Pleural Effusion

Chapter 164 Pleural Effusion



Robert G. Sherding, Stephen J. Birchard


Pleural effusion is an abnormal accumulation of fluid within the pleural space and is a clinical manifestation of conditions such as pyothorax, feline infectious peritonitis, congestive heart failure, intrathoracic neoplasia (e.g., lymphoma, thymoma, pulmonary neoplasia, mesothelioma), chylothorax, heartworm disease, hemothorax, hypoalbuminemia, lung lobe torsion, and diaphragmatic hernia. Pleural effusion is usually suspected from clinical signs and physical findings and is confirmed by thoracentesis or thoracic radiography.



ETIOLOGY


Pleural effusion occurs when one or, more often, a combination of the factors that determine pleural fluid dynamics are altered so as to increase fluid formation, decrease fluid absorption, or both. For example, pleural effusion is often associated with congestive heart failure (CHF) because increased capillary hydrostatic pressure results in increased pleural fluid formation. Extreme hypoalbuminemia may lower systemic colloidal osmotic pressure sufficiently to cause increased formation and decreased absorption of pleural fluid. Inflammation of the pleura may increase the formation of pleural fluid because of increased blood flow (hydrostatic pressure) and permeability of the pleural capillaries along with increased intrapleural colloidal osmotic pressure due to a higher concentration of protein in the fluid. Pleural effusion may also result from lymphatic insufficiency caused by thoracic duct obstruction, intrathoracic neoplasia, pleural thickening, or lymphatic hypertension secondary to CHF. The major causes of pleural effusion in dogs and cats are listed in Table 164-1.



Table 164-1 CAUSES OF PLEURAL EFFUSION AND CRITERIA FOR DIAGNOSIS

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CLINICAL SIGNS



• Dyspnea and exercise intolerance (inactivity) are the most consistent presenting signs of pleural effusion. Dogs and cats generally accommodate to small to moderate increases in the volume of intrapleural fluid by gradually decreasing their level of activity. Thus the signs of early pleural effusion are subtle and often imperceptible to the owner. As the accumulation of intrapleural fluid becomes substantial, however, tachypnea and respiratory distress become apparent during mild exertion and eventually even at rest.

• To facilitate breathing, the animal may prefer a sitting or crouched sternal posture, with the head and neck extended and the elbows abducted away from the thorax. An anxious facial expression and open-mouth breathing with forceful abdominal efforts during inspiration may be observed. Cyanosis may be seen in severe cases.



Key Point


Any struggling or increased distress during examination procedures may worsen dyspnea and induce respiratory arrest because of limited respiratory reserve.


• Other clinical signs associated with pleural effusion depend on the underlying cause but may include anorexia, depression, weight loss, dehydration, pallor, fever, hypothermia, or cough. Cough in animals with pleural effusion may indicate tracheal compression by a mediastinal mass (lymphoma, thymoma), intrapulmonary involvement (e.g., lung tumor, pulmonary edema, heartworm disease, pneumonia), or the presence of pleuritis.


DIAGNOSIS


Suspect pleural effusion on the basis of clinical signs and physical findings. Confirm pleural effusion by thoracentesis (see Chapter 3) or thoracic radiography (see Chapter 159). The cause is often determined through analysis of pleural fluid obtained by thoracentesis, in conjunction with post-thoracentesis radiographs. Depending on the suspected etiology, consider other diagnostic procedures such as laboratory evaluations, cardiac evaluations, ultrasonography, and specialized imaging procedures (contrast radiography, scintigraphy, computed tomography [CT]). Rarely, exploratory thoracotomy is required for definitive diagnosis.



Physical Examination



Thoracic Auscultation and Percussion



• Auscultation generally reveals muffled or inaudible heart and lung sounds ventrally, while breath sounds are preserved dorsally.

• On percussion, pleural effusion causes the thorax to sound dull and hyporesonant, and a horizontal fluid line may be demonstrable.


Other Physical Findings



• Fever suggests an inflammatory, infectious, or neoplastic process.

• Jugular venous distention or pulsations and auscultation of murmurs, gallops, and arrhythmias are signs suggestive of cardiogenic pleural effusion. Pleural effusion can be associated with congestive heart failure in cats with hyperthyroidism; thus, palpate the thyroid region for the presence of thyroid gland nodules or enlargement.

• Decreased compressibility of the cranial thorax is suggestive of a mediastinal mass, usually lymphoma or thymoma. Mediastinal lymphoma can also compress the esophagus, causing signs of dysphagia and regurgitation; it also can impinge on the sympathetic innervation of the eye, causing Horner syndrome.

• Animals with pleural effusion should also be thoroughly examined for evidence of a tumor in any location, since extrathoracic neoplasms may metastasize to the lungs or pleural cavity and cause pleural effusion (e.g., mammary adenocarcinoma).

• Ophthalmoscopy may reveal lesions of chorioretinitis in cats with feline infectious peritonitis (FIP).

• External signs of trauma may indicate hemothorax or diaphragmatic hernia.


Thoracic Radiography


Routine thoracic radiography is generally effective for confirming pleural effusion.




Key Point


If the animal is in extreme respiratory distress, perform pleural drainage to stabilize the patient before radiography. Otherwise, the stress of restraint and manipulation during radiography could prove fatal.


The radiographic signs of pleural effusion include separation of the lung lobes from the parietal pleura and sternum by extrapulmonary fluid density (i.e., compression of lung lobes by a pleural fluid density), fluid-filled interlobar fissures producing a scalloped appearance to the edges of the lungs, and obscuring of the cardiac and diaphragmatic shadows, which is referred to as the silhouette sign (see Chapter 159). There is also widening of the mediastinum and blunting or filling of the costophrenic angles by intrapleural fluid density on a ventrodorsal view. In addition, the various causes of pleural effusion may be associated with other radiographic findings of diagnostic significance (see Table 164-1).


• A rounded contour to the caudal lobar borders, often accompanied by atelectasis of the cranial and middle lobes, is suggestive of a chronic fibrosing reaction of the visceral pleura that is exerting a constrictive and restrictive effect on the lung lobes (especially in cats with chronic chylothorax). Atelectatic lobes may be mistaken for pulmonary masses, hilar masses, or lung lobe torsion.

• Although most pleural effusions are bilateral, unilateral effusion is seen occasionally. This is most suggestive of pyothorax because the natural walling-off response to septic suppuration can cause extensive pleural thickening, which may seal off the mediastinum and limit the effusion to one side. Pleural thickening associated with chylothorax can also result in unilateral effusion. In addition to pyothorax and chylothorax, unilateral effusion is occasionally observed in traumatic hemothorax, diaphragmatic hernia, pulmonary neoplasia, and lung lobe torsion.

• Pleural effusion is sometimes found in combination with ascites. Simultaneous pleural and peritoneal effusions (dual-cavity effusion) occur most often in cats with FIP but also in dogs or cats with severe hypoalbuminemia, diaphragmatic hernia, widely disseminated neoplasia, pancreatitis, and CHF.



Key Point


Because many intrathoracic structures are obscured by the presence of pleural effusion, obtain radiographs after the removal of pleural fluid to facilitate visualization of such abnormalities as mediastinal mass, cardiomegaly, intrapulmonary lesions (masses, infiltrates, vascular changes), lung lobe torsion, or diaphragmatic hernia (see Table 164-1).


• Both right and left lateral radiographic views of the thorax may be indicated when a unilateral lesion is suspected (e.g., focal fluid accumulation, pulmonary mass or focal density, lung lobe torsion). Horizontal beam radiography (e.g., standing lateral view) can be used to confirm small-volume effusions, to demonstrate fluid encapsulation, and to facilitate visualization of thoracic structures by shifting fluid away from the structures of interest.


Thoracentesis and Fluid Analysis


In most animals with pleural effusion, the combination of radiographic findings and fluid analysis establishes the diagnosis or determines the direction for additional diagnostic evaluations. Drainage of the pleural fluid also provides therapeutic benefit and may be lifesaving in patients with hypoxemia.



Thoracentesis



• Thoracentesis is a safe and generally effective method for removal of fluid from the pleural space. See Chapter 3 for a description of thoracentesis techniques.

• Following the collection of diagnostic specimens, enough fluid should be aspirated to relieve respiratory distress. In some animals it may be difficult to obtain an adequate volume of fluid if the fluid is compartmentalized within the pleural space by adhesions or if it is viscid and full of fibrin or debris. In this case, or if repeated pleural drainage is anticipated, place a thoracostomy tube (see Chapter 3).


Pleural Fluid Analysis


Perform the following analyses on the pleural fluid.


• Perform cytologic examination of direct smears and centrifuged cell concentrate smears stained with routine hematologic stains. In scanty aspirates, smears for cytologic examination are the first priority.

• Measure total nucleated cell count (differentiates transudates from exudates).

• Determine physical and biochemical characteristics, such as color, turbidity (correlates with nucleated cell count in non-chylous fluids), odor (foul odor suggests pyothorax), specific gravity and total protein concentration (differentiates transudates from exudates), viscosity and clot formation (indicates fibrinous exudate as in feline infectious peritonitis [FIP]), and presence of chylomicrons (tests for triglyceride indicate chylous effusion).

• Consider non-routine chemical analyses; e.g., pH (<6.9 in pyothorax), glucose (<10 mg/dl in pyothorax), lactic dehydrogenase (>200 IU/L in exudates), adenosine deaminase (elevated in inflammatory exudates), and fibronectin (elevated in neoplastic effusion).

• If infection is suspected, consider Gram stain, acid-fast stain, culture and sensitivity testing for aerobic and anaerobic bacteria, and, in some cases, culture for fungi.

• If FIP is a possibility, set aside an aliquot of fluid in case protein electrophoresis and a polymerase chain reaction test are later warranted based on preliminary findings (see under Feline Infectious Peritonitis).


Classification of Pleural Fluid Patterns


On the basis of these analyses, pleural effusions are generally classified into one of several patterns: transudate, modified transudate, nonseptic exudate, septic exudate, chylous effusion, or hemorrhage (Table 164-2). In addition, any of these can be subcategorized as neoplastic versus non-neoplastic depending on whether or not neoplastic cells are present on cytologic evaluation. There can be considerable overlap between these various categories; nevertheless, they are helpful for understanding the pathogenesis and determining the cause of pleural effusions.



Table 164-2 PLEURAL FLUID ANALYSIS PATTERNS*

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Transudate


Transudative effusions are generally associated with alteration of capillary hydrostatic pressure caused by CHF or decreased plasma colloidal osmotic pressure caused by hypoalbuminemia. The typical features of a pure transudate are low protein concentration and low cell count consisting predominantly of mesothelial cells; this is comparable to the characteristics of normal pleural fluid.



Modified Transudate


Long-standing transudative effusions often become modified transudates, as they acquire greater cellularity and protein content. Modified transudates and non-septic exudates can be very similar.



Septic Exudate


Septic exudates are purulent effusions consisting of numerous degenerating neutrophils, usually in association with intra- and extracellular bacteria. The fluid usually has marked turbidity, high protein concentration, and a foul odor, whereas the color is variable. The presence of a septic exudate within the pleural space indicates septic pleuritis (pyothorax) and is an indication for culture and Gram stain of the effusion (see under Pyothorax).



Nonseptic Exudate


Nonseptic inflammation and disorders that cause lymphatic or venous obstruction, such as neoplasia, diaphragmatic hernia, lung lobe torsion, pulmonary infarction, and thymic branchial cysts may result in a nonseptic exudative effusion that is difficult to distinguish from a modified transudate. Some clinicians subclassify such effusions as neoplastic effusions if neoplastic cells are evident cytologically.


• Feline infectious peritonitis causes a nonseptic pyogranulomatous exudate that has fairly distinctive characteristics—yellow and translucent in appearance with a viscous consistency, high protein concentration (approximating serum levels), high fibrin content, and low-to-moderate cellularity consisting mostly of non-degenerate neutrophils and macrophages (see under Feline Infectious Peritonitis; see also Chapter 10). If FIP is suspected, consider performing protein electrophoresis of the fluid. If gamma globulin is greater than 32% of the protein in effusates, FIP is strongly considered; conversely, if more than 48% of protein is albumin or if the albumin-to-globulin ratio is greater than 0.81, FIP is unlikely.

• Eosinophilic effusion (>10% eosinophils) can be either an exudate or modified transudate. The most frequent cause has been intrathoracic neoplasia (e.g., systemic mastocytosis, lymphoma). Eosinophilic pleural effusion has also been associated with heartworm disease, allergy, bronchointerstitial lung disease, eosinophilic granulomatosis, and trauma.


Chylous Effusion


Chylous effusions are caused by extravasation or leakage of intestinal lymph (chyle) from an obstructed or ruptured thoracic duct. Chylothorax in dogs and cats may be idiopathic or associated with trauma, thoracic lymphangiectasia, intrathoracic neoplasia, heart disease, heartworms, venous thrombosis, diaphragmatic hernia, or lung lobe torsion (see under Chylothorax). The milky white opaque fluid contains mostly lymphocytes accompanied by variable numbers of neutrophils, depending on the duration of the effusion and the extent of the resulting pleuritis (see Table 164-2). Chylous effusions are confirmed by the presence of chylomicrons, which can be demonstrated by the ether clearance test, by the presence of a cream layer on refrigeration, and microscopically by the presence of Sudan-positive orange fat droplets, but these are relatively insensitive methods.




Key Point


The most reliable test for chylous effusion is the comparison of the triglyceride and cholesterol concentrations measured simultaneously in the fluid and serum. Chylous effusion is characterized by a high triglyceride concentration (usually >300 mg/dl) compared with that of serum and a pleural fluid cholesterol-to-triglyceride ratio of less than 1. The fluid cholesterol concentration is less than or equal to that of serum.



Hemorrhagic Effusion


Hemorrhage into the pleural space may be associated with trauma, hemostatic abnormalities, or neoplasia. The physicochemical and cytologic features of the effusion are similar to those of defibrinated peripheral blood. The presence of erythrophagocytosis by macrophages confirms that the hemorrhage is not merely the result of a traumatic collection.



Laboratory Evaluations


Various laboratory evaluations may provide useful diagnostic information depending on the suspected underlying cause of pleural effusion.


• The complete blood count may reveal a neutrophilia with a left shift in pyothorax, FIP, lung lobe torsion, and neoplasia. Lymphopenia is common in animals with chylothorax, but it can also be a nonspecific manifestation of stress. Anemia and various other hematologic abnormalities may be associated with lymphoma and feline leukemia virus (FeLV).

• Serum protein determinations may reveal hypoalbuminemia, a cause of transudation, or hyperglobulinemia, a common finding in FIP and other conditions of chronic immune stimulation. Severe proteinuria would be expected on the urinalysis of a dog or cat with hypoalbuminemia and transudation associated with nephrotic syndrome.

Related posts:

  1. Feline Infectious Peritonitis (Feline Coronavirus)
  2. Borreliosis (Lyme Disease)
  3. Immune-Mediated Dermatoses
  4. Otitis Media and Otitis Interna

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Aug 27, 2016 | Posted by in SMALL ANIMAL | Comments Off on Pleural Effusion

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