Peritoneal Fluid Collections, Peritonitis, and Peritoneal Abscess

Peritoneal Fluid Collections

Etiology

Ascites is the abnormal accumulation of fluid in the peritoneal cavity. There are numerous causes of ascites, including congenital, infective, inflammatory, and neoplastic diseases. In the United States the most common causes are liver disease and malignancy. In many parts of the world, tuberculosis is an important cause. The main causes of ascites and their frequency in the United States are listed in Table 81-1 .

TABLE 81-1

Causes of Ascites in U.S. Clinical Practice

Data from 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; 117:215–220.

Cause	Percentage
Cirrhosis	81
Malignancy	10
Heart failure	3
Tuberculosis	2
Dialysis	1
Pancreatic disease	1
Others	2

Prevalence and Epidemiology

The term ascites does not specify the type of fluid accumulated within the peritoneal cavity. Ascites may be further classified as in Table 81-2 into infected, chylous, hemorrhagic, and neoplastic fluid.

TABLE 81-2

Types and Characteristics of Complex Ascites

Type	Characteristics
Infected	Attenuation of fluid collection ≥20 HU Loculated Rim enhancing Air ±
Chylous (discussed in text)	Fat/fluid level Fluid/fluid level Density equal to that of water
Hemorrhagic (discussed in text)	High density fluid or clotted blood Density ≥50 (Density may vary as blood may be clotted or lysed) Retroperitoneal spread ±
Neoplastic	Solid mass or metastatic nodules Abundant fluid May or may not be loculated

HU, Hounsfield units.

Specific Types

Chylous Ascites

Chylous ascites is a milky fluid that is rich in triglycerides secondary to leakage of lymph into the peritoneal cavity. In the United States the most common cause of chylous ascites is malignancy, of which lymphoma accounts for 30% to 50% of cases. Other associated neoplasms include breast, esophageal, pancreatic, colon, renal, testicular, ovarian, and prostate cancer, as well as lymphangiomyomatosis, a more common cause in children. Approximately 0.5% to 1% of cirrhotic patients with ascites have chylous instead of serous fluid.

Trauma, surgery, or radiotherapy to the abdomen may damage lymphatic channels and lead to chylous ascites. Worldwide, infectious causes such as tuberculosis and filariasis (parasitic infection caused by Wuchereria bancrofti ) are more common than neoplastic causes.

Paracentesis typically shows a cloudy milky aspirate. Triglyceride content of more than 0.1 g/L is diagnostic of chylous ascites. When the cause of chylous ascites is unclear, computed tomography (CT) of the abdomen and pelvis may be useful to evaluate for lymphadenopathy.

Hemorrhagic Ascites

Persons with hemorrhagic ascites have a red blood cell count greater than 50,000/mm ³ . The normal red blood cell count of peritoneal fluid is less than 1000/mm ³ . There are several causes of hemorrhagic ascites. Bloody ascites occurs in approximately 5% of patients with cirrhosis. In such patients, hemoperitoneum may occur spontaneously or after a traumatic paracentesis. In the more common setting of traumatic paracentesis, the ascitic fluid will clot, in contrast to nontraumatic bloody ascites, in which the red cells are lysed and the fluid does not clot on standing. The presence of nontraumatic bloody ascites in a cirrhotic patient raises concern for an underlying malignancy such as hepatocellular carcinoma. Approximately 20% of ascitic fluid aspirations in patients with malignant ascites are bloody.

Trauma is clearly an important cause of hemoperitoneum (to be discussed). Other less common causes of hemorrhagic ascites are peritoneal dialysis, tuberculosis, rupture of vascular tumor such as hepatic adenoma, sarcoidosis, and vasculitis such as Henoch-Schönlein purpura.

Pathophysiology

There are three theories that explain the genesis of ascites. The diminished effective volume theory and overflow theory differ in whether abnormal renal sodium retention precedes or follows the accumulation of ascites. The peripheral arterial vasodilation theory combines aspects of both the volume and overflow theories and is the most widely accepted.

Imaging

Radiography

Plain films are insensitive to ascites until at least 500 mL of fluid has accumulated. Indirect and nonspecific signs are abdominal haziness, bulging of the flanks, indistinct psoas margin, and increased separation of bowel loops. More specific signs include separation of lateral liver contour from the thoracoabdominal wall (Hellmer's sign), separation of the ascending and descending colon from the properitoneal fat line, and symmetric density on either side of the urinary bladder (the “Mickey Mouse” sign).

Ultrasonography

Ultrasonography detects as little as 10 mL of fluid. It is of help in assessing patency and flow pattern of portal or hepatic veins and in guiding paracentesis ( Table 81-3 ). Peritoneal fluid is seen in the pelvic cul-de-sac in normal females in all phases of the menstrual cycle. Features that differentiate simple from complicated ascites on imaging studies are shown in Table 81-4 and illustrated in Figure 81-1 . The findings of simple ascites does not exclude infection or tumor. Gallbladder wall thickening is seen in 82% of cases of benign ascites, whereas only 5% of malignant ascites show this finding. Ascites may cause artifacts as a result of reflection of the ultrasonic sound waves at the liver/fluid interface. Pericolonic epiploic appendages may simulate peritoneal metastases.

TABLE 81-3

Accuracy, Limitations, and Pitfalls of the Modalities Used in Imaging of Peritoneal Fluid

Modality	Accuracy	Limitations	Pitfalls
Ultrasonography	Can detect as little as 10 mL of ascites Can assess portal and hepatic venous flow Portable	Overlying bowel gas, patient body habitus, limited evaluation of peritoneal masses	Artifacts from reflection of fluid-solid interfaces. Epiploic appendages can be mistaken for peritoneal metastases.
CT	The best single test for determining site and nonliver cause of ascites	Cannot detect low-density soft tissue in ascitic fluid Low sensitivity for serosal bowel metastases	A collection with considerable amount of solid tissue (e.g., in setting of pancreatic necrosis) may appear as simple fluid collection.
MRI	Good overview of ascites	Ill patients may not tolerate long scan.	Standing wave artifact on 3.0-T MRI.

CT, Computed tomography; MRI, magnetic resonance imaging.

TABLE 81-4

Differentiation of Simple and Complex Ascites on Imaging Tests

Imaging Type	Simple Ascites *	Complicated Ascites ^†
Ultrasonography (see Figure 81-1 )	Anechoic Acoustic enhancement Fills the space between organs and bowel without mass effect Mobile with patient's position change Compresses with transducer pressure Thickened gallbladder Diffuse smooth thickening of small bowel without nodularity	Internal echoes Septa: Multiple septa suggest tuberculosis or pseudomyxoma Fluid displaces bowel and solid organs Scalloping of solid organ surface (liver, spleen) suggests pseudomyxoma Loops of bowel matted together Fluid in the lesser sac Loculated fluid collections Lack of thickening of gallbladder Peritoneal solid or cystic masses suggest malignant disease or less likely tuberculosis
CT	Uniform attenuation of 0 to 20 HU Bowel floats freely in midabdomen Ascites that does not extend onto the lesser sac	Loculated collections Peritoneal thickening or abnormal enhancement Peritoneal, omental masses or nodularity Attenuation >20 HU or variable attenuation Enhancement of peritoneal fluid on delayed phases
MRI	Fluid that is uniformly low signal on T1 weighting and very high signal on T2 weighting	Higher signal fluid on T1 weighting Debris within fluid Loculated fluid

CT, Computed tomography; HU, Hounsfield units; MRI, magnetic resonance imaging.

* Simple ascites denotes transudative fluid as seen in liver disease and cardiac failure.

† Complex ascites indicates the presence of infection, inflammation, or neoplasm. Hemorrhagic ascites is dealt with separately in the text.

Figure 81-1, Ultrasound images of simple and “complicated” ascites. A and B, A 44-year-old man presented with alcoholic cirrhosis. Peritoneal fluid (arrowhead, A ) is anechoic without debris, consistent with simple ascites of chronic liver disease. The gallbladder ( arrow, B ) is thickened. C and D, A 54-year-old woman presented with ovarian cancer. The ascites shows septation (arrowhead, C ) and debris ( arrow, C ). A peritoneal mass ( dashed arrow, D ) with vascular flow is seen. These findings are consistent with complicated ascites of peritoneal carcinomatosis.

Hemoperitoneum may have differing appearances depending on transducer frequency and duration of hemorrhage. At 2- to 3-MHz acute hemorrhage is anechoic with increased through-transmission. With increasing frequency of transducer, the hemorrhage appears echogenic. After the first 4 days, with hematoma lysis, internal echoes either fill the collection or layer dependently. With time, hematoma becomes an anechoic seroma.

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