Canine Thrombotic Disease Study Guide

Overview and Clinical Importance

Thrombotic disease in dogs represents a significant clinical challenge and is an important topic for the NAVLE examination. Unlike cats, where aortic thromboembolism (ATE) is predominantly cardiogenic in origin, canine thrombotic disease is typically associated with systemic hypercoagulable states and involves in situ thrombus formation rather than embolization from cardiac sources. This fundamental difference in pathophysiology impacts presentation, diagnosis, and management strategies.

Aortic thrombosis (ATh) and pulmonary thromboembolism (PTE) represent the two most clinically significant single-site thrombotic conditions in dogs. Understanding the underlying pathophysiology, recognizing clinical presentations, and implementing appropriate diagnostic and therapeutic strategies are essential competencies for veterinary practitioners.

Pathophysiology of Thrombosis

Virchow's Triad

The foundation of understanding thrombotic disease lies in Virchow's triad, which describes the three primary factors that predispose to pathological thrombus formation:

Key Concept: The simultaneous presence of two or more of these factors significantly increases thrombotic risk. In dogs with protein-losing nephropathy, for example, hypercoagulability (from antithrombin loss) combines with potential endothelial dysfunction to create a highly prothrombotic state.

Hypercoagulable Conditions in Dogs

Protein-Losing Nephropathy (PLN)

PLN is the most common underlying condition associated with canine aortic thrombosis, accounting for approximately 32% of cases. The mechanism involves:

• Urinary loss of antithrombin III (ATIII): ATIII has similar molecular size to albumin and is lost in urine when glomerular filtration is compromised
• Platelet hyperresponsiveness: Secondary to hypoalbuminemia
• Increased procoagulant factors: Some clotting factors are retained while inhibitors are lost
• Hyperfibrinogenemia: Compensatory increase in fibrinogen production

Conditions Associated with Hypercoagulability

Aortic Thrombosis (ATh) in Dogs

Definition and Pathogenesis

Aortic thrombosis (ATh) in dogs refers to the formation of a thrombus in situ within the distal aorta, typically at or proximal to the aortic trifurcation. This is distinct from aortic thromboembolism (ATE), which involves migration of a thrombus from a distant site (usually cardiac).

Canine ATh vs. Feline ATE: Critical Differences

Exam Focus: A key NAVLE concept is distinguishing chronic, insidious pelvic limb dysfunction in dogs (think ATh from hypercoagulable state) from acute paralysis with severe pain in cats (think ATE from cardiomyopathy). When you see a middle-aged to older dog with progressive hindlimb weakness and weak (but present) femoral pulses, consider aortic thrombosis and investigate for underlying PLN, neoplasia, or endocrinopathy.

Signalment and Risk Factors

• Age: Median age 10 years (range 2-17 years); middle-aged to older dogs
• Sex: Slight male predominance
• Size: Mid to large breed dogs most commonly affected; median weight 24.7-26.7 kg

Breed Predispositions

• Greyhounds: Overrepresented; possibly due to large referral population or breed predisposition
• Cavalier King Charles Spaniels: Reported predisposition
• Soft-coated Wheaten Terriers: Association with familial PLN
• Shetland Sheepdogs: Reported in some studies

Clinical Signs of Aortic Thrombosis

Presenting Complaints

• Lameness or hindlimb weakness (most common: approximately 80%)
• Exercise intolerance
• Progressive pelvic limb dysfunction over weeks to months
• Perceived pain (approximately 50%)
• Acute paralysis (less common than in cats)

Physical Examination Findings

Diagnostic Approach

Laboratory Findings

Diagnostic Imaging

Abdominal Ultrasound (Gold Standard for ATh)

Abdominal ultrasonography is the primary diagnostic modality for aortic thrombosis, used in approximately 91% of diagnosed cases.

• Findings: Intraluminal or mural mass in distal abdominal aorta; echogenic material within vessel lumen
• Location: Typically at or proximal to the aortic trifurcation (iliac bifurcation)
• Doppler: Color flow Doppler assesses blood flow past the thrombus; may show turbulence or absent flow
• Additional findings: Evaluate kidneys for PLN changes, adrenals for HAC, check for masses

Echocardiography

• Should be performed if cardiac murmur or arrhythmia detected
• May identify intracardiac thrombi, endocarditis, or structural heart disease
• Spontaneous echo contrast ("smoke") suggests blood stasis
• Primary cardiac disease is RARELY the sole cause of ATh in dogs (unlike cats)

Advanced Imaging

• CT Angiography: Useful for visualizing thrombus extent; filling defects within contrast-enhanced vessel
• MRI: Can detect filling defects in distal aorta; useful for concurrent CNS evaluation

Pulmonary Thromboembolism (PTE) in Dogs

Definition and Pathogenesis

Pulmonary thromboembolism (PTE) is the occlusion of pulmonary vasculature by products of the coagulation cascade, resulting in ventilation-perfusion mismatch, hypoxemia, and potentially life-threatening cardiopulmonary compromise.

Pathophysiologic Consequences

• Ventilation-perfusion mismatch: Perfused alveoli cannot be ventilated, leading to hypoxemia and hypocapnia
• Pulmonary arterial hypertension: Massive embolism and reflex vasoconstriction increase pulmonary vascular resistance
• Right heart failure (cor pulmonale): Increased afterload leads to RV dilation and dysfunction
• Bronchoconstriction: Vagal stimulation and mediators from thrombus cause increased airway resistance
• Loss of surfactant: Ischemia damages type II pneumocytes, leading to atelectasis

Clinical Signs of PTE

Clinical signs are nonspecific and variable, making diagnosis challenging. Signs depend on the extent of vascular occlusion and underlying disease.

Diagnosis of PTE

There is no gold standard for PTE diagnosis in veterinary medicine. Diagnosis is based on clinical suspicion, risk factor assessment, and exclusion of other causes.

Diagnostic Findings

Treatment of Thrombotic Disease

Treatment Principles

• Supportive care: Analgesia, fluid therapy, oxygen supplementation (for PTE)
• Treat underlying cause: Essential for preventing recurrence
• Antithrombotic therapy: Anticoagulants and/or antiplatelet drugs
• Thrombolytic therapy: Reserved for severe, life-threatening cases

Antithrombotic Drug Options

Prognosis

Aortic Thrombosis

• Overall survival to discharge: 50-60%
• Ambulatory dogs: Better prognosis than nonambulatory
• Chronic presentation: Better than acute presentation
• Long-term survival possible with appropriate therapy; warfarin-treated dogs showed improvement

Pulmonary Thromboembolism

• Prognosis is generally poor
• Depends on extent of cardiopulmonary compromise and underlying disease
• Early recognition and treatment improves outcomes