Abscess

Overview

•Abscess = walled-off pus collection with a pyogenic capsule that excludes systemic antibiotics - drainage is always the definitive treatment

•Staphylococcus aureus is the dominant pathogen across most abscess types; anaerobes important in dental, perianal, and lung abscesses

Presentation

•Fluctuant swelling - cardinal sign; palpable fluid shift within a tense, tender lump

•Localised pain, erythema, warmth - surrounding cellulitis reflects ongoing inflammation

•Purulent discharge - spontaneous or on pressure

•Systemic features (fever, rigors, tachycardia, raised CRP) - indicate systemic spread; require urgent review

•Site-specific: trismus (dental), hot-potato voice (quinsy), productive cough/haemoptysis (lung), nipple changes (breast)

⚠️

Cellulitis is firm and diffuse with no fluctuance. A fluctuant, warm, tender lump is an abscess until proven otherwise - do not treat with antibiotics alone and miss the drainage indication.

Investigations

•Clinical diagnosis in most superficial abscesses - fluctuance, warmth, erythema

•Ultrasound - confirms collection in breast abscess or deep soft tissue; first-line imaging

•FBC, CRP, blood cultures - if systemic features present

•Pus swab/culture at drainage - identifies organism and sensitivities; essential for MRSA screening

•CT - deep abscesses (intra-abdominal, hepatic, psoas); gold standard for lung abscess characterisation

•CXR - lung abscess: cavity with air-fluid level

Management

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Antibiotics are NOT definitive treatment for a formed abscess. The pyogenic capsule excludes antibiotics from the cavity - drainage is always required. A patient given antibiotics alone for a dental or skin abscess will not improve.

•First-line (all sites): incision and drainage (I&D) - definitive treatment for most skin, soft tissue, dental, perianal abscesses

•Breast abscess: USS-guided needle aspiration preferred (better cosmesis, can be repeated)

•Analgesia: ibuprofen and paracetamol while awaiting drainage - never sufficient alone

•Antibiotics indicated when: systemic features, surrounding cellulitis, immunocompromised, or high-risk anatomical site

Abscess type	First-line antibiotic	Penicillin allergy / alternative
Skin/soft tissue (MSSA)	flucloxacillin	clarithromycin or doxycycline
Mixed organisms suspected	co-amoxiclav	-
Lactational breast (MSSA)	flucloxacillin	-
Non-lactational breast (anaerobes)	co-amoxiclav	erythromycin or clarithromycin + metronidazole
Dental (systemic upset)	amoxicillin or phenoxymethylpenicillin; add metronidazole if severe	clarithromycin
Lung abscess	co-amoxiclav or metronidazole + beta-lactam (prolonged course)	Tailor to cultures

•Lactational breast abscess: encourage continued breastfeeding to maintain ductal patency - stopping worsens the condition

•Lung abscess: bronchoscopy to restore bronchial patency; percutaneous drainage or surgery for large/refractory cases

Complications

•Sepsis - systemic spread; requires IV antibiotics and resuscitation

•Fistula formation - e.g. mammary duct fistula, anorectal fistula

•Dental abscess spread - osteomyelitis, deep neck space infection, Ludwig's angina, cavernous sinus thrombosis, cerebral abscess

•Necrotising fasciitis - rare, life-threatening spread along fascial planes

•Recurrence - up to 25% within one year for skin/soft tissue; associated with nasal Staph. aureus carriage; decolonisation with nasal mupirocin and chlorhexidine washes may reduce recurrence

•Pleural empyema - complication of lung abscess if pus ruptures into the pleural space

🚨

Ludwig's angina - rapidly spreading bilateral submandibular/sublingual cellulitis from dental infection (classically lower second/third molar). Can cause airway compromise within hours. Floor-of-mouth swelling, dysphagia, or stridor = medical emergency requiring immediate airway assessment.