Soft Tissue Abscess

An abscess is a collection of pus in the dermis or subcutaneous tissue. Abscesses are one of the commonly encountered skin and soft tissue infections. Although abscesses may occur spontaneously, predisposing factors such as abrasions and punctures are often identified. A patient with a skin abscess usually presents with a localized, fluctuant, tender mass that appears red and warm. Incision and drainage are the mainstay of management, but antibiotics can be used depending on the size of the abscess as well as the patient’s risk factors for severe infection.

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Overview

Definition

Soft tissue abscess is a collection of pus in the dermis or subcutaneous tissue.

Epidemiology

  • Incidence of abscesses rose throughout the 1990s; associated with an increase in methicillin-resistant Staphylococcus aureus (MRSA).
  • Since the 2000s, the incidence of abscesses has plateaued.
  • Abscesses can develop anywhere, but commonly are seen on the trunk, extremities, underarms, and buttocks.

Etiology

Causative agents:

  • S. aureus (75% of cases):
    • Methicillin-sensitive S. aureus (MSSA)
    • MRSA
  • Polymicrobial: 
    • S. aureus
    • Streptococcus pyogenes
    • Gram-negative bacilli
    • Anaerobes 
  • Rare organisms:
    • Mycobacteria
    • Nocardia
    • Fungi (cryptococcus, blastomycosis)
  • Sterile: from injecting irritant drugs

Risk factors:

  • Loss of skin integrity due to: 
    • Trauma (e.g., abrasions) 
    • Inflammation (e.g., atopic dermatitis)
    • IV drug use
  • Immunosuppression (e.g., diabetes mellitus)
  • Pre-existing skin infection (e.g., folliculitis)
  • Obesity 
  • MRSA colonization
  • Venous insufficiency
  • Lymphedema   

Pathophysiology and Clinical Presentation

Pathophysiology

  • Break in the skin barrier → introduction of infection/causative agent
  • Local cell injury and necrosis causes release of cytokines.
  • Inflammatory cells are drawn to the source of infection/irritation.
  • Encapsulation by surrounding cells:
    • Prevents the spread of infection
    • Also inhibits penetration by immune cells and antibiotics, however
  • Infection in a closed space (e.g., blocked sweat gland or perianal gland, hair follicle) also facilitates abscess development.
  • Low pH inside the abscess cavity further decreases antibiotic effectiveness.

Clinical presentation

  • Swelling or discrete nodule: 
    • Tender 
    • Fluctuant 
    • Erythematous 
  • Surrounding induration
  • +/- Streaking redness
  • +/- Regional lymphadenopathy 
  • +/- Spontaneous purulent drainage
  • Pain typically improves when the nodule begins to drain.
  • Systemic symptoms:
    • Fever
    • Malaise
    • Chills 
    • Sweats

Diagnosis

Soft tissue abscesses are usually diagnosed based on exam and history.

Laboratory studies

  • Not needed in routine cases
  • CBC may show leukocytosis.
  • ↑ Inflammatory markers (erythrocyte sedimentation rate, C-reactive protein)
  • Blood and wound cultures obtained if:
    • Severe systemic manifestations
    • Presence of extensive cellulitis
    • Extremes of age
    • Recurrent infections
    • Failure of initial antibiotics
    • Immunodeficiency (e.g., neutropenia, diabetes)
    • Special circumstances (animal bite, unusual exposure)

Imaging studies

  • Not needed if abscess is obvious on physical exam
  • Ultrasonography: shows hypoechoic mass
  • CT scan with contrast if:
    • Worried about more extensive cellulitis or necrotizing fasciitis
    • Abscess shows up as a rim-enhancing lesion.
Soft tissue abscess on ultrasound

An ultrasound showing hypoechoic area surrounded by soft tissue swelling consistent with abscess

Image: “Abscess” by Section of Emergency Medicine, Department of Medicine, Louisiana State University Health Sciences Center, New Orleans, LA 70112, USA. License: CC BY 3.0

Management

Uncomplicated cases

  • Incision and drainage are the mainstay of treatment.
  • If abscess is > 5 cm, packing of abscess cavity may help to keep it open and draining.
  • Adjunctive oral antibiotics for 7–14 days should be considered:
    • For abscess > 2 cm
    • Multiple abscesses
    • Immunodeficiency/medical comorbidities
    • Systemic symptoms (fever, tachycardia)
    • Prosthetic valve (risk of endocarditis)
    • Other prosthetic devices (e.g., prosthetic joints, vascular grafts)
  • Antibiotics with staphylococcal and streptococcal coverage:
    • Trimethoprim/sulfamethoxazole
    • Cephalosporins 
    • Clindamycin
  • Adjunctive measures:
    • Elevation of affected extremity to ↓ swelling
    • Warm compresses (to promote drainage)
    • Symptomatic fever and pain management

Cases requiring hospitalization and IV antibiotics

  • Very large abscesses with extensive cellulitis
  • Severe presentation: bullae/blistering, skin necrosis, signs of sepsis
  • Elderly
  • Infants
  • Significant immunodeficiency/poorly controlled diabetes
  • Failure to improve with oral antibiotics
  • IV antibiotics:
    • Streptococcal and staphylococcal coverage
    • Cefazolin, ceftriaxone, cefepime, or clindamycin
  • Surgical management:
    • Drainage is still the mainstay of treatment.
    • May need to be done in the operating room under general anesthesia 
    • Tissue debridement may be required:
      • For extremely large or deep abscesses with tissue necrosis
      • Skin necrosis

Differential Diagnosis

  • Cellulitis: a common and painful bacterial skin infection that affects the deeper layers of dermis and subcutaneous tissue. Presents as an erythematous, edematous area that feels warm and tender to the touch. Caused most commonly by S. aureus and S. pyogenes. Diagnosis is usually clinical and management is with antibiotics based on suspected organisms.
  • Necrotizing fasciitis: a rapidly progressive infection resulting in extensive necrosis of subcutaneous tissue, fascia, and muscle. Most commonly caused by group A Streptococcus but often involves other types of bacteria in a mixed infection. Presents with necrosis, crepitus, bullae, and purple skin discoloration. Management is emergent surgical debridement with broad-spectrum antibiotics.
  • Dermatitis: a general term for an edematous skin rash. Caused by an infection or an allergic reaction, usually not by bacteria. Diagnosis is established clinically. Management is with antihistamines and topical steroids.
  • Folliculitis: a localized inflammation of the hair follicle or sebaceous glands that is primarily caused by S. aureus. Presents as erythema, papules, pustules, and tenderness of the affected area. Diagnosis is made clinically, and treatment involves antibiotic skin cleansers and, sometimes, systemic antibiotics.
  • Impetigo: a highly contagious skin infection of the upper epidermis. Caused by S. aureus or group A Streptococcus. Presents with an erythematous area covered in small vesicles, pustules, and/or honey-colored crusts. Management is with antibiotics.
  • Staphylococcal scalded skin syndrome: a blistering skin disorder caused by a local infection usually due to S. aureus. Presents with fever and diffuse, tender erythema, intraepidermal blisters, and sloughing off of the superficial layer of skin, leaving a red, “scalded” appearance. Management involves IV antibiotics.
  • Epidermoid cyst: the most common cutaneous cyst, usually presenting as a skin-colored dermal nodule, sometimes with a central punctum. Diagnosed clinically and treated with surgical excision or observation. Can become secondarily infected and turn into an abscess, in which case the cysts present as tender, erythematous, fluctuant nodules, often spontaneously draining. 
  • Hidradenitis suppurativa: chronic suppurative infection of sweat glands manifesting primarily in the intertriginous areas (axillae, groins, inframammary folds). Hidradenitis suppurativa usually presents as cutaneous nodules and induration often associated with multiple draining sinuses. Diagnosis is established clinically and treatment involves antibiotics and surgical excision.

References

  1. Spelman D., Baddour L.M. (2020). Cellulitis and skin abscess in adults: Treatment. UpToDate. Retrieved January 30, 2021, from https://www.uptodate.com/contents/cellulitis-and-skin-abscess-in-adults-treatment
  2. Spelman D., Baddour L.M. (2020). Cellulitis and skin abscess: Epidemiology, microbiology, clinical manifestations, and diagnosis. UpToDate. Retrieved January 30, 2021, from https://www.uptodate.com/contents/cellulitis-and-skin-abscess-epidemiology-microbiology-clinical-manifestations-and-diagnosis
  3. Stevens DL et al. (2014). Infectious Diseases Society of America. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clin Infect Dis.

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