Soft tissue abscess
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Soft tissue abscesses are focal or localized collections of pus caused by bacteria or other pathogens surrounded by a peripheral rim or abscess membrane found within the soft tissues in any part of the body 1.
Soft tissue abscesses include subcutaneous abscesses, intramuscular abscesses and intermuscular abscesses or abscesses located in the deep soft tissues within the fascial planes.
Intramuscular abscesses are localized, walled-off fluid collections within the muscle 1.
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Soft tissue abscesses are common and can occur in all age groups 2.
Factors that increase the likelihood of developing a soft tissue abscess include the following 1-3:
- diabetes mellitus
- cardiopulmonary disease
- intravenous drug use
- alcohol abuse
- chronic venous insufficiency
- trauma, lacerations, surgical incisions, skin breach
- infections elsewhere in the body
Soft tissue abscesses have been associated with the following conditions 1-3:
- skin ulcer
- infective endocarditis
The diagnosis of soft tissue abscesses can be made by a combination of typical clinical features and imaging findings and can be verified by fluid aspiration/tissue sampling in the setting of abscesses requiring drainage.
The presentation of soft tissue abscesses will depend on size and location as well as associated clinical conditions.
Local symptoms include swelling, erythema, warmth pain and tenderness with or without lymphadenopathy and possibly spontaneous purulent drainage 2,3.
General and systemic symptoms might include fever, malaise chills and/or sweats 2,3.
Laboratory studies will reveal increased inflammatory markers including leukocyte count, c-reactive protein (CRP) and erythrocyte sedimentation rate (ESR).
Complications of a soft tissue abscess include the following:
- systemic spread
- infective endocarditis
A soft tissue abscess consists of a central core or abscess cavity filled with pus and a peripheral abscess membrane or abscess capsule made up of leukocytes, macrophages, fibrin collagen and granulation tissue 4, that for one thing prevents further spread of infection and then again inhibits the influx of immunoreactive cells and antibiotic effectiveness.
Organisms that cause soft tissue abscesses include the following 1-3:
- Staphylococcus aureus
- streptococcus species such as Streptococcus pyogenes
- Serratia marcescens
- Pseudomonas aeruginosa
- Mycobacteria (avium/ tuberculosis)
Soft tissue abscesses are found within the superficial and deep soft tissue planes that are within and around the subcutaneous and deep fatty tissue, the fasciae and muscles 1; blood vessels and lymphatics might also be involved.
They are commonly seen in the trunk, extremities, buttocks and the perianal region as well as in the axillary region.
X-rays are generally of limited value for the evaluation of a soft tissue abscess but they might show soft tissue gas or foreign bodies increasing suspicion for an infectious process or reveal any other causes for underlying soft tissue swelling 2.
Ultrasound will usually show an irregular, hypoechoic, centrally avascular fluid collection surrounded by a hypervascular rim 2. The central core often shows internal echos that might move on compression 5.
CT will demonstrate an abnormal fluid collection of low attenuation with surrounding rim-enhancement, that might be irregular and thick if compared to the wall of a cyst. It might also reveal soft tissue gas 3.
There are usually inflammatory changes such as soft tissue edema and fat stranding in the surrounding tissues 3.
MRI features an overall high sensitivity and moderate to good specificity for the detection of an acute abscess 1,3 and might show high specificity in subacute, chronic or acute on chronic abscesses if the penumbra sign is present 1,6.
- abscess cavity: low to intermediate signal intensity 1,3
- capsular rim: low signal intensity - high signal intensity if the penumbra sign is present 5
- abscess cavity: high signal intensity 1,3
- capsular rim: low signal intensity
- abscess cavity: diffusion restriction that is hyperintense on high b-value and low signal on ADC 7
- capsular rim: -
T1 C+ (Gd)
- abscess cavity: low signal intensity/no enhancement
- capsular rim: avid enhancement
The radiological report should contain a description of the following 1:
- presence, location and size of the abnormal fluid collection
- tissue plane (subcutaneous, fascial, intramuscular)
- possibly penumbra sign if present
- associated findings
- skin ulcer
- sinus tract
The wording 'drainable soft tissue abscess' is considered poorly defined within the radiological report and has been discouraged if mainly based on imaging features and in the setting where the clinical context is not sufficiently known 1.
Terms such as 'tissue infarction', 'liquefied necrosis' and 'tissue necrosis' are histopathological and have been discouraged within a radiological report1.
Treatment and prognosis
The management will depend on the location, size, clinical symptoms, underlying clinical conditions and/or cause as well as complications 2,8. Incision and drainage is the mainstay in most cases 2. More complex abscesses might require surgical intervention and debridement 2,8.
Supportive measures include fever and pain management, warm compresses and limb elevation if applicable.
Supplementary oral antibiotics might be administered in the setting of multiple abscesses, medium-sized or larger abscesses, systemic symptoms, comorbidities, prosthetic devices or prosthetic heart valves.
Large or very large soft tissue abscesses, the presence of risk factors and/or complications as well as children or elderly patients might require hospitalization and/or intravenous antibiotics 2.
Conditions mimicking the radiological appearance of a soft tissue abscess include 1,2:
- sterile fluid collections
- post-operative seroma
- evolving hematoma
- early myositis ossificans
- foreign body reaction
- necrotic tumor
- devitalized tissue
- compartment syndrome with myonecrosis
- postcontrast imaging increases conspicuity and observer confidence 1,9,10
- DWI/ADC improves soft tissue abscess detection especially if intravenous contrast agents are contraindicated 1,11
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