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Ascites

Published on 19/07/2015 by admin

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 Free-flowing ascites conforms to shape of surrounding structures and flows to dependent recesses

image Simple fluid signal on T1WI (hypointense) and T2WI (hyperintense) MR
image No appreciable complexity within ascites fluid
• Exudative  ascites: Typically demonstrates increasing density of fluid with increasing protein content

image Often mildly hyperdense (15-30 HU) relative to transudative ascites
image May demonstrate complexity, including septations, peritoneal thickening/enhancement, and loculation
image Loculated ascites fluid exerts mass effect and displaces adjacent structures (such as bowel loops)
image Usually simple fluid signal on T1WI and T2WI MR, but internal complexity and protein can result in intermediate T1 and T2 signal
image Chylous ascites can measure < 0 HU or demonstrate fat-fluid levels

PATHOLOGY

• Ascites is typically divided into 2 types

image Transudative ascites is caused by high portal venous pressures and is characterized by low protein, low LDH, normal glucose, and low specific gravity

– Transudative ascites is simple ascites fluid most often caused by cirrhosis, hepatitis, heart failure, renal failure, hypoproteinemia, etc.
image Exudative ascites is characterized by high protein and high specific gravity 

– Exudative ascites related to multiple causes, such as infection, ischemia, peritoneal carcinomatosis, peritonitis, or pancreatitis
image
(Left) Axial CECT shows ascites image due to hepatic cirrhosis, with large varices and splenomegaly. Notice the relatively simple, uncomplicated appearance of this transudative ascites.

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(Right) Axial CECT shows massive ascites due to right heart failure. Like other forms of transudative ascites, note that the fluid appears simple without evidence of complexity, nodularity, or adjacent peritoneal thickening/enhancement.
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(Left) Ultrasound shows large anechoic ascites image in the lower abdomen displacing bowel loops image in a patient with cirrhosis. As in this case, simple transudative ascites is classically anechoic, freely mobile, and shows acoustic enhancement.

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(Right) Ultrasound shows complicated ascites image in a cirrhotic patient. The fluid was non-mobile and loculated on real-time scanning, and there are multiple internal septations image, suggesting this is not simple transudative ascites. The ascites fluid was found to be infected.

TERMINOLOGY

Definitions

• Pathologic accumulation of fluid within peritoneal cavity

IMAGING

General Features

• Best diagnostic clue

image Diagnostic paracentesis (in cases where either infection or tumor is suspected)
• Location

image Free-flowing ascites fluid flows to most dependent recesses of abdomen and pelvis

– Morison pouch (hepatorenal fossa): Most dependent recess in upper abdomen
– Rectouterine or rectovesical space: Most dependent spaces in pelvis
– Paracolic gutters and subphrenic spaces
– Lesser sac usually does not fill with ascites, with the exception of cases with tense ascites or ascites due to a local source (gastric ulcer or pancreatitis)

image Otherwise, lesser sac fluid suspicious for ascites related to carcinomatosis or infection
• Morphology

image Free-flowing ascites: Conforms to shape of surrounding structures and does not deform normal shape of adjacent organs or exert mass effect

– Fluid insinuates itself between organs
image Loculated fluid: Fluid demonstrates rounded contour and appears encapsulated

– Loculated ascites fluid exerts mass effect and displaces adjacent structures (such as bowel loops)
• Key concepts and descriptors

image Ascites typically divided into 2 types

– Transudative ascites is caused by high portal venous pressures and is characterized by low protein, low LDH, normal glucose, and low specific gravity

image Transudative ascites is simple ascites fluid that is most often caused by cirrhosis, hepatitis, heart failure, renal failure, or hypoproteinemia
– Exudative ascites characterized by high protein and high specific gravity

image Exudative ascites related to multiple causes, such as infection, ischemia, peritoneal carcinomatosis, peritonitis, and pancreatitis
image Transudate, exudate, hemorrhagic, pus
image Chylous, bile, pancreatic, urine, cerebrospinal fluid
image Pseudomyxoma peritonei, neonatal ascites

Radiographic Findings

• Abdominal radiograph: Insensitive for fluid, as diagnosis of ascites on radiographs requires a substantial amount of fluid to be present (usually > 500 cc)

image Diffuse haziness and increased density of abdomen
image Poor visualization of normally visualized soft tissue structures, such as psoas and renal outline

– Obliteration of hepatic and splenic angles
image Bulging of flanks due to fluid distending abdomen
image Separation of small bowel loops with centralization of floating gas-containing small bowel
image Hellmer sign: Lateral edge of liver medially displaced from adjacent thoracoabdominal wall
image Symmetric densities on sides of bladder (dog ears sign)
image Medial displacement of ascending and descending colon
image Lateral displacement of properitoneal fat line

CT Findings

• Transudative ascites

image Typical transudative ascites has density of 0-15 Hounsfield units (HU) and appears free flowing
image Small amounts of ascites typically seen in right perihepatic space, Morison pouch, and pouch of Douglas
image Larger amounts of fluid accumulate in paracolic gutters

– Ascites can cause centralization of bowel loops
– Ascites accumulating in central abdomen may have triangular configuration within leaves of mesentery
image Massive ascites distends peritoneal spaces
image Look for associated evidence of liver, heart, or kidney failure (most common causes of transudative ascites)
• Exudative ascites

image Exudative ascites typically demonstrates increasing density of fluid with increasing protein content

– Exudative ascites often mildly hyperdense relative to transudative ascites (15-30 HU)
– May demonstrate complexity, including septations, peritoneal thickening/enhancement, and loculation
image Ascites related to peritoneal carcinomatosis: Complex ascites with nodularity and thickening of adjacent peritoneum and frequent loculation of ascites
image Chylous ascites: Intraperitoneal and extraperitoneal fluid usually of water density, but can be < 0 HU  or demonstrate fat-fluid levels (due to fat content)
image Bile ascites: Low density fluid (usually < 15 HU) most typically localized adjacent to liver or biliary structures

– Bilomas (loculated collections of bile fluid) sharp margins
image Urinary ascites: Low density fluid (< 15 HU) usually localized adjacent to urinary tract (and site of leak)

– IV contrast accumulates within ascites fluid during excretory phase (after renal concentration and excretion)
image Cerebrospinal fluid ascites: Small amount of free fluid is expected with ventriculoperitoneal shunt

– Localized collection near tip of shunt catheter is abnormal and implies malfunction
image Pancreatic ascites: Most often accumulates surrounding pancreas, lesser sac, and anterior pararenal space

– Usually seen in presence of severe pancreatitis or disruption of pancreatic duct
image Pseudomyxoma peritonei: Large low-attenuation collection, multiseptate, loculations

– Multiple cystic-appearing masses, calcification
– Thickening of peritoneal and omental surface
– Scalloping of liver and spleen contour

MR Findings

• Transudative ascites: Fluid signal on T1WI (hypointense) and T2WI (hyperintense)
• Exudative: Usually simple fluid signal on T1WI and T2WI, but internal complexity and protein can result in intermediate T1 and T2 signal 

image Internal complexity (such as debris or septations) usually more apparent on MR compared to CT
• Artifacts

image Flow voids related to bulk motion of fluid usually more evident on SSFSE/HASTE sequences (compared to T2 FSE or steady state free precession/SSFP) and should not be misinterpreted as tumor or carcinomatosis
image Failure of fat suppression adjacent to sites of susceptibility (i.e., gas within bowel) can be misinterpreted as ascites fluid

– Ascites often erroneously diagnosed near diaphragm due to susceptibility from adjacent lung

Ultrasonographic Findings

• Uncomplicated ascites

image Homogeneous, freely mobile, anechoic fluid with posterior acoustic through transmission
image Free fluid forms acute angles where fluid borders organs, should shift with changes in patient position, and should compress with increased transducer pressure
image Polycyclic, lollipop, arcuate appearance: Small bowel loops arrayed on either side of vertically floating mesentery in patients with massive ascites
image Transverse and sigmoid colon usually float on top of fluid (nondependent gas content when patient is supine)
• Complicated exudative ascites

image May demonstrate internal echoes or septations

– Blood often produces coarse echoes and chyle produces fine echoes
image Fluid is often loculated (i.e., not freely mobile with changes in position or transducer compression)

– Loculated ascites implies adhesions, malignancy, or infection of peritoneum
image Internal septations more apparent on US than CT 

– Matted or clumped, infiltrated bowel loops
– Thickened interfaces between fluid and adjacent structures, peritoneal lining, omental thickening
• Thickening of gall bladder wall, > 3 mm in benign ascites, < 3 mm in carcinomatosis
• Sonolucent band; small amounts of fluid in Morison pouch, around liver

Imaging Recommendations

• US: Simple, cost-effective, and very sensitive (can detect volumes of ascites as small as 5-10 mL)
• CT: Valuable for evaluating underlying cause of ascites

DIFFERENTIAL DIAGNOSIS

Hemoperitoneum

• High-attenuation fluid which is typically > 30 HU (30-60 HU), although will gradually decrease in density over time 

image Active hemorrhage results in fluid at site of extravasation which is isodense to contrast-enhanced blood vessels
image Sentinel clot: Highest density (> 60 HU) blood products accumulate near site of bleeding

Malignant Ascites

• Exudative ascites which is often loculated and associated with nodular thickening of peritoneum and carcinomatosis
• Known underlying malignancy and imaging evidence of metastatic disease are keys to correct diagnosis

Infectious Ascites

• Exudative ascites with fluid demonstrating slightly higher attenuation (15-30 HU) than simple fluid
• Presence of peritoneal thickening/enhancement, septations, and frank loculated fluid collections (with peripheral rim enhancement) are clues to correct diagnosis

Cystic Peritoneal Metastases

• Most often result of mucinous neoplasms (ovary, GI tract)
• Nodular cystic lesions in the omentum and mesentery, but usually with associated soft tissue component, thickening/nodularity of omentum, and other evidence of widespread metastatic disease
• Large ascites common, frequently with evidence of loculation and internal complexity
• Adnexal mass of cystic density (ovarian, Krukenberg)

Physiologic Free Fluid

• Small fluid in cul-de-sac is physiologic in premenopausal females and should not be interpreted as pathologic ascites
• Free fluid always considered abnormal in a male patient

PATHOLOGY

General Features

• Etiology

image Hepatic: Cirrhosis, portal hypertension, Budd-Chiari, portal vein thrombosis, hepatitis
image Cardiac: Congestive heart failure, constrictive pericarditis, cardiac tamponade
image Renal: Nephrotic syndrome, chronic renal failure
image Neoplasm: Carcinomatosis due to ovary, colon, gastric, and pancreatic malignancies (amongst many others)

– Meigs syndrome, mesothelioma
image Infections: Bacterial, fungal, parasitic, or TB

– Acquired immunodeficiency syndrome
image Trauma: Blunt, penetrating, or iatrogenic injury

– Diagnostic/therapeutic peritoneal lavage
– Bile ascites: Trauma, cholecystectomy, biliary or hepatic surgery, biopsy, percutaneous drainage
– Urinary ascites: Trauma to bladder or collecting system vs. iatrogenic injury from instrumentation
– Cerebrospinal fluid: Ventriculoperitoneal shunts
– Chylous: Trauma to cisterna chyli or other lymphatics
image Hypoalbuminemia, protein-losing enteropathy
image Miscellaneous: Myxedema, marked fluid overload

Gross Pathologic & Surgical Features

• Transudate: Clear, colorless, or straw colored
• Exudate: Yellowish or hemorrhagic
• Neoplasm: Bloody, clear, or chylous
• Pyogenic: Turbid, chylous, yellowish white, or milky
• Pseudomyxoma peritonei: Gelatinous or mucinous

Microscopic Features

• Ascites fluid may contain blood cells, colloids, protein molecules, or crystalloids (e.g., glucose and water)
• Serum-ascites albumin gradient (SAAG): Difference of < 11 g/L suggests exudative ascites

image Protein: < 2.5 g/dl = transudate; > 2.5 g/dl = exudate
• Polymorphonuclear leukocyte count > 500/mm³ suggests infection or pancreatic ascites

CLINICAL ISSUES

Presentation

• Most common signs/symptoms

image Small ascites may be asymptomatic, but larger volumes result in abdominal discomfort and distension
image Physical examination: Bulging flanks, flank dullness, fluid wave, umbilical hernia, or penile/scrotal edema
• Paracentesis with US guidance or blind tap (to discern nature of ascites) required in all patients with new onset ascites of unknown etiology or chronic ascites with fever, abdominal pain, renal insufficiency, or encephalopathy

image Fluid analysis: Protein, lactate dehydrogenase, amylase, blood cell count with differential, bacteriology, cytology, pH, triglycerides

Natural History & Prognosis

• Complications: Spontaneous bacterial peritonitis, respiratory compromise, anorexia

Treatment

• Sodium restriction and diuretics for transudative ascites
• Cases with refractory transudative ascites may require large volume paracentesis

image Some patients may be candidates for peritoneovenous shunting (LeVeen, Denver catheters) or transjugular intrahepatic portosystemic shunt (TIPS)
• Patients with exudative ascites will not respond to salt restriction or diuretics, and only treatment is therapeutic paracentesis or treatment of underlying cause of ascites

DIAGNOSTIC CHECKLIST

Consider

• Recognize signs of exudative ascites and consider paracentesis for specific diagnosis

Image Interpretation Pearls

• High-attenuation ascites can result from vicarious excretion

image
(Left) Axial T2 FS MR demonstrates loculated T2 hyperintense ascites image. Notice the presence of discrete nodules image within the ascites fluid, reflecting the fact that this is malignant ascites in a patient with peritoneal carcinomatosis from appendiceal cancer.
image
(Right) Axial NECT in a patient after small bowel transplantation demonstrates fat-fluid levels image within fluid collections that indicate the chylous nature of the fluid and that it represents leakage from small bowel lymphatics.
image
(Left) Axial CECT shows complex, loculated ascites from bile peritonitis due to a biliary injury. Lesser sac ascites, which displaces the stomach image and duodenum image in this case, is usually due to a local source (e.g., pancreatitis, gastric ulcer), peritonitis, or carcinomatosis.

image
(Right) Axial T1 FS C+ MR demonstrates loculated ascites image in the pelvis. Notice the extensive peritoneal thickening image and hyperenhancement more posteriorly in the pelvis, consistent with this patient’s malignant ascites and peritoneal carcinomatosis.
image
(Left) Axial CECT shows marked thickening of the distal stomach image, compatible with malignancy. Extensive ascites is present with nodular thickening of the parietal peritoneum image, indicating malignant ascites from gastric carcinoma.

image
(Right) Axial CECT in the same patient shows the tumor extending along the gastrocolic ligament to involve the transverse colon image. Note the nodular thickening of the parietal peritoneum image. GI malignancies are the most common source of malignant ascites in men.
image
Axial CECT shows loculated ascites due to peritoneal dialysis. Note the mass effect and contrast-enhancing wall.

image
Axial CECT in a cirrhotic patient shoes spontaneous bacterial peritonitis, loculation, and enhancing rim and gas image.
image
Axial NECT in a patient with pancreatitis shows a prominent lesser sac collection of fluid image.
image
Axial CECT shows fluid in lesser sac and the “scalloped” surface of the liver and spleen. Diagnosis was peritoneal carcinomatosis.
image
Axial CECT shows ascites, including lesser sac, along with a thickened, “smudged” omentum image. Diagnosis was malignant ascites.
image
Axial CECT shows loculated ascites, peritoneal thickening image, and nodular omentum. Diagnosis was ovarian carcinoma metastases.
image
Axial CECT shows a nodular, cirrhotic liver with signs of portal hypertension, including splenomegaly, ascites image, and varices image. The intrahepatic ducts are dilated with an abnormal arborization due to primary sclerosing cholangitis in this case of transudative ascites.
image
Axial CECT shows a shattered spleen with a sentinel clot image (higher density, heterogeneous) in the perisplenic region and a large hemoperitoneum. The free-flowing blood around the liver image measured 35-45 HU.
image
Axial CECT shows long segment of jejunal thickening image, mesenteric infiltration, and ascites image in this patient with Henoch Schönlein purpura. Steroid treatment resulted in prompt resolution of symptoms.
image
Axial CECT shows fluid and nodular thickening on both sides of the diaphragm image. Ascites lies medial to the diaphragm and nodular thickening on the diaphragm undersurface represents malignant implants on the parietal peritoneum image. Note pleural implant image.
image
Axial CECT in this case of TB peritonitis shows marked thickening of the omentum and peritoneum image with infiltration of the mesentery, loculated ascites, and enlargement of mesenteric nodes image.
image
Axial NECT shows dense persistent enhancement of small kidneys image compatible with contrast-induced nephropathy. Ascites image measured 60 HU due to vicarious excretion of the contrast medium from the peritoneum as a means of compensating for failed renal excretion.

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