Ascites and Peritoneal Fluid Collections

Introduction

Ascites is the pathologic accumulation of fluid in the peritoneal cavity. It is a common finding that can be associated with a large number of diseases. Imaging plays an important role in detecting and characterizing ascites. Although ascites may merely reflect generalized third-space fluid loss in conditions such as congestive heart failure, it is more commonly related to intra-abdominal factors that produce peritoneal fluid more rapidly than it can be absorbed. Cirrhosis and neoplasm are the two most common causes of ascites in the Western world. Tuberculosis and cirrhosis are the predominant causes worldwide. ^,

The physical diagnosis of ascites is difficult unless there is at least 1.5 to 2 L of fluid in the peritoneal cavity; the details are beyond the scope of this chapter. Diagnostic paracentesis is indicated for any patient who develops ascites for the first time and for patients with chronic ascites who develop fever, encephalopathy, or abdominal pain. Paracentesis for small volumes should be performed under sonographic guidance with an 18- or 20-gauge, plastic-sheathed catheter to avoid injury to the liver, spleen, or gut. With massive ascites, a blind tap is often performed. ^,

Types of Peritoneal Fluid Collections

The various causes of ascites ( Box 65.1 ) are classically divided into two broad pathophysiological categories: transudates and exudates. Transudates are clear and colorless fluid collections with a protein content less than 2.5 g/dL and specific gravity less than 1.016. They are due to pathologic alteration of the Starling forces, and are most commonly seen in patients with portal hypertension (see Box 65.1 ), chronic renal failure, hypoproteinemia, and anasarca. ^{, ,} Exudates are yellow-colored or sanguineous protein-rich fluid collections, occurring in patients with a wide range of peritoneal diseases including metastases, infection, tuberculosis, or pancreatitis. While certain imaging features can help differentiate transudates from exudates ( Table 65.1 ), fluid analysis is ultimately necessary to make the diagnosis. ^{, , , ,}