Ascites, itself, usually does not cause a problem, but the primary disease process causing the ascites can be a serious condition. Ascites is usually produced slowly and in small amounts; however, if a large amount of fluid is produced, or rapidly produced, an emergency situation may exist. Large amounts of free fluid in the abdomen may compress the diaphragm, leading to respiratory compromise and difficulty breathing. Fortunately, this is a rare occurrence.
Another potential emergency situation is the rapid onset of ascites. If the ascites develops quickly, it is often associated with profound weakness or shock. The loss of intravascular (within the blood vessels) blood volume may lead to acute anemia and shock as fluid leaves the blood vessels and moves freely into the abdominal cavity. The most common cause of such an event is the rupture of a blood vessel within the abdominal cavity. Bleeding tumors within the abdomen or traumatic injury are likely causes.
Most of the time, the fluid buildup is more gradual and an emergency situation is not present. Any amount of ascites is a significant finding, however, and steps should be taken to find a diagnosis. Ascites has many different causes and establishing a diagnosis is usually not difficult. A good physical exam, basic blood tests and evaluation of the ascitic fluid often lead to the diagnosis, or they provide a direction upon which further diagnostics need to be done.
Hypoalbuminemia is a decrease in serum albumin levels. Albumin is available through the diet, but it is also produced in the liver. The kidney functions to maintain albumin concentration in the plasma by preventing its excretion in the urine. Normal gastrointestinal function is needed for proper absorption. Albumin is responsible for much of the colloidal osmotic pressure of the blood, and thus is an important factor in regulating the exchange of water between the plasma and the interstitial compartment (space between the cells). Decreases in the albumin level (usually under 1.5 gm/dl) lead to a pressure gradient that causes fluid to leak out of the blood vessels, producing ascites. Protein-losing nephropathy. Disease of the kidney's glomerulus (filtration system) may produce a condition where protein is excreted through the urine resulting in very low plasma albumin levels. This may be caused by infectious, immune, neoplastic or idiopathic (unknown etiology) mechanisms.
Gastrointestinal diseases that prevents the absorption of albumin. If albumin is not being absorbed, low blood levels are to be expected. Some diseases causing this include gastrointestinal lymphoma, inflammatory bowel disease and intestinal lymphangiectasia.
Severe liver disease may cause ascites by decreased albumin production or by increased resistance to blood flow within the liver. Severe hepatitis and liver cirrhosis are examples.
Obstruction of blood flow within the abdomen can result in ascites. This obstruction may cause a leakage of fluid from the veins or lymphatics and into the peritoneal (abdominal) cavity depending on the location. Obstruction of the blood flow from the liver to the heart (hepatic venous outflow obstruction) may cause a more chronic ascites. This occurs in right-sided heart failure, as the heart cannot handle the venous blood return from the liver.
Right sided heart failure. This may be caused by primary cardiac disease, pulmonary disease or severe heartworm infestation.
Abdominal masses. Tumors, abscesses or cysts may cause ascites by several mechanisms. They may rupture, causing acute ascites. Hemangiosarcoma is the most common tumor to potentially rupture and bleed. Other masses may compress or cause obstruction of vessels and lymphatics causing a more chronic and slower buildup of ascites.
Other Causes Trauma. Rupture of the spleen may cause leakage of blood; rupture of the gall bladder may cause leakage of bile, a bile peritonitis; injury to the urinary tract (kidney, ureter, bladder, or urethra) may cause leakage of urine, resulting in a uroabdomen (urine within the abdomen).
Peritonitis is inflammation of the inner lining of the abdomen. This inflammation can produce fluid in varying amounts with very specific characteristics. Acute pancreatitis is a potential cause. Peritonitis can also occur in cases of bowel (stomach, small and large intestines) compromise or perforation, necessitating emergency surgical intervention.
Bleeding disorders caused by a variety of conditions (poisoning, metabolic, inherited, cancer) might result in a bloody ascites.
Diseases of the lymphatics may cause lymph fluid to form in the abdomen. Ruptured lymphatics due to tumors, trauma or idiopathic (unknown etiology) conditions are the usual causes.