rigors, and raised inflammatory markers.
Skin abscesses are overwhelmingly caused by bacteria which spread into the subcutaneous tissues through breaches in the epidermis. Streptococcus has traditionally been the most common pathogen. Fungal abscesses are rare. In some cases sterile abscess formation has been described where a irritant drug or substance is injected into the skin, resulting in aseptic inflammation and abscess formation.
Subcutaneous abscesses are usually diagnosed clinically and do not routinely require any imaging. In complex cases where possible radical surgical treatment is being considered imaging may be undertaken to determine the extent of soft tissue involvement. CT remains the preferred modality for most abscess evaluation. Ultrasound is being shown to be increasingly useful in differentiate cellulitis with gross cutaneous swelling from a true abscess with one study reporting that ultrasound evaluation changed management in half of all emergency cases of cellulitis.
- anechoic or hypoechoic spherical collections of echogenic fluid
- there may be an echogenic capsule but often the borders are poorly defined
- septae, sediment, or even gas may be seen within the central fluid collection.
- compression with the transducer may induce movement or swirling of the abscess contents
- cobblestone appearance of surrounding subcutaneous tissues due to edema from associated cellulitis
Possible imaging differential considerations include
- lymph node enlargement
- sebaceous cyst
- herniated bowel
- blood vessel