Overview
Ascites describes the abnormal build-up of fluid in the peritoneal cavity of the abdomen. For ascitic fluid to be noted on clinical examination, around 1.5 L has to accumulate.
Ascitic fluid can be differentiated based on its serum-ascites albumin gradient (SAAG) into high SAAG (sometimes known as transudative) causes and low SAAG (exudative) causes.
Pathophysiology
Ascitic fluid with a high SAAG (transudates) occurs due to portal hypertension, where the fluid can be thought of as ‘squeezed out’ of blood vessels. Ascitic fluid with a low SAAG (exudates) occurs due to secretion in inflammation or malignancy.
SAAG is calculated using SAAG=serum albumin concentration – ascitic fluid albumin concentration. Portal hypertension leads to fluid leakage, but albumin remains in the serum. This leads to an increased serum albumin concentration and a lower ascitic fluid albumin concentration, increasing the SAAG value, therefore representing transudates.
In causes of exudates, inflammation leads to increased capillary permeability and water along with albumin leak out into the ascitic fluid. This decreases the serum albumin concentration and increases the ascites fluid albumin concentration, giving a low SAAG.
Causes
High SAAG (transudate)
Where SAAG > 11g/L:
- Liver disease – cirrhosis is the most common
- Heart failure
- Budd-Chiari syndrome
- Portal vein thrombosis
Low SAAG (exudates)
Where SAAG < 11g/L:
- Causes of hypoalbuminaemia:
- Nephrotic syndrome (albumin leaks out of the kidneys)
- Protein-losing enteropathy (e.g. inflammatory bowel disease)
- Malignancy
- Tuberculosis
- Pancreatitis
- Myxoedema
Presentation
The main feature of ascites is abdominal distention. Patients may have discomfort and pain depending on how tense the ascites is. Features of an underlying cause may be present such as stigmata of liver disease.
Investigations
- FBC
- U&Es:
- May be deranged in renal dysfunction
- LFTs:
- May be deranged in liver disease
- The albumin concentration is needed to calculate SAAG
- TFTs:
- Abdominal ultrasound:
- Confirms the presence of ascites
- Ascitic tap (diagnostic paracentesis):
- Used to calculate SAAG and establish the underlying cause
Management
High SAAG
Since fluid is being moved from the blood vessels into the peritoneal cavity, aldosterone is secreted in an attempt to retain more water and increases the blood pressure further, which can lead to more accumulation. Management involves attempting to counteract these processes:
- Limit dietary sodium:
- Aldosterone acts to increase salt (and hence, water) retention, therefore salt restriction allows diuresis and relief of symptoms
- Diuretics, particularly spironolactone:
- This inhibits aldosterone’s action and leads to less salt and water retention
- Abdominal paracentesis:
- Should be performed if the patient has tense ascites
- If large amounts of ascitic fluid are drained, this may reduce the amount of albumin in the serum and lead to circulatory dysfunction, therefore supplementary albumin should be given
- Transjugular intrahepatic portosystemic shunt (TIPS):
- May be considered in some patients, particularly those with advanced cirrhosis
Low SAAG
- Management involves treating the underlying cause
