Assessment of ascites

The aetiology of ascites comprises two major categories, non-peritoneal and peritoneal diseases.

Non-peritoneal diseases causing ascites are more common and include the following:

• Conditions causing portal hypertension: cirrhosis, alcohol-related liver disease, fulminant hepatitis (e.g., due to paracetamol toxicity), subacute hepatitis, hepatic veno-occlusive disease, massive liver metastasis, congestive heart failure, constrictive pericarditis, Budd-Chiari syndrome.

• Conditions causing hypo-albuminaemia: nephrotic syndrome, protein-losing enteropathy.

• Miscellaneous conditions: myxoedema, ovarian tumours (cancer, benign tumours, and ovarian hyperstimulation syndrome), pancreatic ascites, chylous ascites and biliary ascites, urogenital surgical trauma.

Peritoneal diseases causing ascites include the following:[12]Royal College of Obstetricians and Gynaecologists. Management of ascites in ovarian cancer patients. Nov 2014 [internet publication]. https://www.rcog.org.uk/globalassets/documents/guidelines/scientific-impact-papers/sip45ascites.pdf [13]Elkassem S, Dixon E, Conly J, et al. Primary peritonitis in a young healthy woman: an unusual case. Can J Surg. 2008 Apr;51(2):E40-1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2386325 http://www.ncbi.nlm.nih.gov/pubmed/18377740?tool=bestpractice.com ​

• Malignant ascites: peritoneal mesothelioma, peritoneal carcinomatosis, and ovarian cancer​.

• Infectious peritonitis: tuberculosis, Chlamydia trachomatis, fungal and parasitic infections (Candida albicans, Histoplasma capsulatum, Coccidioides immitis, Cryptococcus neoformans, Schistosoma mansoni, Strongyloides stercoralis, Entamoeba histolytica). Primary peritonitis is caused by the spread of an infection from the blood and lymph nodes to the peritoneum and is rare. Secondary peritonitis (infection from the gastrointestinal or biliary tracts) is more common.

• Other peritoneal diseases: systemic lupus erythematosus.

Approximately 5% of patients with ascites have two or more causes of ascites formation (mixed ascites).[14]Runyon BA, Montano AA, Akriviadis EA, et al. The serum-ascites albumin gradient is superior to the exudate-transudate concept in the differential diagnosis of ascites. Ann Intern Med. 1992 Aug 1;117(3):215-20. http://www.ncbi.nlm.nih.gov/pubmed/1616215?tool=bestpractice.com Usually these patients have cirrhosis and another cause for ascites, such as peritoneal carcinomatosis. Many patients with enigmatic ascites may have more than one predisposing factor for sodium and water retention (e.g., heart failure, nephrotic syndrome, metabolic dysfunction-associated steatohepatitis [previously known as non-alcoholic steatohepatitis]), which cumulatively can cause fluid overload and ascites.

Portal hypertension

By definition, portal hypertension is the development of increased pressure within the portal venous system as a result of inflow resistance at the level of the portal vein. To determine the presence of portal hypertension, hepatic vein catheterisation can be used to calculate the hepatic venous pressure gradient (HVPG), or the difference between the wedged and free hepatic venous pressures.[15]Veldhuijzen van Zanten D, Buganza E, Abraldes JG. The role of hepatic venous pressure gradient in the management of cirrhosis. Clin Liver Dis. 2021 May;25(2):327-43. http://www.ncbi.nlm.nih.gov/pubmed/33838853?tool=bestpractice.com A normal gradient is considered ≤5 mmHg. Levels above this suggest the presence of portal hypertension, but clinically significant portal hypertension is not typically seen until the pressure gradient reaches 10 mmHg.[15]Veldhuijzen van Zanten D, Buganza E, Abraldes JG. The role of hepatic venous pressure gradient in the management of cirrhosis. Clin Liver Dis. 2021 May;25(2):327-43. http://www.ncbi.nlm.nih.gov/pubmed/33838853?tool=bestpractice.com

The serum to ascitic fluid gradient (also known as the serum-ascites albumin gradient [SAAG]) can help to delineate the aetiology of ascites. It is calculated by comparing serum albumin with albumin found in the ascitic fluid during a diagnostic paracentesis. In portal hypertension, the calculated SAAG will be ≥11 g/L (≥1.1 g/dL).

Autoimmune and metabolic liver diseases may cause portal hypertension. Non-cirrhotic liver diseases (i.e., alcohol-related liver disease, fulminant hepatic failure, or massive hepatic metastasis) may also cause portal hypertension.

Non-hepatic causes of portal hypertension can lead to ascites. These include congestive heart failure (most common) and constrictive pericarditis. Severe tricuspid regurgitation with right-sided heart failure can cause ascites.

Causes unrelated to portal hypertension

When ascites is unrelated to portal hypertension, the calculated SAAG is <11 g/L (<1.1 g/dL). Causes of ascites unrelated to portal hypertension include bacterial and fungal infections, as well as infection with Chlamydia, tuberculosis, and HIV that may result in peritonitis. Peritoneal carcinomatosis, nephrotic syndrome, systemic lupus erythematosus, or myxoedema can also cause ascites. Additionally, surgery or trauma to lymphatics or the urogenital system can cause ascites. Rare causes are pancreatic, bile, or chylous ascites, and ovarian tumours (cancer, benign tumours, and ovarian hyperstimulation syndrome).