Abscess

Definition

An abscess is a focal accumulation of pus within body tissues, representing the end result of tissue destruction by a pyogenic infection. The surrounding tissue undergoes liquefactive necrosis, forming a walled-off cavity filled with dead cells, bacteria, and inflammatory exudate. — Rosen's Emergency Medicine, p. 1250

Pathophysiology

Bacteria breach the skin or mucosal barrier (usually via trauma, follicular invasion, or haematogenous seeding) and trigger an acute inflammatory response. Polymorphonuclear neutrophils migrate to the site, attempt phagocytosis, and die, releasing proteases. The resulting liquefied material (pus) accumulates under pressure, creating the characteristic fluctuant cavity enclosed by a pyogenic membrane.

Common Types

Type	Key Features
Furuncle	Staphylococcal infection of a hair follicle + surrounding tissue; localized fluctuant nodule
Carbuncle	Multiple confluent furuncles in thick skin (nape of neck, back, thigh); severe pain, fever; risk of septicaemia
Hidradenitis suppurativa	Recurrent apocrine gland abscesses in axillae/groin; often resistant to therapy
Perianal/perirectal	May indicate fistula-in-ano; requires surgical referral
Collar-button (web space)	Subfascial hand infection; palmar + dorsal swelling, fingers held in abduction
Peritonsillar	Deep neck space; risk of airway compromise
Intra-abdominal	Hepatic, subphrenic, pelvic; usually post-operative or complicating visceral perforation
Brain abscess	Neurosurgical emergency; headache, focal deficit, fever triad
Epidural	Spinal emergency; back pain + neurological signs

Clinical Features

Local: Erythema, warmth, swelling, tenderness, central fluctuance (the hallmark distinguishing it from cellulitis)
Systemic: Fever, malaise, leukocytosis (more common in carbuncles or deep abscesses)
MRSA lesions often show central necrosis and are frequently mistaken by patients for spider bites
Adjacent fingers held in abduction suggests collar-button (web space) abscess

Diagnosis

Clinical Examination

Fluctuance on palpation is the key distinguishing feature from cellulitis, which lacks a drainable fluid collection.

Point-of-Care Ultrasound (POCUS)

Ultrasound is highly useful when clinical examination is equivocal:

MRSA Abscess with surrounding cellulitis — Rosen's Emergency Medicine

Abscess: discrete, focal, hypoechoic-to-anechoic fluid collection surrounded by thickened subcutaneous tissue; may contain internal echoes if debris-laden
Cellulitis: "cobblestoning" — hypoechoic fluid interwoven in a reticular pattern between tissue planes, without a discrete cavity
Ultrasound also determines depth and size, critical before I&D
For peritonsillar abscess: intraoral probe shows rounded hypoechoic/anechoic collection; carotid artery location can be assessed

— Roberts & Hedges' Clinical Procedures in Emergency Medicine, p. 869

Microbiology

Most common organism: Staphylococcus aureus (including MRSA in community settings)
Perianal, intra-abdominal: mixed flora including anaerobes
IV drug users: polymicrobial; Streptococcus spp.
Send wound cultures in moderate–severe cases

Management

1. Incision & Drainage (I&D)

I&D is the definitive treatment — antibiotics alone are inadequate once pus has localized.

Key principles (Roberts & Hedges):

Do not incise prematurely — incision before pus localizes is not curative and may spread infection or cause bacteraemia
Apply warm compresses for up to 24–36 hours if fluctuance is not yet present, then reassess
Incise at the point of maximum fluctuance; make the incision long enough to allow complete drainage and prevent premature closure
Break up loculations with a haemostat or gloved finger
Irrigate the cavity
Pack the wound to prevent premature skin closure (remove packing at 12–24 hours; replace with warm soapy soaks)
Most wounds heal by secondary intention; delayed primary closure only after repeat washouts in large wounds with confirmed infection control

— Schwartz's Principles of Surgery, p. 1976; Roberts & Hedges', p. 870

2. Antibiotics

Uncomplicated abscess in an immunocompetent host: I&D alone is typically curative. Routine post-procedural antibiotics are not standard.

Antibiotics ARE indicated when:

Severe or extensive disease (multiple sites, rapid spread)
Associated cellulitis with systemic signs
Immunosuppression, extremes of age
Comorbidities (diabetes, peripheral vascular disease)
Difficult-to-drain location (face, hand, genitalia)
Associated septic phlebitis
Unresponsive to I&D alone
IV drug user with fever (rule out endocarditis before discharge)

Antibiotic selection:

Scenario	Agent
Mild outpatient, MRSA coverage	TMP-SMX or clindamycin (5–7 days)
Mild outpatient, non-MRSA	Dicloxacillin, cephalexin
Doxycycline/minocycline	Alternative for MRSA (oral)
Severe / hospitalised	Vancomycin, daptomycin, linezolid, telavancin, or ceftaroline IV
IVDU with major abscess	IV ampicillin/sulbactam (2g/1g q6h)

Duration: 3–5 days (uncomplicated outpatient); 7–14 days (complicated/hospitalised)
Cephalosporins and macrolides are typically ineffective against MRSA

3. Bacteraemia Prophylaxis

Transient bacteraemia can occur during I&D
Patients at high risk for endocarditis should receive prophylactic antibiotics before the procedure
Immunocompromised patients: IV broad-spectrum antistaphylococcal coverage (cefazolin 1g IV or vancomycin) 30 minutes before I&D

— Roberts & Hedges', pp. 870–872

Special Considerations

MRSA

Community-associated MRSA is now the most common pathogen in ED skin/soft tissue infections in many US cities
MRSA survives on surfaces for weeks and spreads via household contacts
Recurrences are common; patients and contacts require hygiene education
First-line: TMP-SMX, clindamycin, minocycline, or doxycycline

Hidradenitis Suppurativa

Drain if fluctuant, painful, and large
Mild: topical clindamycin × 3 months
Moderate/severe: oral clindamycin + rifampicin × 3–6 months
Antiandrogen therapy if antibiotics fail
Recurrent/refractory: surgical referral

Recurrence Prevention

Personal hygiene education (regular bathing, hand washing)
Keep wounds covered with clean dry dressings
Decolonisation (mupirocin nasal ointment, chlorhexidine body washes) for recurrent MRSA

When to Refer / Admit

Deep-space infections (hand, neck, mediastinum)
Brain or spinal epidural abscess
Sepsis or septic shock
Failed outpatient I&D
Immunocompromised or systemically unwell patients
Intra-abdominal or pelvic abscess (typically require CT-guided drainage or surgical intervention)

Sources: Rosen's Emergency Medicine 9e; Roberts & Hedges' Clinical Procedures in Emergency Medicine 7e; Schwartz's Principles of Surgery 11e

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