Peritonitis

Definition

• Harrison's Principles of Internal Medicine 22E, p. 1113
• Bailey and Love's Short Practice of Surgery 28th Ed., p. 1109

Classification

• Localised - inflammation confined to a specific area; patient may have normal vital signs but shows local guarding and rebound tenderness
• Diffuse (generalised) - inflammation of large regions of the parietal peritoneum; life-threatening, typically from viscus perforation or major vascular catastrophe

• Primary (spontaneous) bacterial peritonitis (SBP/PBP) - no identifiable intra-abdominal source
• Secondary peritonitis - from a definable intra-abdominal source
• CAPD-associated peritonitis - from peritoneal dialysis catheter

Pathogenesis

• Upper and lower cavities divided by the transverse mesocolon
• The greater omentum lines the lower peritoneal cavity
• Pancreas, duodenum, and ascending/descending colon are in the anterior retroperitoneal space
• Kidneys, ureters, and adrenals are in the posterior retroperitoneal space

Primary (Spontaneous) Bacterial Peritonitis (PBP)

Associations

• Most common in cirrhosis (alcoholic, post-necrotic, chronic active hepatitis)
• Also: metastatic malignancy, congestive heart failure, SLE, lymphoedema, nephrotic syndrome in children
• Occurs in ≤10% of cirrhotic patients with ascites

Pathogenesis

• Haematogenous spread facilitated by impaired hepatic filtration
• Deficient complement cascade proteins in cirrhotic ascitic fluid
• Reduced PMN opsonic/phagocytic activity in advanced liver disease
• Gut microbiota alterations (increased Enterobacteriaceae), small bowel bacterial overgrowth → bacterial translocation

Microbiology (PBP)

• Usually single organism
• Enteric gram-negative bacilli: E. coli (most common)
• Gram-positives: streptococci, enterococci, pneumococci
• Anaerobes: uncommon (multiple organisms + anaerobes → reconsider diagnosis, seek secondary source)
• Emerging concern: ESBL-producing Enterobacteriaceae and gram-positive organisms from quinolone prophylaxis

Diagnosis of PBP

• Fever in up to 80% of patients
• Abdominal pain, ascites, peritoneal irritation
• Subtle cases: malaise, fatigue, encephalopathy, jaundice, AKI without another cause
• Key criterion: >250 PMNs/µL in ascitic fluid (diagnostic)
• Single organism on culture

Secondary Peritonitis

• Dense neutrophils and fibrinopurulent debris coat the viscera and abdominal wall
• Serous fluid accumulates, becomes suppurative as infection progresses
• Subhepatic and subdiaphragmatic abscesses may form
• Reaction generally remains superficial (except in tuberculous peritonitis)

CAPD-Associated Peritonitis

• Most common reason for discontinuation of CAPD
• Organisms: skin flora - Staphylococcus species (~45%), predominantly coagulase-negative; S. aureus in nasal carriers; gram-negative bacilli; Candida spp.
• Pathogenesis: skin organisms migrate along the catheter
• Diagnosis: cloudy dialysate with >100 WBCs/µL with >50% neutrophils (or >50% PMNs regardless of absolute count if dwell time is short)
• Multiple organisms on culture should prompt search for secondary peritonitis

Clinical Features

Signs may be limited or absent in obese patients or those on immunosuppressive medications.

Investigations

• CT scan (contrast-enhanced) - investigation of choice; identifies underlying cause, perforation, abscess
• Erect chest X-ray - identifies subdiaphragmatic free gas (perforation)
• Lateral decubitus X-ray - alternative if CT unavailable and patient too unwell to sit erect
• Ultrasound - useful for tubo-ovarian pathology; lacks specificity otherwise
• Ascitic fluid tap - PMN count and culture (essential in PBP)
• Laboratory - FBC, CRP, LFTs, renal function, lactate, blood cultures (biomarkers support but are rarely diagnostically specific)
• Laparoscopy - if above investigations inconclusive

Treatment

General management (Bailey & Love summary)

1. General care of the patient
2. Correction of fluid and electrolyte imbalance (IV resuscitation)
3. Nasogastric tube (decompression) + urinary catheter (monitoring)
4. Broad-spectrum antibiotics (IV)
5. Analgesia
6. Surgery - to remove the underlying cause and lavage residual contamination

Antibiotic regimens

• Cefotaxime 2 g IV every 8 h (or another third-generation cephalosporin) for 5 days
• Albumin infusion (1.5 g/kg at diagnosis, 1 g/kg on day 3) reduces risk of hepatorenal syndrome
• Consider ESBL coverage in nosocomial or quinolone-prophylaxis patients

• Norfloxacin 400 mg/day orally (long-term) in high-risk cirrhotic patients
• TMP-SMX (double-strength once daily) as alternative

• Cover S. aureus, coagulase-negative staphylococci, and gram-negative bacilli
• Often: first-generation cephalosporin (e.g. cefazolin) for gram-positives + fluoroquinolone or third-generation cephalosporin (e.g. ceftazidime) for gram-negatives
• High MRSA areas: vancomycin substituted for cefazolin
• Administered intraperitoneally

Surgical management

• Aim: remove underlying cause + lavage/dilute residual contamination
• Operative findings: reddened, thickened peritoneum with velvety texture; yellow/white fibrin plaques causing bowel adhesion; turbid exudate ± frank pus

Complications

• Intra-abdominal abscess (subphrenic, pelvic, paracolic)
• Septic shock and multi-organ failure
• Hepatorenal syndrome (in PBP/cirrhosis)
• Ileus (paralytic)
• Adhesions and bowel obstruction (long-term)
• Death - without treatment, diffuse peritonitis is uniformly fatal

Special Types

• Harrison's Principles of Internal Medicine 22E (2025), Chapter 137
• Bailey and Love's Short Practice of Surgery 28th Edition, Chapter 65
• Robbins, Cotran & Kumar Pathologic Basis of Disease, Chapter on Peritoneal Cavity