Soft Tissue Infection
- Etiology - many ways - inhalation, ingestion, direct inoculation, hematogenous, lymphatic, direct invasion
- Factors that increase risk of infection - host immune system, poor vascular supply, necrotic tissue, tissue injury, collections of blood or lymph, implanted devices, foreign bodies
- Tumors can become secondarily infected
- Systemic disorders or immune suppression may alter how infection presents
- Staphylococcus Aureus - most common etiology of soft tissue infection; Streptococcus A is number 2
- Some occupations predispose to certain infections - rice field workers and meliodosis; tropical fish enthusiasts - mycobacterium marinum
- Presentation dictated by type of infection
- Typical - early is erythematous, warm, increased blood flood and dilated capillaries - complain of pain, swelling an heat; Febrile; subsequently may have tissue destruction and necrosis; septic shock - hypotension, tachycardia
- Slow growth infections - local destruction and abscess formation; microcavities to larger cavities; may invade adjacent structures; sinus track to skin
- Invade joint - can cause early irreversible articular damage; low blood supply to joint naturally
- Chronic infections - stable or steady state; chronic reactive changes and acute infection changes; fibrous tissue adjacent an abscess; develop cloacae or sinus tracks lined by epithelium
- Chronic - may eventually have amyloid development or malignant transformation.
- X-ray - limited - may show nothing; may show soft tissue edema or secondary effects such as mass effect on adjacent structures, periosteal reaction in adjacent bone or bone changes from an adjacent abscess
- Ultrasound useful for abscess versus cellulitis.
- MRI - find abscesses, determine sinus tracks, determine involvement of adjacent bone
- Cellulitis - superficial; can involve tendons
- Abscesses - many sources - s. aureus, bowel flora etc; if sinus tracks can cause further spread and can be associated with Squamous Cell Carcinoma in long term skin infections
- Acute septic arthritis - in kids most are not infectious but transient synovitis - but they look the same - so aspirate; if transient - it will help pain; If infection - will help find source
- Acute septic arthritis - Adults - Diff Dx - acute arthropathy, gout, pseudogout, polyarthropathy and systemic arthropathies. Strep pyogenes, staph aureus, N. gonorrhea, Beta-hemolytic strep
- Chronic Septic Arthritis - Organism must be determined; prosthesis - often skin contaminates;
- Pyomyositis - fever, pain with passive motion; elevated markers; A muscle or other muscles; gas gangrene is severe and often fatal; often fluid collections; staphylococcus and clostridial myonecrosis
- Necrotizing fasciitis - group A strep or polymicrobacterial; gas is possible; fascia will be thickened; muscles and muscle groups will have abnormal signal
- Tuberculosis - 3rd most common infection worldwide - "cold" abscess - may not have a pyrexial component; when invading bone can look like a malignancy; mycobacterium marinum - nodular lymphadenitis
- Fungal - often immunocompromised; variable presentation
- Ultrasound - useful for determining if fluid collection or just soft tissue edema
- MRI - definitive study for both dx and progression
Reference: Thomas Pope et al Expert Consult Musculoskeletal Imaging 2nd Edition
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