What are the causes of fluid collection in the peritoneum aside from bowel rupture?

Causes of Peritoneal Fluid Collection (Excluding Bowel Rupture)

Peritoneal fluid accumulation has multiple etiologies that can be broadly categorized by the presence or absence of portal hypertension, with cirrhosis being the most common cause (80% of cases in the United States), followed by malignancy, cardiac disease, infectious processes, and less common inflammatory conditions. 1, 2, 3

Portal Hypertension-Related Causes (SAAG ≥1.1 g/dL)

Hepatic Cirrhosis

• Cirrhosis is the most frequent cause of ascites, accounting for approximately 80% of cases in developed countries 1, 2, 3
• The mechanism involves increased hydrostatic pressure at the sinusoidal level, splanchnic vasodilation, and sodium retention mediated by the renin-angiotensin-aldosterone system 2
• Portal hypertension leads to increased vascular permeability and fluid sequestration in the peritoneum 2

Cardiac Ascites

• Right heart failure produces ascites with high ascitic fluid protein (>2.5 g/dL) and SAAG ≥1.1 g/dL 1
• Results from elevated central venous pressure transmitted to hepatic sinusoids 1

Massive Liver Metastases

• Extensive hepatic tumor infiltration can cause portal hypertension and ascites with SAAG ≥1.1 g/dL 1

Non-Portal Hypertension Causes (SAAG <1.1 g/dL)

Peritoneal Carcinomatosis

• Malignant neoplasia is the second most common cause of ascites after cirrhosis 3, 4
• Tumor implants on peritoneal surfaces increase vascular permeability and produce exudative fluid 1
• Cytology of ascitic fluid should be obtained when malignancy is suspected 1

Tuberculous Peritonitis

• Common in developing countries and immunocompromised patients 3, 4
• Adenosine deaminase (ADA) levels in ascitic fluid can aid diagnosis 3
• Culture for mycobacteria should be guided by clinical context 1

Pancreatic Ascites

• Results from pancreatic duct disruption or pancreatic pseudocyst rupture 1
• Ascitic fluid amylase concentration is markedly elevated and should be measured when pancreatic origin is suspected 1

Sepsis and Inflammatory Causes

Systemic Inflammatory Response Syndrome (SIRS)

• In patients with generalized peritonitis and sepsis, the systemic inflammatory response syndrome, increased vascular permeability, and aggressive crystalloid resuscitation predispose to fluid sequestration and collection in the peritoneum 1
• Advanced sepsis commonly causes bowel edema, which along with fluid overload can lead to intra-abdominal hypertension (IAH) 1

Secondary Peritonitis (Non-Perforation Sources)

• Gangrenous appendicitis without frank perforation can produce peritoneal fluid 1
• Infected intra-abdominal viscera may cause localized peritonitis with fluid accumulation 1
• Anastomotic dehiscences in the postoperative period are common causes 1

Autoimmune Ascites

• Rare cause characterized by inflammatory changes on peritoneal biopsy 5
• May respond to immunosuppressive therapy such as mycophenolate mofetil 5

Gynecologic Causes

Ruptured Ovarian Cyst

• Produces sudden-onset pelvic pain with free fluid 6
• Can be mistaken for traumatic fluid on imaging 1, 6

Pelvic Inflammatory Disease

• Presents with free fluid and adnexal tenderness 6

Ruptured Ectopic Pregnancy

• Most concerning cause when free fluid occurs in reproductive-age women with positive pregnancy test and no visible intrauterine pregnancy 6
• Echogenic fluid (suggesting blood) is highly concerning 6

Renal Causes

Nephrotic Syndrome

• Severe hypoalbuminemia leads to decreased oncotic pressure and ascites formation 3, 4

Diagnostic Approach

Initial Paracentesis Analysis

For new-onset ascites, abdominal paracentesis with ascitic fluid analysis is the most cost-effective method of determining etiology 3:

• SAAG (serum-ascites albumin gradient): SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy; SAAG <1.1 g/dL excludes portal hypertension 1
• Total protein concentration: >2.5 g/dL suggests cardiac source 1
• Cell count and differential: PMN count >250 cells/mm³ indicates spontaneous bacterial peritonitis 1
• Culture: Indicated for inpatients with new or recurrent ascites 1

Optional Testing Based on Clinical Context

• Amylase: Only when pancreatic origin suspected 1
• Cytology: Only when causes other than cirrhosis suspected 1
• Adenosine deaminase (ADA): For suspected tuberculous peritonitis 3
• Tumor markers: When malignancy suspected 3

Critical Pitfalls to Avoid

• Fluid overload during resuscitation can itself cause peritoneal fluid accumulation through increased vascular permeability and should be avoided in patients with generalized peritonitis 1
• Pre-existing ascites from cirrhosis, ruptured ovarian cysts, or pelvic inflammatory disease may be mistakenly attributed to trauma in emergency settings 1, 6
• Ultrasound typically does not detect free fluid until at least 500 mL is present, so negative exams do not exclude early fluid accumulation 1, 6
• In critically ill patients with ascites and intra-abdominal hypertension, consideration should be given to draining ascites to prevent abdominal compartment syndrome 1