Portosystemic shunt

What it is

Normally the portal vein carries blood from the stomach, intestines, pancreas and spleen to the liver, which metabolises ammonia and other substances absorbed from the gut. A portosystemic shunt (PSS) is a vessel that connects the portal system to the caudal vena cava or the azygos vein, so that portal blood skips the liver. The liver receives less portal flow and remains small (atrophy), and ammonia and other toxins accumulate in the blood.

• Congenital shunts are usually single. Intrahepatic shunts (often a patent ductus venosus) are typical of large breeds; extrahepatic shunts (for example portocaval or portoazygos) are typical of small and toy breeds.
• Acquired shunts are multiple, small extrahepatic vessels that open secondarily in response to portal hypertension, for example in cirrhosis.

Who it affects

Congenital shunts usually cause signs in young dogs, often in the first year. Extrahepatic shunts: Yorkshire terrier, Maltese, Havanese, miniature schnauzer, pug, shih tzu and other small breeds. Intrahepatic shunts: Irish wolfhound, Labrador and golden retrievers, Australian cattle dog, Old English sheepdog. Acquired shunts appear in adults with chronic liver disease.

Anatomy involved

• Liver: small, with hypoplasia of the intrahepatic portal branches (microhepatia).
• Portal vein and caudal vena cava: the anomalous vessel joins the portal system or one of its tributaries (left gastric, splenic, gastroduodenal veins) to the caudal vena cava or the azygos vein.
• Brain: ammonia and other toxins cause hepatic encephalopathy.
• Kidneys and urinary bladder: hyperammonaemia and hyperuricaemia lead to ammonium urate crystals and uroliths, with renomegaly in some dogs.

Clinical signs

Stunted growth and poor body condition; neurological signs of hepatic encephalopathy that often worsen after eating a protein-rich meal: dullness, disorientation, head pressing, circling, apparent blindness, ataxia and seizures. Prolonged recovery from sedation or anaesthesia. Urinary signs from urate stones: haematuria, pollakiuria, stranguria or obstruction. Vomiting, diarrhoea, polyuria and polydipsia.

Diagnosis

• Blood work: microcytosis, low urea, low albumin, low cholesterol and glucose, raised ammonia.
• Liver function tests: increased fasting and postprandial bile acids or a raised ammonia (fasting or after an ammonia tolerance test).
• Urinalysis: ammonium biurate crystals.
• Imaging: abdominal ultrasound with Doppler identifies many shunts; CT angiography is the reference method to map the vessel and plan surgery. Portovenography is used intraoperatively.

Treatment

Medical management stabilises the patient and controls encephalopathy: a moderately protein-restricted, highly digestible diet, lactulose (which acidifies the colon and traps ammonia), and antibiotics such as metronidazole or neomycin to reduce ammonia-producing bacteria; seizures are treated with anticonvulsants. Single congenital shunts are ideally attenuated surgically, gradually, with an ameroid constrictor or cellophane band (extrahepatic) or by percutaneous transvenous coil embolisation (intrahepatic). Gradual closure prevents acute portal hypertension. Acquired shunts are not closed; the underlying liver disease is managed.

In the cat

Congenital portosystemic shunts are less common in the cat than in the dog. They are usually single and extrahepatic, and mainly affect domestic shorthair cats, although the Persian, Himalayan, Siamese and Burmese are represented. Signs appear in kittens and young cats, and some are characteristic of the species:

• Intermittent ptyalism (excessive salivation), very typical of the cat with hepatic encephalopathy.
• Copper-coloured irises in cats without blue or green eyes.
• Neurological signs (dullness, ataxia, apparent blindness, behavioural changes, seizures), stunted growth and slow recovery from anaesthesia.
• Ammonium urate uroliths, with haematuria or urethral obstruction in males.

Diagnosis (bile acids, ammonia, Doppler ultrasound, CT angiography) and initial medical management are similar to those in the dog. Gradual surgical attenuation or ligation of the anomalous vessel is the treatment of choice, but in the cat postoperative neurological complications, including seizures, are more frequent than in the dog and worsen the prognosis; even so, many cats that undergo surgery do well in the long term.

Key studies in the cat

• Weisse et al. (2024), CT angiography of 231 cats with congenital extrahepatic shunts from 29 institutions. Twenty-five different anatomies were found, five of which accounted for 78% of shunts, and the left gastric vein was involved in the origin of the shunt in 75% of the classified types.
• Lipscomb et al. (2007), 49 cats with surgical ligation of a congenital shunt. Two died perioperatively; of 36 with long-term follow-up, 83% were alive at a median of 47 months, most without signs, medication or a special diet. The main complication was neurological signs, in 37% of cats, which did not depend on shunt type, degree of attenuation, age or preoperative seizures; 56% of these cats recovered normal neurological function.
• Strickland et al. (2021), retrospective cohort of 50 cats with a single congenital shunt. Post-attenuation neurological signs appeared in 62%, including seizures in 11 cats, and 5 of the affected cats died before discharge. Preoperative levetiracetam did not prevent them; the only risk factor found was a lower osmolality 24 hours after surgery.
• Otero Balda et al. (2025), 59 cats with post-attenuation neurological signs from 10 institutions. 78% were alive at 30 days, but only 50% of cats with seizures; seizures and the need for propofol were associated with lower survival.
• Mohammadi et al. (2024), meta-analysis of 12 studies: post-attenuation neurological signs occurred in 38.9% of cats and seizures in 20.2%, and mortality among affected cats was 17% and 37.2%, respectively. The evidence supporting preventive antiepileptics such as levetiracetam is limited.