Ascites is defined as an abnormal amount of intraperitoneal fluid.
Ascitic fluid is traditionally characterised as either a transudate (thin, low protein count and low specific gravity) or an exudate (high protein count and specific gravity).
More recently, the concept of the serum-ascites albumin gradient has been shown to be more accurate in classification of the causes of ascites5. For the purposes of simplicity however, we maintain the former classification.
Causes of transudative ascites
- hepatic cirrhosis
- alcoholic hepatitis
- heart failure (CCF)
- portal vein thrombosis
- peritoneal dialysis
Causes of exudative ascites
- peritoneal carcinomatosis
- nephrotic syndrome
- ischaemic bowel
- bowel obstruction
Detection of intraperitoneal fluid on plain film requires at least 500 mL to be present.
Plain film findings of ascites include:
- diffusely increased density of the abdomen
- poor definition of the the soft tissue shadows, such as the psoas muscles, liver and spleen
- medial displacement of bowel and solid viscera (away from properitoneal fat stripe)
- bulging of the flanks
- increased separation of small bowel loops
May detect smaller volumes especially if its adjacent diaphragm or the anterior margin of the liver 3.
Assessment of fluid type:
- simple ascites = anechoic
- exudative, haemorrhagic or neoplastic ascites contains floating debris
- septations suggest inflammatory or neoplastic cause
CT is most sensitive to small amounts of fluid in the peritoneum which collects preferentially in the dependent regions, such as Morrison's pouch and the pelvis. The CT density of intraperitoneal fluid may give a clue to the underlying aetiology:
- transudative ascites density should be approximate to that of water (-10 to +10HU).
- exudative ascites density > 15 HU.
- haemoperitoneum density is higher still (~ 45HU).
Of course, other intra- or extra-abdominal CT features may give further evidence to the orgin of the ascites (e.g. features of heart failure, features of cirrhosis, peritoneal catheter in situ, etc)
Consider other causes of intraperitoneal fluid:
- physiological : small amount of pelvic fluid may be normal in young females
- chylous ascites
- bile leak
- urine : bladder trauma
- 1. Love L, Demos TC, Reynes CJ et-al. Visualization of the lateral edge of the liver in ascites. Radiology. 1977;122 (3): 619-22. doi:10.1148/122.3.619 - Pubmed citation
- 2. Goerg C, Schwerk WB. Peritoneal carcinomatosis with ascites. AJR Am J Roentgenol. 1991;156 (6): 1185-7. AJR Am J Roentgenol (citation) - Pubmed citation
- 3. Thoeni RF. The role of imaging in patients with ascites. AJR Am J Roentgenol. 1995;165 (1): 16-8. AJR Am J Roentgenol (abstract) - Pubmed citation
- 4. Jolles H, Coulam CM. CT of ascites: differential diagnosis. AJR Am J Roentgenol. 1980;135 (2): 315-22. AJR Am J Roentgenol (abstract) - Pubmed citation
- 5. 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;117 (3): 215-20. - Pubmed citation
- 6. Brant WE, Helms CA. Fundamentals of diagnostic radiology. Lippincott Williams & Wilkins. (2007) ISBN:0781765188. Read it at Google Books - Find it at Amazon
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