15. Hot abscess

1. Definition

• Hot (acute) abscess → localised collection of pus in a tissue cavity formed by necrosis, bounded by a pyogenic membrane
• Cold abscess → tuberculous; no signs of acute inflammation

2. Aetiology

• Staphylococcus aureus → most common; including meticillin-resistant S. aureus (MRSA)
• Streptococci, Escherichia coli, anaerobes, mixed flora
• Sources:
  • Wounds, injections (injection abscess in drug users)
  • Infected haematoma
  • Hair follicle or gland infection
  • Haematogenous spread → metastatic abscess
  • Infected cyst
• Risk factors → diabetes, obesity, immunosuppression, poor hygiene

3. Pathogenesis: structure of the abscess

1. Cavity → filled with pus
2. Pyogenic membrane (abscess wall) → granulation tissue lining the cavity
   • Inner layer → fibrin, necrotic tissue, neutrophils
   • Outer layer → granulation tissue, fibroblasts; later fibrous capsule
3. Perifocal zone → inflammatory infiltration and oedema of the surrounding tissue
4. Membrane → limits spread and absorption of toxins; also limits antibiotic entry
5. Pressure in the cavity → throbbing pain, spread along least resistance

4. Composition of non-specific pus

• Liquid part (pus serum) → plasma exudate, proteins, enzymes, toxins
• Cellular part → living and dead neutrophils (pus cells), lymphocytes, macrophages
• Tissue debris
• Bacteria
• Features by organism:
  • Staphylococcus → thick, creamy, yellow, odourless
  • Streptococcus → thin, serous, blood-stained
  • Escherichia coli → brown, faecal smell
  • Pseudomonas → blue-green, sweet smell
  • Anaerobes → grey, foul-smelling, with gas bubbles
  • Actinomyces → yellow sulphur granules

5. Classification

• By site → superficial (skin, subcutaneous), deep (muscle, organs), cavity (subphrenic, pelvic)
• By organ → breast, perianal, liver, lung, brain, tubo-ovarian
• By origin → primary (local), secondary (metastatic, haematogenous)
• Specific forms:
  • Furuncle → infection of a hair follicle and sebaceous gland
  • Carbuncle → coalescing furuncles, several openings; nape of neck, diabetics
  • Hidradenitis suppurativa → apocrine glands (axilla, groin)
  • Pilonidal abscess

6. Clinical picture

Local symptoms

• Pain → throbbing, worse at night
• Swelling, redness, heat
• Fluctuation → elastic, wave-like movement of fluid on palpation with 2 fingers
• Pointing → thinning of the skin, white or yellow centre before spontaneous rupture
• Loss of function

General symptoms

• Fever, often swinging (hectic)
• Chills, sweating
• Malaise, tachycardia
• Deep abscesses → general signs predominate; local signs may be absent
• Swinging pyrexia → hidden pus somewhere, pus needs drainage

7. Diagnosis

Laboratory

• Leucocytosis, neutrophilia, raised CRP
• Blood glucose
• Pus Gram stain, culture and sensitivity

Imaging

• Ultrasound → hypoechoic collection, guides aspiration
• CT → deep abscesses (abdominal, pelvic)
• Diagnostic needle aspiration → pus confirms the diagnosis

8. Differential diagnosis

• Cellulitis without collection
• Infected cyst
• Haematoma
• Aneurysm → pulsatile; never incise without imaging in the groin, neck, axilla
• Tumour, soft tissue sarcoma
• Cold (tuberculous) abscess

9. Treatment

Conservative

• Early stage, no collection → antibiotics, rest, elevation, warmth
• Small abscess < 5 cm in a healthy patient → drainage alone usually enough

Surgical

• Incision and drainage → treatment of choice
• Steps:
  1. Anaesthesia → local, regional or general
  2. Incision over the point of maximal fluctuation, along skin creases
  3. Pus sent for culture
  4. Break down loculi with a finger
  5. Irrigation with saline
  6. Drain or loose packing; no primary closure
  7. Daily dressings until healing by secondary intention
• Deep abscess → percutaneous drainage under ultrasound or CT guidance
• Recurrent or chronic → excise the wall (fistula, cyst)
• Antibiotics → surrounding cellulitis, systemic signs, diabetes, immunosuppression, face and hand

10. Complications

• Spread → phlegmon, lymphangitis, lymphadenitis
• Thrombophlebitis
• Sepsis, metastatic abscesses
• Spontaneous rupture → into a cavity (peritonitis) or the skin (fistula)
• Facial abscess above the upper lip → cavernous sinus thrombosis