Soft tissue abscesses are focal or localized collections of pus caused by an immune response to pathogenic microorganisms. They are surrounded by a peripheral rim or abscess membrane and can be found within the soft tissues in any body part 1.
Soft tissue abscesses include subcutaneous abscesses, intramuscular abscesses, intermuscular abscesses, and abscesses located in the deep soft tissues within fascial planes.
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Epidemiology
Soft tissue abscesses are common and can occur in all age groups 2.
Risk factors
Factors that increase the likelihood of developing a soft tissue abscess include the following 1-3:
alcohol excess
trauma, lacerations, surgical incisions, skin breach
infections elsewhere in the body
Associations
Soft tissue abscesses have been associated with the following conditions 1-3:
Diagnosis
The diagnosis of soft tissue abscess can be made by a combination of typical clinical features and imaging findings and can be verified by fluid aspiration/tissue sampling if drainage is indicated.
Clinical presentation
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
Complications of soft tissue abscess include the following:
systemic spread
infective endocarditis
sepsis
Pathology
A soft tissue abscess consists of a central core or abscess cavity filled with pus and a peripheral abscess membrane or capsule made up of leukocytes, macrophages, fibrin, collagen and granulation tissue 4.
These factors combine to inhibit further spread of infection while simultaneously impairing the influx of immune cells and antibiotic effectiveness.
Etiology
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)
Nocardia spp.
fungi
Location
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.
Radiographic features
Plain radiograph
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
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 may move with compression 5.
CT
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
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.
Signal characteristics
-
T1
abscess cavity: low to intermediate signal intensity 1,3
capsular rim: low signal intensity - high signal intensity if the penumbra sign is present 5
-
T2/T2FS/IMFS
abscess cavity: high signal intensity 1,3
capsular rim: low signal intensity
-
DWI/ADC
abscess cavity: diffusion restriction that is hyperintense on high b-value and low signal on ADC 7
capsular rim: -
-
T1 C+ (Gd)
abscess cavity: no enhancement
capsular rim: avid enhancement
Radiological report
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
osteomyelitis
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 report 1.
Treatment and prognosis
Management of soft tissue abscess 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 of treatment in most cases 2. More complex abscesses may require surgical intervention and debridement 2,8.
Supportive measures include analgesia and antipyretics, 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.
Patients more likely to require hospitalization and/or intravenous antibiotics include 2:
those with large or very large soft tissue abscesses
risk factors for severe disease and/or complications (e.g. immunocompromise)
pediatric and elderly patients
Differential diagnosis
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
Practical points
MRI
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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