The ascitic cat

Introduction

Ascites is a term used to describe the accumulation of free fluid within the peritoneal cavity. Based on cell counts, total protein, specific gravity, and cellular content, the fluid may be classified as a transudate (pure or modified) or an exudate (Table 1). This classification can ultimately be helpful in identifying the etiology of ascites and directing proper treatment. Chylous or pseudochylous effusions, hemorrhagic, bilious, neoplastic effusions, and uroabdomen are specific exudates which many clinicians prefer to differentiate from true ascites 1.

Pathophysiology

Fluid can accumulate within the peritoneal cavity via several mechanisms. These include:

1. Increased hydrostatic pressure within the vasculature (as in right-sided congestive heart failure or portal hypertension)
2. Decreased colloid osmotic pressure (as in hypoproteinemia secondary to intestinal malabsorption, hepatic failure or protein-losing diseases)
3. Increased vascular permeability (as in vasculitis or inflammatory conditions)
4. Viscus, vessel or mass rupture, or coagulopathy
5. Lymphatic obstruction/rupture or lymphoproliferative disease 2

While the character of the fluid may shed important diagnostic insight on the source of effusion, a thorough history and physical examination is essential before a fluid sample is obtained to help differentiate these possibilities.

History

An owner of a cat with ascites may describe a primary complaint of abdominal distension or clinical signs commonly associated with ascites. These include lethargy, decreased appetite, or tachypnea (this last sign results from the increased abdominal volume exerting pressure on the diaphragm). The clinician should obtain a full medical history, including any past or current conditions or operations, and all medications. A history of urethral obstruction may indicate a concern for uroabdomen. Known or suspected cardiac disease (i.e., history of a murmur or arrhythmia) may portend a suspicion of rightsided congestive heart failure (CHF). The clinician should also ascertain whether or not the cat has experienced any recent trauma, which would raise suspicion of a visceral rupture or hemoabdomen. An understanding of the cat’s origin, normal environment, and potential exposure to other animals may increase one’s suspicion of a primary infectious pathogen such as feline infectious peritonitis (FIP). This virus has a predilection for younger cats (< 3 years), many of whom live in crowded or stressful environments or have a history of fever that did not resolve with antibiotics 3.

Physical examination

The physical exam of an ascitic cat often (but not always) reveals abdominal distension (Figure 1); small volumes may not distend the abdominal wall. Ascites can be difficult to definitively differentiate from other causes of abdominal distension on physical exam alone, since organomegaly (including an enlarged urinary bladder), mass effects, pregnancy, and obesity can produce the same appearance. The presence of ascites may allow detection of a palpable fluid wave; this is ascertained by placing a hand flat against one side of the abdominal wall whilst the other hand gently taps the opposite flank to stimulate fluid movement 3.

Physical exam may elicit a variety of other findings that can help direct a clinician’s suspicion toward the primary etiology. Particular attention should be paid to the potential presence of icterus (yellow-tinged sclera, mucous membranes, or integument), which indicate hepatopathy or coagulopathy. Subcutaneous edema may signify hypoproteinemia. Peripheral lymphadenopathy may indicate lymphoma or infectious agents. Evidence of cardiac disease can include an auscultable heart murmur, arrhythmia, or gallop sound, although it is important to remember that the lack of such findings does not exclude cardiac disease. Jugular vein distension and/or pulsation (Figure 2) suggests elevated central venous pressure secondary to right-sided CHF. Loss of auscultable air movement in some or all lung fields would suggest concurrent pleural effusion, which may occur with neoplasia, hypoproteinemia, CHF, or lymphoma. Palpable hepatomegaly can occur secondary to right-sided congestive heart failure or with primary hepatic pathology (cholangiohepatitis or infiltrative/neoplastic disease).

Diagnostic tests

Diagnostic investigation should not be limited to peritoneal fluid analysis and cytology, but this is often one of the highest yield tests for narrowing a list of differentials and so is frequently the first diagnostic performed. A sample of peritoneal fluid can be obtained via abdominocentesis. This procedure should be performed as aseptically as possible. Cats can be restrained in lateral, sternal, or dorsal recumbency, whichever is most likely to limit their motion and allow for atraumatic retrieval of a fluid sample. A small amount of hair should be clipped around a ventral location (often just ventral to midline for a cat in lateral recumbency). Ideally, ultrasound guidance is used to identify an anechoic fluid pocket. In the absence of ultrasound, it is advisable to restrain the cat in lateral recumbency and approach the area approximately 1 inch (2.5 cm) ventral and caudal to the umbilicus. The skin should be gently scrubbed with a chlorhexidine-based or similar cleaning agent and wiped clean with isopropyl alcohol. With or without ultrasound guidance, a 22-25 G needle, butterfly catheter, or over-the-needle (OTN) catheter is introduced directly through the abdominal wall into the peritoneal cavity, and gentle traction is applied to an attached syringe (Figure 3). A sterile sample should be preserved in both EDTA and plain tubes for laboratory analysis. Therapeutic abdominocentesis (retrieval of a large volume of ascitic fluid) is ideally not performed until the etiology is identified because, in some cases, this may be counterproductive. One exception to this is a cat with marked tachypnea or other discomfort where abdominocentesis can make the patient more comfortable and stable.

Fluid analysis and cytology

A diagnostic sample should be analyzed for total and nucleated cell counts, total protein, specific gravity, and microscopic evaluation of the cellular components (Figure 4). As outlined in Table 1, describing ascitic fluid as a pure or modified transudate, as an exudate, or one of several non-septic exudative fluids can be extremely helpful in determining the cause of the ascites.

Pure transudates occur most commonly in the setting of hypoproteinemia (secondary to hepatic failure, chronic cholangiohepatitis, lymphocytic cholangitis, renal disease) or increased hydrostatic pressure (right-sided CHF) 4. The cellular count and total protein may rival that of modified transudates because chronic ascites can inflame the mesothelial lining of the peritoneum and increase cell counts 2. This creates an “overlap” in the causes of pure and modified transudates, which is why additional diagnostics (described below) are helpful.

Modified transudates are the most frequent finding in cats with ascites, and the most common causes include congestive heart failure, neoplasia, and hepatopathy 4. With regard to hepatopathy, lymphocytic cholangiohepatitis is more likely to produce a pure transudate, whereas portal hypertension and cirrhosis are more likely to produce modified transudates, since the latter two increase hydrostatic pressure 2.

Exudative fluids can be septic or non-septic, with positive bacterial culture results being the definitive diagnostic test for a septic exudate. Such cases warrant prompt treatment, but because culture takes several days to finalize, the fluid should be evaluated cytologically inhouse immediately after retrieval. Cytologically, septic exudates are characterized by degenerative neutrophils and intracellular bacteria, and there may be foreign material. These effusions may occur secondary to FIP, trauma, rupture of gastrointestinal viscera, or in conjunction with other causes of peritonitis. Non-septic exudates, by contrast, have higher cell counts than pure or modified transudates but lack the degenerate neutrophils or bacteria of a septic exudate. Causes of non-septic exudates include FIP, cholangitis, pancreatitis, biliary or urinary tract rupture, or neoplasia. Effusion secondary to biliary rupture often includes visible biliary crystals.

Many clinicians are tempted to diagnose chylous effusions if the ascitic fluid has a milky, opaque appearance, but true classification of chyle depends on comparing the triglyceride and cholesterol levels of the fluid to concurrent serum concentrations. In chylous effusions, the triglyceride concentration of the effusion is higher and the cholesterol concentration lower than serum concentrations. Chylous effusions contain mostly small, mature lymphocytes. Some clinicians also recognize pseudochylous effusions which have a similar gross appearance but higher cholesterol and lower triglyceride concentrations when compared with serum 1. Chylous effusion can be caused by lymphoma, lymphangectasia, congestive heart failure, or cirrhosis: it can also be idiopathic in nature.

Hemorrhagic effusion may occur in cats secondary to trauma, coagulopathy, rupture of a vessel or mass, or secondary to recent surgery. In comparison to dogs, cats are more likely to have mass rupture that involves the liver rather than the spleen 5. The fluid aspirated during abdominocentesis in these cases appears similar to frank blood and the packed cell volume and total solids concentration should closely match those of the peripheral blood.

Urine accumulation in the abdomen can result in a pure or modified transudate or an exudate, with cell counts increasing in cases that induce inflammation. Definitive identification of uroabdomen requires that the effusion has greater than twice the concentration of creatinine than the peripheral blood 6. If the effusate creatinine concentration is between one and two times greater than the peripheral blood, this is suggestive (but not definitive) for uroabdomen. An effusate potassium concentration that is greater than the potassium level in peripheral blood is similarly suggestive, but not definitive, for uroabdomen 6.

Other tests

In addition to abdominal fluid analysis and cytology, the following diagnostic tests may play an important role in the evaluation and treatment of ascitic cats.

Complete blood cell count: A blood cell count (including a reticulocyte count if applicable) should be reviewed to determine if acute blood loss or anemia linked to chronic disease is present. A neutrophilia or stress leukogram (mature neutrophilia, lymphopenia, with or without deviations in monocyte count) may raise suspicion of infectious or inflammatory disease, particularly FIP. Review of a blood smear can be additionally helpful to identify band neutrophils, toxic change, or a left shift which may indicate an acute or robust inflammatory response.

Serum biochemical profile: Serum total protein levels should be carefully evaluated. Increased total proteins (specifically, hyperglobulinemia) may indicate an infectious agent such as FIP, whereas decreased total proteins may result from hepatic failure, protein-losing enteropathy or nephropathy, or neoplasia. Hepatic disease may be further heralded by increased liver enzymes (AST, ALT, and GGT) which, if present, warrants evaluation of coagulation times, since coagulant proteins are produced within the liver and may or may not contribute to ascites. Azotemia and/or hyperkalemia may raise suspicion of renal disease or uroabdomen.

Urine testing: Urinalysis may reveal elevated protein levels which would implicate protein-losing nephropathy as a cause for hypoproteinemia. Protein within the urine should be quantified via a urine protein/creatinine ratio, provided that urine culture is negative. 

Abdominal imaging: Based on the results of baseline blood testing, abdominal imaging may provide additional specific information regarding etiology. Radiographs are not particularly sensitive or specific for identifying the presence, volume, or cause of ascites (as small volumes may not be evident at all), but large volumes commonly manifest as non-specific and gross loss of serosal detail (Figure 5). Radiographs may depict hepatomegaly (which can occur with right-sided CHF or primary hepatopathy), whilst cirrhotic livers may appear small. The classically described “ground glass” appearance of abdominal viscera may indicate peritonitis. Advanced radiographic studies involving contrast may be useful for evaluating the integrity of the urinary bladder, urethra, or lymphatic vessels.

More specific abdominal imaging is obtained via ultrasonography, which is more useful than radiography. Ultrasound can definitively identify fluid accumulation (which often appears as anechoic fluid or – with increasing cellularity of the fluid – partially “speckled” fluid), allow subjective estimation of the severity/volume of ascites, and aid assessment of potential etiologies. Primary hepatopathy may be reflected in abnormal liver size or echotexture, or by the presence of a hepatic mass or biliary obstruction. Hepatic venous distention is highly suggestive of elevated central venous pressures secondary to right-sided CHF. The intra-abdominal lymph nodes can be observed and measured for evidence of lymphoma or lymphatic obstruction. The integrity of the urinary tract can be assessed and, specifically, the appearance of the kidneys interrogated for changes in echotexture that might indicate glomerulopathy-inducing proteinuria.

Echocardiography: Echocardiography is pursued when the clinical picture and diagnostics suggest rightsided CHF or, less commonly, pericardial effusion as the cause for ascites. In cats, the most common diseases affecting the right side of the heart and leading to congestive heart failure include restrictive cardiomyopathy, tricuspid valve dysplasia, or arrhythmogenic right ventricular cardiomyopathy (Figure 6). Pericardial effusion causing cardiac tamponade is uncommon in cats. Hypertrophic cardiomyopathy is more likely to affect the left side of the heart, and the previously high prevalence of dilated cardiomyopathy has drastically declined since commercial cat foods were supplemented with taurine. 

Currently, both these forms of cardiomyopathy rarely cause ascites in cats. 

Additional diagnostic testing: FIP is an important differential for cats with ascites but it can present a diagnostic challenge to clinicians. Definitive diagnosis requires immunofluorescence or immunohistochemical staining of viral RNA or proteins within macrophages of diseased tissue or fluid.

A common constellation of laboratory findings that suggest a high likelihood of FIP include a leukocytosis (neutrophilia and lymphopenia), serum hyperproteinemia with relatively high globulin and low albumin concentrations, hyperbilirubinemia and hyperbilirubinuria, and sometimes a non-regenerative anemia 78. The peritoneal fluid obtained from cats with the “wet” or effusive form of FIP has a total protein content that is characteristically high for an exudate (greater than 3.5 g/dL), and globulins may comprise more than 50% of these proteins 9.

Serum antibody titers for the causative feline coronavirus, a common and ubiquitous pathogen among cats, are sensitive but poorly specific, since approximately only 10% of cats exposed to the virus progress to clinical FIP 10. Furthermore, a negative antibody test does not rule out FIP.

The Rivalta test has been reported to have a sensitivity of 91%, a specificity of 66%, a positive predictive value of 58%, and a negative predictive value of 93% for the diagnosis of FIP 11. This involves adding a drop of ascitic fluid to an acetic acid solution and evaluating the mixture for a white flocculent material (which occurs as a result of the high concentrations of protein and inflammatory mediators).

Where available, a conventional polymerase chain reaction (PCR) test can identify the virus in blood but does not differentiate between exposed cats and those affected with FIP. A new PCR-based test for detection of the mutated virus has been developed, and whilst preliminary results are promising, the clinical value of this test has yet to be fully elucidated 12.

Treatment

Treatment for ascites in cats is entirely dependent on the identified cause. In general, therapeutic abdominocentesis can be useful if it improves patient comfort. As for the diagnostic sampling method, cats requiring this procedure should be restrained in lateral, sternal, or dorsal recumbency, and the entry site into the abdomen aseptically prepared. A 22-25 G butterfly catheter or OTN catheter may be used to percutaneously enter the abdomen and slow, careful suction applied to an attached syringe to aspirate fluid. The author prefers an OTN catheter (possibly attached to two extension sets connected via a 3-way stopcock) for relief of a large volume of ascites. In doing so, the stylet can be removed while leaving the catheter in place, thereby avoiding a sharp needle in the abdominal cavity for a prolonged period of time.

In patients with right-sided heart failure, it is important to remember that diuretics will not quickly mobilize or evacuate ascites, so acute discomfort should be addressed first with therapeutic abdominocentesis. Diuretic therapy (furosemide 0.5-2 mg/kg PO Q12h) and ACE inhibitor therapy (enalapril or benazepril 0.25-0.5 mg/kg PO Q12-24h) should be initiated for chronic management in an attempt to prevent or reduce the rate of recurrent fluid accumulation. Ideally, serum electrolytes and renal values, as well as systemic blood pressure, should be evaluated prior to and after starting these therapies.

Lymphoma is best addressed with one of a variety of chemotherapeutic protocols, the most common among them being COP (cyclophosphamide, vincristine, prednisolone or prednisone) or CHOP (cyclophosphamide, doxorubicin, vinca alkaloid, prednisolone or prednisone) -based protocols. Recent evaluation of a 25-week modified protocol (including L-asparaginase, a vinca alkaloid, cyclophosphamide, doxorubicin, and prednisolone) provided another promising protocol to extend quality and quantity of life in cats with lymphoma 12.

Treatment for cholangitis or cholangiohepatitis depends on the underlying etiology but often includes antibiotics (amoxicillin-clavulanate 15 mg/kg PO Q12h or enrofloxacin 5 mg/kg Q24h along with metronidazole 7.5 mg/ kg Q12h), hepatoprotectants (S-adenosylmethionine 20 mg/kg PO Q24h), choleretics (ursodeoxycholic acid (10-15 mg/kg Q12h), and vitamin E (10-30 IU/kg Q24h). Immunosuppressives (prednisolone 2-4 mg/kg/day) are a necessary cornerstone of treatment in chronic lymphocytic cholangitis. Supportive care (intravenous fluids, antiemetics, nutritional support) is necessary in acutely ill patients, as is specific treatment for any co-morbidities (inflammatory bowel disease, pancreatitis).

Regrettably, ascitic cats with FIP have a poor prognosis, but short-term therapy to improve quality of life commonly includes therapeutic abdominocentesis and/or thoracocentesis, immunosuppressants (dexamethasone 1 mg/kg Q24h IP or IV followed by prednisolone 2 mg/kg Q24h) and/or immunomodulating drugs (human interferon-alpha 30 U/cat PO Q24h). Supportive care is necessary in acutely ill patients 13. Initial stabilization and surgical therapy is likely necessary in patients with septic effusion, uroabdomen, or hemorrhagic effusion.

Cats presenting with ascites warrant thorough diagnostic evaluation to identify the primary etiology. The most common causes of ascites in cats include congestive heart failure, neoplasia, hepatopathy, and FIP, and the treatments and prognoses for these conditions vary greatly, further emphasizing the importance of proper diagnosis.