14. Acute purulent infection

1. Definition

• Acute purulent (pyogenic) surgical infection → acute inflammation caused by pyogenic bacteria, with pus formation, needing or likely to need surgical treatment
• Develops when bacterial number and virulence overcome host defence

2. Aetiology

Causative organisms

• Staphylococcus aureus → most common; localised abscesses, furuncle, carbuncle
• Streptococcus pyogenes → spreading infection (cellulitis, erysipelas, lymphangitis)
• Escherichia coli and other Gram-negative bacilli → abdominal, biliary, urinary sources
• Pseudomonas aeruginosa → burns, hospital infection; blue-green pus
• Anaerobes → Bacteroides, Clostridium; foul smell, gas
• Mixed (polymicrobial) flora → bowel-related infections

Conditions for infection

• Virulence and dose of bacteria → > 10^5 organisms per gram of tissue
• Local factors → dead tissue, haematoma, foreign body, poor blood supply
• General factors → diabetes, malnutrition, immunosuppression, steroids, old age

3. Pathways of penetration and spread

Entry (portal of entry)

• Damaged skin → wounds, abrasions, injections, insect bites
• Mucosa → mouth, gut, urinary tract
• Hair follicles, sweat and sebaceous glands
• Endogenous → from the body's own flora (bowel perforation)
• Exogenous → from the environment
• Hospital-acquired → hands, instruments, catheters

Spread in the body

• Contiguous → along tissue planes, fascia, tendon sheaths
• Lymphatic → lymphangitis, lymphadenitis
• Haematogenous → bacteraemia, sepsis, metastatic abscesses (septicopyaemia)
• Along natural canals → bile ducts, ureter

4. Pathogenesis: tissue processes

Microscopic changes

1. Bacterial invasion, toxin release
2. Vasodilatation and increased capillary permeability → exudate, oedema
3. Neutrophil migration (diapedesis), phagocytosis
4. Tissue necrosis from toxins, enzymes and thrombosis
5. Liquefaction of dead tissue and neutrophils → pus
6. Demarcation → granulation tissue forms a wall (pyogenic membrane)

Macroscopic stages

1. Serous infiltration → swelling, redness, no pus; may resolve
2. Purulent stage → pus collects; fluctuation appears
3. Outcome:
   • Resolution
   • Abscess → localised collection
   • Phlegmon → diffuse spread
   • Spontaneous rupture → fistula, sinus
   • Systemic spread → sepsis

5. Classification

By spread

• Localised → abscess, furuncle, carbuncle
• Diffuse (spreading) → phlegmon (cellulitis), erysipelas, lymphangitis

By tissue

• Skin → furuncle, carbuncle, erysipelas
• Subcutaneous tissue → abscess, phlegmon
• Glands → hidradenitis, mastitis, parotitis
• Lymphatics → lymphangitis, lymphadenitis
• Bone → osteomyelitis
• Serous cavities → peritonitis, pleural empyema
• Hand → paronychia, felon (panaritium)

By course

• Acute, subacute, chronic

6. Clinical picture

Local symptoms (Celsus and Galen)

1. Rubor → redness
2. Tumor → swelling
3. Calor → heat
4. Dolor → pain, throbbing
5. Functio laesa → loss of function
6. Fluctuation → sign of a pus collection
7. Regional lymphangitis (red streaks) and lymphadenitis

Systemic symptoms

• Fever, chills, rigors
• Malaise, headache, loss of appetite
• Tachycardia, tachypnoea
• SIRS criteria (2 or more):
  1. Temperature > 38 °C or < 36 °C
  2. Heart rate > 90/min
  3. Respiratory rate > 20/min
  4. White cell count > 12 or < 4 × 10^9/L
• Severe → sepsis, septic shock

7. Diagnosis

Laboratory

• Leucocytosis with a left shift
• Raised CRP, erythrocyte sedimentation rate, procalcitonin
• Pus culture and sensitivity, Gram stain
• Blood cultures with fever or systemic signs
• Glucose → exclude diabetes

Imaging

• Ultrasound → collection, guides aspiration
• CT → deep or abdominal collections
• X-ray → gas, foreign body, bone involvement

8. Treatment

Conservative (serous infiltration stage)

• Rest, immobilisation, elevation
• Antibiotics → empirical, then by culture
• Analgesia
• Local warmth

Surgical (purulent stage)

• Ubi pus, ibi evacua → where there is pus, drain it
• Incision over the point of maximal fluctuation, along skin lines, avoiding vessels and nerves
• Break down loculi; remove necrotic tissue
• Irrigation, drainage; wound left open
• Percutaneous drainage under ultrasound or CT guidance → deep collections
• Antibiotics → spreading cellulitis, systemic signs, immunosuppression, diabetes

General

• Treat sepsis, fluids
• Control blood glucose
• Tetanus prophylaxis

9. Complications

• Lymphangitis, lymphadenitis
• Thrombophlebitis
• Sepsis, septic shock, metastatic abscesses
• Chronic infection, fistula, osteomyelitis