Animal Care Clinic
PLEURAL EFFUSION IN CATS
Critical Care & Emergency Medicine - Respiratory Diseases
Pleural effusion is an abnormal accumulation of fluid in the pleural space. Normally, a small amount of fluid is present, which serves to lubricate the surfaces and prevent friction when the lung expands and contracts. If there is a disturbance in the production or drainage of this fluid, pleural effusion develops.
Cats compensate for small amounts of excess pleural fluid by reducing their activity and increasing their respiratory rate. As the fluid continues to accumulate, lung expansion is restricted, atelectasis may occur, and the cat becomes dyspneic. Eventually, a life-threatening crisis occurs.
DIAGNOSIS OF PLEURAL EFFUSION IN CATS
ETIOLOGY AND RISK FACTORS
- Causes - A variety of diseases may lead to the development of pleural effusion. Analysis of the fluid and other diagnostic tests help determine the underlying cause of the effusion.
- Transudative fluid may accumulate in the pleural space in association with decreased oncotic pressure or increased hydrostatic pressure within the blood vascular system. Animals with marked hypoalbuminemia (< 1.5 g/dl) have low oncotic pressures that may result in the leakage of fluid into the pleural space. Increased hydrostatic pressure is most often associated with cardiomyopathy and congestive heart failure in the cat. Overzealous fluid therapy theoretically increases hydrostatic pressure, but pulmonary edema is a more common complication of overhydration than is pleural effusion in the cat. With chronicity, transudates may become modified transudates because of secondary inflammation associated with the fluid accumulation.
- Inflammation or infection of the pleura and adjacent tissues may result in the accumulation of exudative pleural fluid. These fluids may be septic, such as those associated with penetrating wounds and secondary pyothorax, or nonseptic such as those associated with feline infectious peritonitis (FIP). Other potential causes include pneumonia, esophageal diseases, and diaphragmatic hernias.
- Neoplastic effusions may develop with mediastinal lymphoma, thymoma or squamous cell carcinoma, with pulmonary neoplasms, and rarely with mesothelioma.
- Obstruction or rupture of the lymphatic thoracic duct leads to the accumulation of chyle in the pleural space (chylothorax). Chylous effusions may also occur secondary to heart disease, certain infections, and neoplasia.
- Hemorrhagic effusions may occur following trauma and surgery, or in association with clotting abnormalities (particularly warfarin toxicity).
- Eosinophilic effusions have occasionally been recognized in association with pulmonary parasites, pneumothorax, and neoplasia.
- Combined pleural and abdominal effusions also develop in the cat, and are most commonly associated with feline infectious peritonitis; heart, liver and kidney disease; and neoplasia. These effusions are frequently modified transudates.
- Risk factors
- Age - No known risk
- Breed/genetics - Purebred cats, particularly the oriental breeds are predisposed to chylothorax.
- Sex - No known risk
- Geographic/environmental - Outdoor cats are more prone to traumatic and infectious causes of pleural effusion, especially feline infectious peritonitis and pyothorax.
- Other medical disorders - Because pleural effusion is often associated with an underlying condition, a variety of illnesses predispose the cat to the disease. These include liver and kidney disease, neoplasia, heart disease, heartworm disease, pneumonia, diaphragmatic hernias, and various causes of hypoalbuminemia.
- Prevention - In general, pleural effusion cannot be prevented. Prompt diagnosis and treatment of the predisposing conditions reduce the risk of developing pleural effusion. Consider keeping cats indoors to decrease the risk of traumatic and infectious causes.
HISTORY AND CLINICAL SIGNS
- Species affected - Dogs and cats (See also Pleural Effusion in Dogs)
- Presenting signs and historical problems - Signs associated with pleural effusion may be subtle and vague, particularly early in the disease process or when only small amounts of fluid have accumulated. Animals tend to be lethargic and intolerant of exercise. Eventually, dyspnea, tachypnea and respiratory distress may be evident. Affected animals may sit or crouch in a sternal position with the head and neck extended. The elbows are often abducted away from the chest in an attempt to help breathing. Some animals may exhibit open-mouth breathing and have a forceful abdominal component to each inspiration. Other signs include anorexia, weight loss, pallor, cyanosis, hypothermia, fever or cough. Distention of the abdomen may be noted with concurrent abdominal effusions.
PHYSICAL EXAMINATION FINDINGS
- Attitude - Most affected animals are lethargic. Some may present moribund or comatose.
- Body condition - Some animals are in poor body condition.
- Vital signs - Depending on the underlying cause of the effusion, some animals may be febrile and some may be hypothermic. Tachycardia may be present. Dyspnea, tachypnea and open mouth breathing are common.
- Mucous membranes - Some affected animals may have pale mucous membranes and some may be cyanotic. Petechiae and ecchymoses may be seen with clotting disorders and icterus may occur with some forms of liver disease.
- Hydration status - Many animals are dehydrated.
- Head and neck - The thyroid glands are palpated to determine if nodules or enlarged glands are present. Cats may develop hypertrophic cardiomyopathy and secondary congestive heart failure with hyperthyroidism. If the cause of the pleural effusion is cardiac disease, jugular venous distention and/or jugular pulses may be present.
- Eyes - Cats with feline infectious peritonitis may also have anterior uveitis and/or chorioretinitis.
- Oral cavity - Usually unremarkable
- Thorax (cardio-pulmonary) - Heart and breath sounds are usually muffled or dull ventrally. Breath sounds dorsally are often normal or only slightly muffled. Chest percussion reveals dullness in the area of fluid accumulation. Auscultation of the heart may reveal a murmur, gallop rhythm or an arrhythmia, especially if the pleural effusion is cardiac in origin. The cranial thoracic cavity may be less compressible than normal if a cranial mediastinal mass is present.
- Abdomen (gastrointestinal/urinary) - Abdominal palpation may reveal other organ abnormalities. Abdominal effusion may also be evident, particularly in cats with feline infectious peritonitis.
- Reproductive system - Often unremarkable
- Lymph notes - Peripheral lymph nodes may be enlarged, particularly if lymphosarcoma is responsible for the effusion.
- Integumentary system - Abrasions or lacerations may be present if the pleural effusion is due to trauma. Ecchymoses and petechiae may be present if the pleural effusion is due to clotting abnormalities.
- Neurologic examination - Mental status varies from dullness and depression to coma.
- Musculoskeletal examination - Weakness is common.
- Special examination techniques -Analysis of the pleural fluid is crucial in determining the underlying cause of the effusion. The fluid is obtained by thoracocentesis. Cytology is performed on both a direct smear and a centrifuged sample.
Fluid analysis involves evaluation of the color, turbidity, odor, specific gravity, viscosity, pH, and total protein content of the fluid. If chylothorax is suspected, triglyceride and cholesterol levels are determined. Other chemical analyses can also be performed. If feline infectious peritonitis is suspected, the fluid may be submitted for protein electrophoresis or polymerase chain reaction assay for coronavirus. A portion of the fluid sample is also submitted for aerobic and anaerobic culture and sensitivity.
- Clinical laboratory tests
- CBC - A neutrophilia with a left shift may be present if pyothorax, feline infectious peritonitis, or neoplasia is the cause of the effusion. Lymphopenia is often associated with chylothorax. Anemia may occur with lymphoma.
- Serum biochemical tests - Biochemical tests may reveal a variety of abnormalities, depending on the underlying cause. A low albumin may be present in animals with hypoalbuminemia. Hyperglobulinemia, with a polyclonal gammopathy may indicate feline infectious peritonitis. Thyroxine levels may be elevated in cats with hyperthyroidism and cardiogenic pleural effusion.
- Urinalysis - Animals with protein-losing nephropathy have severe proteinuria.
- Coagulation profile - PT, PTT and ACT are often prolonged in animals with pleural effusion due to a coagulopathy.
- Pleural fluid analysis- Fluid analysis helps classify the fluid as a transudate, modified transudate, septic exudate, nonseptic exudate, chylous effusion, hemorrhagic effusion, and sometimes a neoplastic effusion. These categories are based upon the gross appearance of the fluid, on the protein content and viscosity of the fluid, on the white blood cell count and the cytologic features of the fluid. Although categorizing the fluid does not always lead to a specific diagnosis, it does allow the differential diagnoses to be narrowed.
- A transudate is defined as a fluid with a protein level < 1.5 g/dl and a white blood cell count < 1,000/ul. Viscosity of the fluid is low and specific gravity is < 1.018. The fluid is usually clear and colorless, and has no odor. A small number of lymphocytes, mesothelial cells, neutrophils and macrophages may be seen on cytology. Transudates arise most commonly with hypoalbuminemia and heart disease.
- A modified transudate is defined as a fluid with a protein level of 2.5 to 4 g/dl and a white blood cell count of 1,000 to 5,000/ul. Specific gravity often ranges from 1.018 to 1.030. The fluid may be clear to moderately cloudy, with a serous to serosanguineous color. Variable numbers of red blood cells, neutrophils, lymphocytes, macrophages and mesothelial cells are seen. Modified transudates can be difficult to distinguish from nonseptic exudates. Diseases associated with modified transudates in the cat include congestive heart failure, chronic hypoalbuminemia, neoplasia, and diaphragmatic hernia.
- Septic exudate is defined as a fluid with a protein level of 3 to 7 g/dl and a white blood cell count of 5,000 to 300,000/ul. The fluid is typically turbid and opaque and may have a foul odor. The color is variable from white to yellow to red. Numerous degenerative neutrophils are found and may contain bacteria. Free bacteria may also be noted. Diseases associated with septic exudates include pyothorax, bacterial infections of the lungs, mediastinum and esophagus, migrating foreign bodies and bacteremia.
- Nonseptic exudate is defined as a fluid with a protein level of 3 to 6 g/dl and a white blood cell count of 5,000 to 20,000/ul. The color ranges from serous to serosanguineous to yellow. The fluid may be hazy or turbid. It is thick and viscous with FIP. With FIP, protein levels may be > 6.0 g/dl. No bacteria are seen on cytology. The distribution of cells is variable, and samples may contain neutrophils, plasma cells, lymphocytes, macrophages and red blood cells. Nonseptic exudates may arise from lymphatic or venous obstruction, or aseptic inflammation. They can be difficult to distinguish from modified transudates. Diseases associated with nonseptic exudates include feline infectious peritonitis, inflammation in adjacent pulmonary or mediastinal or abdominal organs, neoplasia, and diaphragmatic hernia.
- Chylous effusion is defined as a fluid containing chylomicrons and with a triglyceride concentration greater than the serum triglyceride concentration. In true chylous effusions, the pleural fluid cholesterol:triglyceride ratio is < 0.15. Chylous effusions typically have a protein level of 2 to 6.5 g/dl and a variable white blood cell count (often 1,000 to 20,000/ul). The fluid is opaque and either white or pink in color. Lymphocytes predominate, and some neutrophils may also be seen on cytology. True chylothorax is extravasation or leakage of intestinal lymph from an obstructed or ruptured thoracic duct. It may also be idiopathic in nature.
- Pseudochylous effusions have a greater cholesterol concentration than serum cholesterol, but the pleural triglyceride level is less than the serum triglyceride concentration. Chylous effusions may not look as milky or have a lower triglyceride level when the animal is anorexic or fasted. Pseudochylous effusions may be associated with heart disease, mediastinal neoplasia, and certain infections (e.g. tuberculosis, particularly in dogs). It may also be idiopathic in nature.
- Hemorrhagic effusion contains predominately red blood cells. It is red in color. It has similar cell counts, hematocrit, and protein levels to that of peripheral blood. It does not clot because fluid in the pleural space becomes rapidly defibrinated. On cytology, erythrophagocytosis and an absence of platelets help confirm the presence of true effusion and not active hemorrhage. Diseases associated with a hemorrhagic effusion include trauma, coagulopathies, neoplasia and pulmonary infarction.
- Other effusions that may be discovered upon fluid analysis include peritoneal fluid that has crossed the diaphragm, and eosinophilic effusions.
- Pleural fluid biochemistry analysis - Assays of fluid pH, glucose and other enzymes may be helpful in further characterizing the effusion.
- A pH < 6.9, a glucose value < 10 mg/dl, and a high neutrophil count are consistent with a pyothorax.
- A pH > 7.2 and a neutrophil count < 45 percent of the total WBC count are consistent with a malignant effusion.
- Lactate dehydrogenase concentrations < 200 IU/L are consistent with exudates.
- Cardiogenic effusions often have fibronectin concentrations < 31.5 percent of plasma fibronectin values, whereas malignant effusions often have levels > 31.5 percent.
- Serology/immunologic tests - Feline leukemia and feline immunodeficiency virus testing should be performed to rule out immunosuppression of the cat. Feline corona virus titers are obtained in an attempt to identify the presence of FIP.
- Microbiology - Blood cultures may be indicated if bacteremia is suspected, especially in the presence of pyothorax.
- Diagnostic imaging
- Radiographs (thoracic/abdominal) - Thoracic radiographs are indicated to confirm the diagnosis of pleural effusion. Radiography may need to be delayed until after thoracocentesis is performed in animals with severe respiratory distress.
Evidence of pleural effusion on radiography includes separation of the lung lobes from the parietal pleura and sternum with a fluid density between these structures, scalloping of the edges of the lungs, interlobar fissure lines, rounding of the lung margins at the costophrenic angles on the ventrodorsal view, and blurring of the cardiac and diaphragmatic silhouettes.
The effusion is commonly bilateral, but may be unilateral in cases of pyothorax, chylothorax, and diaphragmatic hernias. The mediastinum may be widened if anterior mediastinal neoplasia is present. Cardiomegaly may be noted with heart disease.
Radiographs are repeated after pleural drainage to evaluate the thoracic structures. If a unilateral lesion is suspected, both left and right lateral radiographs are taken, along with ventrodorsal or dorsoventral views. Horizontal beam radiography allows detection of small amounts of fluid. Radiographs of the abdomen may reveal concurrent abdominal effusion.
- Ultrasound (thoracic/ECHO) - Thoracic ultrasonography is helpful prior to removal of pleural effusion to visualize structures within the thorax. It may identify pulmonary, mediastinal or pleural masses, and the presence of a diaphragmatic hernia. Ultrasonography can also be useful to guide the positioning of the needle during thoracentesis.
Echocardiography is performed after thoracentesis and when the cat is stable. It helps to confirm the presence, and to characterize the type of heart disease.
- Biopsy - Exploratory thoracotomy with biopsy of abnormal tissue may be needed to confirm the cause of the pleural effusion in some cases. Biopsy of anterior mediastinal masses may be performed under ultrasonography.
- ECG - An electrocardiogram is recommended if cardiac arrhythmias are ausculted.
DIAGNOSIS AND PROGNOSIS
- Differential diagnosis - A variety of illnesses can cause dyspnea and lethargy in the cat. Some of these include:
- Feline asthma
- Congestive heart failure
- Pulmonary edema
- Pulmonary contusions
- Pulmonary neoplasia
- Diaphragmatic hernia
- Heartworm disease
- Recommended tests - Thoracic radiographs, thoracic ultrasonography, CBC, biochemistry profile, urinalysis and fluid analysis
- Summary of diagnostic criteria - Thoracic radiographs confirm the presence of pleural effusion. Fluid analysis helps determine the class of fluid present and may provide important information as to the underlying cause of the effusion.
- Prognosis - The prognosis for pleural effusion varies, depending upon the underlying cause. For most cases, the prognosis is poor to guarded. Animals with a diaphragmatic hernia or hemothorax due to trauma or anticoagulant rodenticide toxicity have a fair to good prognosis. Animals with heart disease, chylothorax and pyothorax have a fair to poor prognosis. The prognosis for some cats with pyothorax, not complicated by fibrinous atelectasis of the lungs, can be good. Cats with anterior mediastinal neoplasia have a poor prognosis, although some tumors respond favorably to chemotherapy. For cats with FIP, the prognosis is grave because the disease is invariably fatal.
TREATMENT OF DISEASE
The principles of therapy for pleural effusion involve relief of any respiratory distress by evacuation of pleural fluid and institution of emergency procedures, as well as treatment of the underlying cause of the effusion.
- Emergency therapy - Emergency therapy involves the following:
- Oxygen therapy
- Thoracocentesis - If continued production of pleural fluid is anticipated, then an indwelling chest tube is inserted so that intermittent evacuation of fluid is possible.
- Sedation and intubation in severe cases
- Repeated radiographs of the chest to monitor response to therapy
- Specific therapy - Definitive therapy requires the establishment of an etiologic diagnosis. A brief overview of therapy is discussed below. For more specific details regarding therapy, see the in-depth content articles pertaining to each specific disease.
For animals with congestive heart failure, furosemide, oxygen and nitroglycerine are often recommended. If the cat is hyperthyroid, consider methimazole therapy.
For animals with pyothorax, broad-spectrum antibiotics are administered until culture and sensitivity results return. Then, antibiotic therapy is guided by sensitivity results. Intravenous fluids are often required to treat dehydration. Thoracostomy tubes are used to lavage the chest.
For animals with hemorrhagic effusions, blood product transfusions or autotransfusion may be necessary. A thoracostomy tube may also be required.
For animals with chylothorax, an indwelling thoracostomy tube is often necessary. Surgical ligation of the thoracic duct and/or surgical shunting procedures are often performed. Dietary modification and medical therapy with the benzopyrones may also be instituted.
Long-term therapy depends on the underlying cause of the pleural effusion. It may include antibiotics, furosemide for heart disease and repeated thoracocenteses.
Frequent rechecks are recommended, especially early in the course of therapy. Radiographs are initially taken every 48 to 72 hours to monitor response to therapy.
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