Canine Pleuritis Study Guide

Overview and Clinical Importance

Pleuritis (inflammation of the pleura) and pleural effusion (abnormal accumulation of fluid in the pleural space) represent significant clinical conditions in canine medicine. Understanding the pathophysiology, diagnostic approach, and treatment of these conditions is essential for NAVLE success and clinical practice. The pleural space is the potential space between the visceral pleura (covering the lungs) and parietal pleura (lining the thoracic wall), normally containing only a small amount of serous fluid for lubrication during respiration.

Pleural disease is always considered a medical emergency when it causes respiratory distress, as fluid or inflammatory exudate accumulation compresses the lungs and significantly impairs ventilation. Prompt recognition and treatment are critical for patient survival.

Pleural Anatomy and Physiology

The pleura is a serous membrane consisting of a single layer of mesothelial cells supported by connective tissue. It forms a closed sac around each lung.

Key Anatomical Components

• Visceral pleura: Covers the lung parenchyma and dips into the fissures between lung lobes; receives blood supply from bronchial arteries; lacks sensory innervation for pain
• Parietal pleura: Lines the thoracic wall, diaphragm, and mediastinum; receives blood supply from intercostal arteries; innervated by intercostal and phrenic nerves (pain-sensitive)
• Pleural cavity: Potential space between visceral and parietal pleura; normally contains less than 10 mL of serous fluid in dogs
• Mediastinum: In dogs, the mediastinum has fenestrations allowing communication between right and left pleural cavities (most effusions are bilateral)

Classification of Pleural Effusions

Pleural effusions are classified based on their physical characteristics, protein content, and cellularity. This classification helps narrow the differential diagnosis list.

Clinical Signs and Physical Examination

Presenting Signs

Dogs with pleural effusion or pleuritis typically present with signs of respiratory compromise that may range from subtle to life-threatening:

• Tachypnea and dyspnea: Rapid, shallow breathing pattern (restrictive pattern); normal respiratory rate at rest is less than 30 breaths per minute
• Orthopnea: Difficulty breathing while lying down; dogs may stand or sit with elbows abducted and neck extended
• Open-mouth breathing: Indicates severe respiratory distress
• Cyanosis: Blue-tinged mucous membranes indicating hypoxemia (medical emergency)
• Exercise intolerance: May be noted before overt respiratory distress develops
• Coughing: Less common than with pulmonary parenchymal disease
• Systemic signs: Lethargy, anorexia, weight loss, fever (especially with pyothorax)

Physical Examination Findings

• Muffled heart sounds: Fluid between lungs and chest wall attenuates sound transmission
• Decreased lung sounds: Especially ventrally where fluid accumulates
• Dull percussion: Ventrally (fluid) versus hyperresonant dorsally (pneumothorax)
• Paradoxical abdominal breathing: Abdominal muscles move inward during inspiration as the diaphragm cannot descend normally

Diagnostic Approach

Thoracic Radiography

Radiography is the primary imaging modality for identifying pleural effusion. Three-view thoracic radiographs are the standard of care.

Radiographic Signs of Pleural Effusion

• Widened pleural fissure lines (widest laterally, thinning toward the hilum)
• Retraction of lung lobes from the thoracic wall
• Silhouetting (border effacement) of cardiac silhouette and diaphragm
• Scalloped or leaf-like appearance of lung lobe margins
• Blunting of costophrenic angles on VD/DV views
• Dorsal displacement of trachea (severe effusions)

Thoracic Ultrasound (TFAST)

Point-of-care ultrasound is increasingly used for rapid detection of pleural effusion and can be performed with minimal stress to dyspneic patients.

• Highly sensitive for detecting small volumes of effusion
• Echogenic fluid suggests exudate or hemorrhage
• Can guide thoracocentesis for safer needle placement
• Loss of glide sign indicates pneumothorax

Thoracocentesis: Technique and Interpretation

Thoracocentesis is both diagnostic and therapeutic. It should be performed immediately in any dyspneic patient suspected of having pleural space disease.

Technique

• Position patient in sternal recumbency or standing (allows fluid to pool ventrally)
• Clip and aseptically prepare the 7th-9th intercostal space at the costochondral junction
• Insert needle CRANIAL to the rib (vessels and nerves run caudal to each rib)
• Use butterfly catheter or over-the-needle catheter connected to extension tubing, three-way stopcock, and syringe
• Advance at 45-degree angle until fluid is obtained (feel a pop entering pleural space)
• Aspirate as much fluid as possible; may need to tap both sides
• Submit samples for cytology and culture (aerobic and anaerobic)

Equipment Selection

Pyothorax (Septic Pleuritis)

Pyothorax is a bacterial infection of the pleural space characterized by accumulation of purulent exudate. It is a life-threatening condition requiring aggressive treatment.

Etiology and Pathophysiology

• Migrating foreign bodies: Most commonly identified cause in dogs (grass awns); hunting and working breeds overrepresented
• Penetrating wounds: Bite wounds, thoracic trauma, iatrogenic (post-surgical)
• Extension from pneumonia: Parapneumonic effusion
• Esophageal perforation: Foreign body, endoscopic injury, stricture dilation
• Idiopathic: Cause identified in only 2-22% of canine cases

Bacterial Organisms

Diagnosis

• Fluid analysis: Septic suppurative exudate with degenerative neutrophils, intracellular bacteria
• Cytology: Toxic neutrophils with karyolysis, karyorrhexis; bacteria may be seen intra- or extracellularly
• Culture: Aerobic and anaerobic cultures essential; negative culture does not rule out pyothorax
• Blood work: Inflammatory leukogram, neutrophilia with left shift, toxic changes

Treatment

Prognosis

• Overall survival rate approximately 83% in dogs with appropriate treatment
• Actinomyces infections have higher recurrence rates and require prolonged antibiotic therapy (greater than 6 weeks)
• Fibrosing pleuritis is the most serious complication, preventing lung expansion

Memory Aid - PYOTHORAX = P.U.S.: Polymicrobial (mixed aerobes/anaerobes), Urgent drainage needed (chest tubes), Six weeks minimum antibiotics. Remember: Springer Spaniels and Labrador Retrievers are overrepresented due to their outdoor/hunting activities and exposure to plant material.

Chylothorax

Chylothorax is the accumulation of chyle (lymphatic fluid from the intestines) in the pleural space due to leakage or obstruction of the thoracic duct.

Etiology

• Idiopathic: Most common in dogs
• Cardiac disease: Right-sided heart failure, pericardial disease, heartworm disease
• Thoracic lymphangiectasia: Dilation and dysfunction of lymphatic vessels
• Neoplasia: Lymphoma, thymoma causing obstruction
• Trauma: Ruptured thoracic duct (often heals spontaneously)
• Lung lobe torsion: May cause or result from chylothorax

Diagnosis

• Gross appearance: Milky white, opaque fluid (may be pink to red if hemorrhagic; remains opaque after centrifugation)
• Cytology: Predominantly small lymphocytes (chronic cases may have more neutrophils)
• Triglyceride comparison: Fluid triglyceride greater than serum triglyceride (ratio greater than 2:1 is diagnostic)
• Cholesterol comparison: Fluid cholesterol less than serum cholesterol (opposite of pseudochylous effusion)

Treatment

Hemothorax

Hemothorax is the accumulation of blood in the pleural space.

Etiology

• Anticoagulant rodenticide toxicosis: Most common cause in dogs (brodifacoum, bromadiolone)
• Trauma: Blunt or penetrating thoracic injury
• Neoplasia: Hemangiosarcoma, other bleeding tumors
• Coagulopathies: Thrombocytopenia, DIC, liver failure, hemophilia

Diagnosis

• Fluid PCV within 5-10% of peripheral blood PCV
• Fluid total protein within 10-20% of serum protein
• Fresh blood may clot; older hemorrhage undergoes fibrinolysis and will not clot
• Coagulation panel (PT/PTT) essential to evaluate for coagulopathy

Treatment

• Anticoagulant rodenticide: Vitamin K1 therapy (2.5-5 mg/kg PO divided BID for 4-6 weeks for second-generation anticoagulants)
• Autotransfusion: Collected pleural blood can be administered back to the patient if fresh and uncontaminated
• Blood products: Fresh frozen plasma, whole blood transfusion as needed
• Small volume hemothorax: May be left to reabsorb if not causing respiratory compromise

Complications of Pleural Disease

Fibrosing (Constrictive) Pleuritis

The most serious long-term complication of chronic pleural effusion, particularly pyothorax and chylothorax. Chronic inflammation causes thickening and fibrosis of the visceral pleura, preventing normal lung expansion even after fluid removal.

• Radiographic signs: Rounded, attenuated lung lobes that fail to expand post-thoracocentesis
• Treatment: Decortication (surgical removal of fibrous peel) may be attempted but carries guarded prognosis

Lung Lobe Torsion

Rotation of a lung lobe on its bronchus and vasculature, causing venous congestion and eventual necrosis. Can cause or result from pleural effusion.

• Breeds predisposed: Afghan Hound, Borzoi, deep-chested breeds; right middle lobe most commonly affected
• Radiographic sign: Vesicular or granular pattern within affected lobe, lobe fails to collapse normally
• Treatment: Surgical lung lobectomy

Summary: Differentiating Pleural Effusions