What Is a Portosystemic Shunt?
In a healthy dog, blood from the gastrointestinal tract travels through the portal vein to the liver, where it is detoxified before returning to the general circulation. A portosystemic shunt (PSS) is an abnormal blood vessel that bypasses this route entirely, allowing portal blood — laden with ammonia, toxins, and digestive by-products — to enter the systemic circulation without being filtered by the liver.
The consequences of this bypass are far-reaching. The liver, starved of the blood flow and growth factors it depends on, fails to develop or function normally. The brain, exposed to substances it would not ordinarily encounter in significant concentrations, is particularly susceptible to the toxic effects. The result is a condition that can appear neurological in nature but is fundamentally hepatic in origin.
Congenital Versus Acquired Shunts
Shunts are broadly classified as congenital or acquired, and as intrahepatic or extrahepatic depending on where the abnormal vessel is located.
Congenital Shunts
Congenital portosystemic shunts are present from birth and account for the majority of cases seen in dogs. They arise when foetal blood vessels that should close at or shortly after birth fail to do so, or when a developmental anomaly creates an abnormal connection between the portal and systemic venous systems.
Extrahepatic shunts, where the aberrant vessel lies outside the liver, are more common in small and toy breeds. Intrahepatic shunts, where the vessel persists within the liver parenchyma, are more typical in large breeds. The distinction matters for surgical planning.
Acquired Shunts
Acquired shunts develop secondary to portal hypertension — most often as a result of chronic liver disease causing increased resistance to portal blood flow. Multiple small vessels open up to decompress the portal system. These are generally more difficult to treat and carry a less favourable prognosis than congenital single shunts.
Breeds Most Commonly Affected
Congenital portosystemic shunts have a clear breed predisposition, suggesting a hereditary component in many cases. Breeds with elevated risk include:
- Yorkshire Terriers — one of the most frequently affected small breeds
- Maltese
- Miniature Schnauzers
- Pugs
- Shih Tzus
- Irish Wolfhounds — predisposed to intrahepatic shunts
- Labrador Retrievers
- Golden Retrievers
Recognising the Signs
Clinical signs typically appear in puppies or young adults, often noticed when a dog seems smaller or slower to develop than its littermates. The neurological manifestations are frequently the most striking and the reason owners seek veterinary attention.
Neurological Signs (Hepatic Encephalopathy)
- Disorientation and aimless wandering
- Head pressing against walls or furniture
- Abnormal vocalisation
- Circling behaviours
- Blindness (which may be transient)
- Seizures
- Stupor or coma in severe episodes
Crucially, these signs often fluctuate and may worsen after meals — particularly high-protein meals — because the digestive process increases ammonia production. Some owners notice their dog seems normal in the morning but abnormal an hour after eating.
Other Common Signs
- Poor growth and small body size
- Excessive thirst and urination
- Recurrent urinary tract issues or struvite/ammonium urate bladder stones
- Vomiting and drooling, particularly after eating
- Reduced tolerance to sedative medications
The reduced drug tolerance is clinically important. Dogs with PSS cannot metabolise drugs normally, meaning standard anaesthetic doses may produce dangerously prolonged effects. Any dog of a predisposed breed should be flagged for PSS assessment before elective procedures.
Diagnosis
Routine blood work often shows a small liver (microhepatica) on imaging, low blood urea nitrogen (BUN), mildly elevated liver enzymes, and low albumin — a pattern that, while not diagnostic alone, is highly suggestive in a young animal. Ammonium urate crystals in urine sediment provide additional support.
Bile acid stimulation testing — measuring pre- and post-prandial bile acid levels — is a sensitive functional test that is usually markedly abnormal in PSS. A fasting blood ammonia level is also informative.
Definitive identification of the shunt vessel requires imaging. Abdominal ultrasound in experienced hands can often locate the vessel. CT angiography has become the gold standard at specialist centres, providing detailed anatomical information essential for surgical planning.
Medical Management
Medical therapy aims to reduce the clinical impact of the shunt, particularly hepatic encephalopathy, while the dog is stabilised prior to surgery or in cases where surgery is not an option.
Dietary Modification
A reduced-protein diet using highly digestible, lower-ammoniagenic protein sources is the primary dietary intervention. The goal is not to eliminate protein — which would cause muscle wasting — but to reduce the ammonia load reaching the systemic circulation. Frequent small meals reduce postprandial ammonia spikes. Many veterinary hepatic diets are formulated with these principles in mind.
Lactulose
Lactulose is an osmotic laxative that acidifies the colon, trapping ammonia as ammonium ions and reducing its absorption. It also accelerates intestinal transit, further decreasing the time available for ammonia production. It is one of the most consistently useful medications in PSS management.
Antibiotics
Oral antibiotics such as metronidazole or neomycin reduce the population of ammonia-producing bacteria in the gut. They are typically used intermittently or during acute encephalopathic episodes rather than as long-term continuous therapy.
Surgical Treatment
For congenital single extrahepatic shunts, surgical attenuation is the treatment of choice and offers the best chance of a normal life. The goal is to progressively reduce blood flow through the shunt, allowing the liver to adapt to increased portal blood flow over time.
Several techniques exist, including ameroid constrictors and hydraulic occluders, which allow gradual rather than acute closure — reducing the risk of portal hypertension developing too rapidly. Complete or near-complete attenuation results in significantly improved quality of life and, in many cases, resolution of clinical signs.
Intrahepatic shunts carry greater surgical risk due to their anatomical position, and outcomes are somewhat less predictable. Dogs with multiple acquired shunts secondary to portal hypertension are generally not candidates for shunt ligation.
Long-Term Outlook
Dogs that undergo successful surgical correction often go on to lead normal lives, with many no longer requiring lifelong dietary restriction or medication. Post-operative monitoring is essential, including repeat bile acid testing several months after surgery. Some dogs require ongoing management even after attenuation if residual shunting persists or if the liver does not fully recover its function.
The earlier a shunt is identified and treated, the better the potential outcome — which underscores the value of awareness among owners and breeders of predisposed breeds.