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Ascites: Definition, Pathophysiology, and Causes
What Is Ascites?
Ascites is the accumulation of free fluid within the peritoneal (abdominal) cavity. Normally, the peritoneal cavity contains less than 150 mL of fluid. When this volume increases significantly, it becomes clinically detectable. It is the most common complication of cirrhosis and carries a poor prognosis: median survival is about 2 years for non-malignant ascites and only 4 months for malignant ascites.
Pathophysiology
The development of ascites involves three key overlapping mechanisms:
1. Portal Hypertension
Portal hypertension is the central driver in cirrhotic ascites. Increased hydrostatic pressure within the portal circulation and hepatic sinusoids forces fluid out into the peritoneal cavity. In cirrhosis, the space of Disse is "capillarized" (loss of sinusoidal fenestrations), reducing sinusoidal permeability and hepatic lymph flow. As cirrhosis advances, splanchnic lymph (low in protein) becomes more abundant than hepatic lymph, making ascitic fluid protein-poor.
- Mulholland and Greenfield's Surgery, p. 2938
2. Splanchnic and Peripheral Vasodilation - Relative Arterial Underfilling
Portal hypertension triggers release of vasodilators (especially nitric oxide) that cause splanchnic and peripheral vasodilation. This leads to a state of relative arterial underfilling despite total body sodium overload. The body misinterprets this as hypovolemia, activating:
- The renin-angiotensin-aldosterone system (RAAS)
- The sympathetic nervous system
- Antidiuretic hormone (ADH/vasopressin)
These compensatory mechanisms drive renal sodium and water retention, expanding total body fluid volume and fueling ascites formation. This is actually the main underlying pathophysiology - the kidneys retaining sodium and water despite fluid overload.
- Brenner and Rector's The Kidney, p. 2826-2828
3. Hypoalbuminemia
Reduced hepatic synthesis of albumin lowers plasma oncotic pressure. This impairs the ability of plasma to hold fluid within the vasculature, promoting fluid transudation into the peritoneal cavity.
Causes of Ascites
Ascites causes are best classified by whether or not they involve portal hypertension, using the Serum-Ascites Albumin Gradient (SAAG):
SAAG = Serum albumin - Ascitic fluid albumin
- SAAG ≥ 1.1 g/dL (≥ 11 g/L): portal hypertension-related
- SAAG < 1.1 g/dL (< 11 g/L): non-portal hypertension etiology
HIGH SAAG (≥1.1 g/dL) - Portal Hypertension Related
| Cause | Notes |
|---|---|
| Cirrhosis | Most common cause overall - accounts for ~80% of all ascites |
| Alcoholic hepatitis | Even without fully established cirrhosis |
| Fulminant hepatic failure | Acute liver failure |
| Fatty liver disease (NAFLD/NASH) | Increasingly common |
| Congestive heart failure | Increased systemic venous pressure, high protein ascites |
| Constrictive pericarditis | Venous congestion; mimics cirrhotic ascites |
| Budd-Chiari syndrome | Hepatic vein occlusion - blocks hepatic outflow |
| Inferior vena cava obstruction | Blocks venous return from the abdomen |
| Portal vein thrombosis | Intrahepatic or extrahepatic |
| Veno-occlusive disease (sinusoidal obstruction syndrome) | Often after bone marrow transplant |
| Nodular regenerative hyperplasia | Non-cirrhotic portal hypertension |
The three main causes (cirrhosis, heart failure, malignancy) can be distinguished by combining SAAG and ascitic protein level:
| Condition | SAAG | Ascitic Protein |
|---|---|---|
| Cirrhosis | High (>1.1) | Low (<2.5 g/dL) |
| Cardiac ascites | High (>1.1) | High (>2.5 g/dL) |
| Malignant ascites | Low (<1.1) | High (>2.5 g/dL) |
- Goldman-Cecil Medicine, p. 1619
LOW SAAG (<1.1 g/dL) - Non-Portal Hypertension Related
| Cause | Notes |
|---|---|
| Peritoneal carcinomatosis | Accounts for ~10% of all ascites; colon, gastric, pancreatic, ovarian, breast, endometrial cancers |
| Peritoneal tuberculosis | Exudative inflammation of peritoneum |
| Nephrotic syndrome | Severe hypoalbuminemia from urinary protein loss |
| Pancreatic ascites | Leakage of pancreatic secretions (amylase markedly elevated in fluid) |
| Protein-losing enteropathy | Loss of serum protein via gut |
| Malnutrition/hypoalbuminemia | Without portal hypertension |
| Connective tissue diseases | e.g., lupus serositis |
| Biliary ascites | Bile leak following surgery or trauma |
| Urine ascites | Bladder perforation or ureteral injury |
| Chylous ascites | Lymphatic disruption (lymphoma, trauma, surgery) |
| Myxedema (hypothyroidism) | Rare cause |
| Meigs syndrome | Ovarian fibroma + ascites + pleural effusion |
| End-stage renal disease | Dialysis-related |
- Mulholland and Greenfield's Surgery, Table 59.17
Clinical Detection
- Shifting dullness on percussion: detects ~1.5 L of fluid
- Fluid wave: usually requires ~10 L
- Bulging flanks: visible abdomen distension
- Abdominal ultrasonography is the initial, most cost-effective, and least invasive confirmation method
- Respiratory compromise may occur from increased intra-abdominal pressure and pleural effusion (typically right-sided)
Key Diagnostic Test: Paracentesis + SAAG
Diagnostic paracentesis should be performed in every patient with new-onset ascites - even those with coagulopathy. Fluid should be sent for:
- Albumin (for SAAG calculation)
- Total protein
- PMN cell count (to exclude spontaneous bacterial peritonitis - SBP)
- Bacteriologic cultures
- Cytology (if malignancy suspected)
- Amylase (if pancreatic ascites suspected), ADA/AFB (if TB suspected)
Summary
The vast majority of ascites (about 80%) is non-malignant, with cirrhosis being the single most common cause. The core mechanism in cirrhotic ascites is portal hypertension leading to vasodilation, perceived arterial underfilling, and renal sodium/water retention. Non-portal causes are less common and are identified by a low SAAG, pointing to processes like malignancy, infection, or hypoalbuminemia.
Recent guideline update: The 2025 AGA Clinical Practice Update (PMID: 41114681) covers management of ascites, volume overload, and hyponatremia in cirrhosis, affirming the central role of SAAG and updated evidence on diuretics and albumin infusion.
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