Emergent Soft Tissue Conditions

Cellulitis

Cellulitis is a localized infectious process that affects the dermal layers and subcutaneous fat. The infection often begins with minor disruption of the skin that allows the invasion of microorganisms into the subcutaneous tissues. There is an increased risk in patients with diabetes mellitus, peripheral vascular disease, human immunodeficiency virus (HIV) infection, immune deficiency from other disease processes, or a retained foreign body. The most common organism involved is Staphylococcus aureus . Typically patients present with fever, focal pain and swelling, erythema and warmth, and leukocytosis.

Radiographically cellulitis manifests with soft tissue swelling causing induration of the subcutaneous fat. On computed tomography (CT), swelling may focal or diffuse, with altered soft tissue attenuation that enhances variably with intravenous administration of iodinated contrast. Skin thickening can accompany the underlying inflammation. Although magnetic resonance imaging (MRI) rarely is required for diagnosis of cellulitis, this modality is indicated when the character of the disease is atypical, either because of suspected abscess formation, neoplastic disorder, or a rapidly progressive nature that can compromise a limb or become life threatening. Foreign bodies are frequently the nidus of infection in the foot and hand. Ultrasonography is the preferred modality for identifying retained foreign bodies, particularly those of wooden composition.

On MRI, cellulitis is characterized by focal or diffuse regions in the subcutaneous fat of low signal intensity on T1-weighted images with corresponding striated high signal intensity on fluid-sensitive sequences ( Fig. 17-1 ). Enhancement occurs after administration of intravenous gadolinium, differentiating these findings from nonspecific soft tissue edema. The adjacent fascia may also enhance.

Abscess

A focal soft tissue infection may become diffuse and evolve into a phlegmon or become more localized into an abscess. A phlegmon is a purulent inflammatory process that may either spread or become walled off into an inflammatory mass, often occurring without a bacterial infection. An abscess is a well-demarcated collection of purulent fluid that has accumulated as a result of an infectious process. It is differentiated from a phlegmon by the formation of a capsule in an attempt to contain the infected fluid from adjacent structures. The clinical presentation of both processes may be similar to that of cellulitis. An abscess, however, requires drainage in addition to appropriate antibiotic treatment. Frequently S. aureus is isolated as the infectious agent, and as many of 51% of infections may be methicillin resistant.

Unless gas is conspicuous, an abscess may be difficult to identify on radiographs. An abscess has a distinctive appearance on contrast-enhanced CT, with a brightly rim-enhancing fluid collection with an irregularly thickened wall ( Fig. 17-2 ). Internal attenuation is variable and influenced by the amount of cellular debris and protein in the fluid. The hallmark of an abscess is gas within the fluid collection. On MRI an abscess is characterized by a localized fluid collection with intermediate to low signal intensity on T1-weighted images and bright signal intensity on fluid-sensitive sequences. The developed wall is often irregular and thick and enhances peripherally after intravenous administration of gadolinium contrast material.