DermatologyTier 1Disease (DEADMAN)

Bacterial infections - abscess

Description

Collection of pus within the dermis/subcutaneous tissue - walled off by inflammatory tissue
May arise from a folliculitis/furuncle, a wound, or de novo in deeper soft tissue
Fluctuant, tender, erythematous nodule +/- surrounding cellulitis
Distinguish from hidradenitis suppurativa (recurrent abscesses in flexures - a distinct chronic follicular occlusion disease, not a simple one-off infection)

Very common presentation to primary care and emergency departments
Risk factors: diabetes, obesity, IV drug use, immunosuppression, skin trauma/shaving, poor hygiene, overcrowding
Community-acquired MRSA rates vary by region - relevant to empirical antibiotic choice where used

Staphylococcus aureus (incl. MRSA) - commonest cause overall
Polymicrobial (incl. anaerobes, gram-negatives) - perianal/perineal, IV drug injection sites, bite wounds
Pathogenesis: bacterial entry (via follicle, break in skin, haematogenous) -> local tissue necrosis -> neutrophil recruitment -> liquefactive collection walled off by fibrous capsule
IV drug use - "skin-popping" abscesses often polymicrobial including oral flora

Clinical

Fluctuant on palpation is the key sign of a drainable collection; point tenderness, warmth, erythema, +/- overlying pointing/discharge
Bedside ultrasound - useful where fluctuance is equivocal (deep/obese patients), distinguishes abscess from cellulitis alone

When to investigate further

Send pus for MCS if: recurrent, failing treatment, immunosuppressed, systemic symptoms, or high local MRSA prevalence - guides de-escalation
Consider underlying cause if recurrent: diabetes screen, immunodeficiency work-up, occult foreign body, hidradenitis suppurativa

A. Incision and drainage - the primary treatment for any fluctuant abscess

I&D is definitive management once pus has collected - antibiotics alone are inadequate and do not substitute for drainage
Adequate incision, break loculations, +/- pack/leave open to heal by secondary intention for larger cavities

B. Add antibiotics when

Extensive surrounding cellulitis, systemic symptoms (fever, tachycardia), immunosuppression, high-risk site (face - danger triangle, hand, perineum), multiple lesions, or failure to improve after drainage
Empirical choice - flucloxacillin (or cephalexin); add MRSA cover (trimethoprim-sulfamethoxazole, clindamycin, or doxycycline) if risk factors/local prevalence warrant, pending culture
Uncomplicated small abscess in an otherwise well patient after adequate drainage - antibiotics generally not required

C. Recurrent abscesses

Decolonisation protocol (nasal mupirocin, chlorhexidine wash) if recurrent staphylococcal disease
Investigate for hidradenitis suppurativa, diabetes, immunodeficiency if pattern suggests it

Diabetes, obesity, immunosuppression
IV drug use - injection-site abscesses, often polymicrobial
Hidradenitis suppurativa - recurrent flexural abscesses, a distinct chronic disease requiring different long-term management
MRSA colonisation - risk factor for recurrence and treatment failure with beta-lactams

Adequately drained simple abscess - resolves within days to a couple of weeks
Untreated/inadequately drained - can progress to spreading cellulitis, necrotising soft tissue infection (rare but important not to miss), or bacteraemia/sepsis
Facial abscesses in the "danger triangle" (nose to corners of mouth) - risk of retrograde spread via valveless facial veins to cavernous sinus
Scarring common after drainage, especially with larger/recurrent collections

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