Canine Congenital Portosystemic Shunts (CPSS)

Congenital portosystemic shunts are vascular anomalies that allow portal blood to bypass the liver, leading to hepatic insufficiency and accumulation of toxins such as ammonia. Affected dogs, often small or toy breeds (e.g., Yorkshire Terriers), show signs of hepatic encephalopathy, poor growth, vomiting, and urinary issues due to ammonium biurate uroliths. Diagnosis involves bile acids testing, ammonia levels, and advanced imaging (ultrasound, CT angiography).

Definition

Congenital Portosystemic Shunts (CPSS): Vascular anomalies connecting the portal vein to systemic circulation, bypassing the liver, leading to hepatic dysfunction and systemic toxin accumulation.

Subtypes

Extrahepatic Portosystemic Shunts (EHPSS)
- Predominantly affect small dog breeds (e.g., Yorkshire Terrier, Cairn Terrier, Maltese).
Intrahepatic Portosystemic Shunts (IHPSS)
- Primarily occur in large breed dogs (e.g., Irish Wolfhound, Golden Retriever).

Etiology

Genetic Basis: Evidence of hereditary components, especially in predisposed breeds. Complex modes of inheritance are suspected, possibly polygenic.
Embryonic Development:
- EHPSS: Result from abnormal development of vitelline veins, leading to functional shunts bypassing the liver.
- IHPSS: Often associated with the persistence of the ductus venosus, a fetal structure that normally closes after birth.

Pathophysiology

Hepatic Dysfunction: Bypass of portal blood flow results in reduced liver perfusion, leading to liver atrophy, decreased metabolic function, and toxin accumulation.
Hepatic Encephalopathy (HE): Toxins (e.g., ammonia, aromatic amino acids) bypass the liver and reach the brain, leading to neurological signs.
Histological Findings: Liver atrophy, portal vein hypoplasia, biliary hyperplasia, sinusoidal dilatation, and hepatic lipidosis are common findings.

Clinical Signs

Neurological Signs: Hepatic encephalopathy, seizures, disorientation, and behavioral changes.
Gastrointestinal Signs: Vomiting, diarrhea, and poor weight gain.
Urinary Signs: Polyuria, polydipsia, and ammonium biurate crystalluria.
Other Signs: Growth retardation, lethargy, and prolonged recovery from anesthesia.

Diagnostics

Blood Tests: Elevated bile acids, hyperammonemia, and abnormal liver enzymes.
Imaging:
- Ultrasound: Identification of shunts and liver atrophy.
- Computed Tomography Angiography (CTA): Precise mapping of shunt anatomy.
Liver Biopsy: Confirms liver histopathological changes associated with CPSS.
Bile Acid Stimulation Test: Elevated postprandial bile acids are indicative of CPSS.

Treatment

Surgical Options:
- Partial Ligation: Gradual closure of the shunt to avoid portal hypertension.
- Ameroid Ring Constrictors: Gradually occlude the shunt over time.
- Cellophane Banding: A less invasive method that also leads to gradual shunt closure.
- Thrombogenic Coils: Intravascular method to promote gradual occlusion.
Medical Management:
- Dietary Adjustments: High-quality, low-protein diets to reduce ammonia production.
- Lactulose: Reduces ammonia absorption from the gut.
- Antibiotics: Metronidazole or ampicillin to reduce ammonia-producing bacteria.

Prognosis

Surgical: Higher survival and quality of life compared to medical management alone.
Medical: Supportive therapy but does not correct the underlying condition, with variable long-term outcomes.

Breed-Specific Considerations

IHPSS: Most commonly found in large breeds like Irish Wolfhounds and Labrador Retrievers, with a suspected genetic predisposition.
EHPSS: More prevalent in small breeds like Yorkshire Terriers, Cairn Terriers, and Maltese, indicating a hereditary component.

Complications

Portal Hypertension: A risk with complete shunt ligation.
Recurrence of Symptoms: May occur if shunt occlusion is incomplete or if new shunts form.
Postoperative Complications: Include seizures, ascites, and delayed recovery.