CONDITION
Portosystemic Shunt
A portosystemic shunt is an abnormal blood vessel that allows blood from the intestines and other abdominal organs to bypass the liver. Normally, this blood travels through the liver first, where toxins are filtered and nutrients are processed. When a shunt is present, unfiltered blood flows directly into the general circulation, and the liver itself may remain underdeveloped. Owners often arrive at this page because their young dog or cat has been slow to grow, seems unusually quiet or uncoordinated after meals, or has had episodes of confusion, drooling, or apparent disorientation. Some animals show digestive signs (vomiting, diarrhoea, or poor appetite), while others may have urinary tract problems or subtle behavioural changes that come and go. The pattern can be vague at first, and the pieces may only connect over time. This page explores the signals that can accompany a portosystemic shunt, the mechanisms that produce those signals, how the condition is investigated through blood tests and imaging, and the medical and surgical approaches that exist. The goal is to help you understand what may be happening and what shape the pathway forward can take.
Why this matters now
Congenital portosystemic shunts are present from birth and often become apparent in young animals, typically within the first year of life. Certain breeds are predisposed, including Yorkshire Terriers, Maltese, Pugs, and Irish Wolfhounds in dogs, and Himalayan and Persian cats. Acquired shunts can develop at any age, usually secondary to liver disease. Signs may be subtle initially and often fluctuate, sometimes leading to delayed recognition.
Clinical signs typically develop gradually and may wax and wane, often relating to meals. The accumulation of toxins, particularly ammonia, affects brain function, causing a range of neurological signs from subtle behavioural changes to more obvious symptoms. Affected animals often fail to thrive, remaining smaller than littermates. Without management, episodes may become more frequent and severe, and hepatic encephalopathy can progress to collapse, seizures and coma.
Signals & patterns
Early signals
Smaller size compared to littermates
Reduced liver function and nutrient processing often results in poor growth and smaller body size.
Intermittent lethargy or dullness
Episodes of unusual tiredness, particularly after meals, as toxin levels fluctuate.
Excessive drinking and urination
Altered kidney function and reduced ability to concentrate urine lead to increased water intake and output.
Prolonged recovery from sedation or anaesthesia
The liver's reduced ability to metabolise drugs results in prolonged drug effects.
Later signals
Obvious neurological signs
Disorientation, aimless wandering, circling, head pressing, or apparent blindness, particularly after eating.
Seizures
Severe toxin accumulation can trigger seizure activity. Hepatic encephalopathy runs from dullness, through circling and head pressing, to collapse, seizures and coma. Seizure activity lasting beyond 5 minutes is defined as status epilepticus, which carries a reported mortality of 25.3% to 38.5% across all causes. How far an episode will progress cannot be judged at home.
Drooling
Excessive salivation, particularly in cats, may occur during neurological episodes.
Urinary problems
Urate bladder stones are common and can cause straining or blood in the urine. A stone can also lodge in the urethra. Straining that produces little or no urine can be mistaken for constipation, and whether the urethra is blocked cannot be judged at home. With complete blockage, vomiting, dehydration and severe dullness develop rapidly, and high blood potassium can cause dangerous heart rhythm changes.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis involves blood tests to detect signs of reduced liver function, followed by imaging to identify the abnormal vessel. Thorough evaluation helps plan management.
Blood tests including bile acids
Blood ammonia levels
Abdominal ultrasound
CT angiography
Urinalysis
Options & trade-offs
Management can be medical, surgical, or a combination. The approach depends on shunt type, severity, and individual circumstances. Many animals can lead good quality lives with appropriate management.
Dietary management
Feeding a moderately protein-restricted, highly digestible diet to reduce ammonia production.
Trade-offs: Forms the foundation of medical management. Requires dietary compliance. May need adjustment based on response.
Medications to reduce toxin absorption
Lactulose and antibiotics to trap ammonia in the gut and reduce ammonia-producing bacteria. Lactulose is UK-authorised for dogs and cats for constipation. Its use to lower ammonia, like that of the antibiotics chosen, is outside the authorised indications (off-licence, under the veterinary medicines cascade).
Trade-offs: Can significantly reduce clinical signs. Requires ongoing medication. Dose often needs adjustment.
Surgical correction
Procedures to gradually close the shunt vessel, redirecting blood flow through the liver.
Trade-offs: Can be curative for congenital single shunts. Requires specialised expertise. The liver must develop capacity to handle blood flow, which may take time. Seizures in the first 7 days after surgery were reported in 75 of 940 dogs (8%) in one multi-centre study, and in a separate series only 30 of 93 dogs (32.3%) that developed them survived to 30 days. Rates of about 20% to 30% are reported in cats. A sudden rise in portal vein pressure after closure is a further recognised complication and is often fatal.
Interventional radiology
Minimally invasive placement of devices to close the shunt from within the blood vessel.
Trade-offs: Alternative to open surgery. Requires specialised facilities and expertise. May be appropriate for certain shunt types.
Long-term medical management
Continued dietary and medical therapy without surgical intervention.
Trade-offs: May be chosen when surgery is not feasible or preferred. Many animals maintain reasonable quality of life, although survival in dogs with a congenital shunt has been shorter than after surgery. Requires commitment to ongoing management.
Common misconceptions
"Neurological signs are always severe and constant"
Signs often fluctuate, ranging from subtle behavioural changes to more obvious episodes. They may worsen after eating and improve between meals.
"Surgery is always necessary"
Surgery is not always possible or chosen, and diet and medication alone can give long survival: one study of 78 dogs with a congenital shunt managed without surgery reported an estimated median survival of 38.5 months. Outcomes are not equal, though. In a prospective study of 124 dogs with a congenital shunt, 24 of 27 medically managed dogs died or were euthanased during follow-up (median 1,936 days), compared with 21 of 97 treated surgically. Acquired shunts are almost always multiple vessels and are not closed surgically, so medical management is the only option for them.
"Affected animals cannot live good quality lives"
With appropriate management, whether medical or surgical, many animals with portosystemic shunts can live comfortable lives, though monitoring remains important.
Related conditions
Hepatic Encephalopathy
Hepatic encephalopathy describes the neurological signs that can arise when toxins bypass or overwhelm the liver's filtering capacity. In animals with a portosystemic shunt, blood carrying ammonia and other metabolic waste flows directly into the circulation without being processed, and this is one of the principal mechanisms by which the neurological signs (confusion, disorientation and drooling) develop.
The page on this condition is being checked again and is offline for now.
Chronic Hepatitis in Dogs
Chronic hepatitis involves ongoing inflammation in the liver itself, and can also lead to reduced liver function over time. While the underlying cause differs, both conditions may present with similar patterns of vague ill health, digestive upset, or behavioural changes, and both can require careful investigation to distinguish one from the other.
Urinary Stones (Urolithiasis)
Dogs and cats with portosystemic shunts can be predisposed to forming certain types of urinary stones, particularly ammonium urate (also called ammonium biurate) stones, because the unfiltered blood carries higher levels of ammonia and uric acid. This shared urinary finding can sometimes be an early clue that prompts further investigation of liver function.
Hepatic Lipidosis
Hepatic lipidosis in cats describes fat accumulation in the liver that impairs its function, often following a period of reduced food intake. While the mechanism differs from a portosystemic shunt, both conditions can present with similar signs (loss of appetite, weight loss and lethargy), and both require blood tests and imaging to distinguish the underlying cause.
Cholangiohepatitis in Cats
Cholangiohepatitis involves inflammation of the bile ducts and liver tissue in cats, and can lead to impaired liver function over time. Both this condition and portosystemic shunt may produce vague signs such as poor appetite, vomiting, and weight loss, and the distinction often rests on careful blood work, imaging, and sometimes biopsy.
Noting whether signs seem to follow any pattern, such as worsening after meals, can provide important diagnostic clues. Withholding food to test the pattern is not a safe home check: low blood sugar, though rare, is reported mainly in young, very small toy-breed dogs with shunts. Documenting the specific behaviours observed during episodes helps distinguish neurological signs from other causes. Keeping track of water intake and urination patterns may also be useful for veterinary discussions.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS