What is a portosystemic shunt?
A portosystemic, or liver, shunt is an abnormal blood vessel that results in blood from the abdominal organs being diverted straight to the heart without entering the liver first. Normally, blood enters the liver from the abdominal organs and is then cleared of nutrients and toxins that accumulate. The liver processes these toxic chemicals to make them safe and prevents them from continuing to circulate. When the liver is bypassed due to a shunt, these toxins are not metabolised and can have harmful effects around the body often resulting in neurological signs due to a condition known as ‘hepatic encephalopathy’. Affected animals also will not be able to store energy compounds in the liver, resulting in poor or stunted growth.
A portosystemic shunt can either be congenital (a birth defect) or acquired (develops later in life). Shunts can either be inside the liver itself (intrahepatic) or outside the liver (extrahepatic).
Are certain breeds predisposed?
Portosystemic shunts are most commonly seen in small or toy breed dogs, such as Pugs, Miniature Schnauzers, Maltese and Yorkshire Terriers, however other breeds can be affected also. In cats, Domestic Shorthairs, Persians, Siamese, Himalayans, and Burmese are most commonly affected.
What are the clinical signs of a portosystemic shunt?
Most pets will present at a young age, typically with a failure to thrive since birth, poor growth, weight loss or failure to gain weight and intermittent dullness, lethargy or strange behaviour (stargazing, head-pressing, walking in circles, staring into walls, random barking, intermittent blindness/pacing/aggression, seizures).
Some animals experience periods of significantly increased urination, abnormalities with urination (blood in the urine, difficulty urinating, urinary tract obstruction), diarrhoea, vomiting or a bloated abdomen.
In many animals, the signs are often associated with or worsened by eating, particularly when their meal is high in protein.
In some cases, shunts are discovered after an animal takes a longer time than expected to recover from a routine sedation or anaesthetic, such as following their desexing procedure.
How is a portosystemic shunt diagnosed?
A portosystemic shunt will be suspected in animals presenting with the clinical signs described above. A multitude of diagnostic tests are required to diagnose portosystemic shunts. This begins with blood testing, which can increase the suspicion for a portosystemic shunt, but are not enough to diagnose a shunt on blood results alone. Routine blood tests, as well as tests to measure bile acid and ammonia levels, will be performed initially. Urinalysis will also be recommended which may also increase suspicion of a portosystemic shunt.
Definitive diagnosis relies on abdominal ultrasonography, CT scanning, or exploratory surgery to diagnose and treat a portosystemic shunt. Prior to surgical procedures being undertaken, a coagulation analysis of your pet’s blood will be performed to ensure that their blood can clot, otherwise additional measures such as a blood transfusion may be required during surgery.
How is a portosystemic shunt treated?
Surgical intervention gives the best chance of recovery in affected animals, however there are medical options available if surgery cannot be elected. Medical management helps to control or minimise clinical signs but does not treat the underlying condition. Surgery is normally recommended as the best option in patients with a treatable shunt. Medical management is ideally reserved for emergency stabilisation of a severe episode of hepatic encephalopathy, and involves intravenous fluid therapy, enemas, lactulose administration, antibiotics, and anticonvulsant therapy, often performed to stabilise the patient prior to performing surgery.
Surgical correction involves exploratory abdominal surgery to enable the surgeon to locate the abnormal vessel. This vessel is then closed to enable blood to be redirected to the liver. The vessel must be closed slowly over a four-to-six-week period to prevent complications associated with rapid closure. Vessels in the liver that have not been used due to the abnormal vessel need time to develop, and closure of the abnormal vessel too quickly can result in excessively high blood pressure in the portal vein. The most common method of achieving this is with the use of a cellophane band secured with staples around the abnormal vessel. This causes a fibrous tissue reaction around the vessel and results in a gradual closure. Other materials can be used for this procedure including placement of silk ligatures, hydraulic occluders, or ameroid constrictors; all of which promote gradual closure.
Some portosystemic shunts can also be treated with a coil placed inside the vessel which promotes the development of a blood clot inside the vessel, effectively sealing it closed. This is performed in a closed manner with the device being placed up through the femoral artery. At this stage we are not able to perform this procedure in house and would need to refer you to a clinic in Brisbane to have this performed if necessary.
What does the post-operative management involve?
After surgery, your pet will be hospitalised for 2-3 days for intensive care. They will be maintained on intravenous fluid therapy, pain relief, and monitored for signs of hypoglycaemia, delayed anaesthetic recovery, haemorrhage, seizures, and signs of portal hypertension (high pressure in the liver).
We recommend feeding your pet a protein-restricted diet, and we will dispense oral medications whilst they are recovering from their surgery and whilst their liver function improves. Antibiotics may need to be continued depending on how your pet recovers from surgery. Within 4-6 weeks post-surgery we are hopeful that all medications can be weaned and eventually discontinued in 2-3 months.
Repeat assessment of bile acids and a biochemistry panel is recommended 2-3 months following surgery. In some cases, the blood results do not always return to normal. More importantly, clinical response is helpful in guiding our assessment of recovery.
What complications can occur?
There are a number of complications that can occur after shunt surgery, and include hypoglycaemia, haemorrhage, coagulation abnormalities, anaemia, portal hypertension, blood clot formation, seizures, arrhythmias (abnormal heart rhythm), bacterial hepatitis (liver infection), sepsis, sudden death, or recurrence of clinical signs. This is why it is important for your pet to remain in hospital under intensive care in the post-operative period, and medications are continued for weeks to months following surgery.
What is the prognosis?
Mortality rates are reported as 6-9% following cellophane banding, the procedure we perform most commonly here. The majority of animals undergoing surgical correction experience complete closure of the shunt vessel and results in normalisation of blood flow to their liver. These cases we expect to lead a normal life, with good to excellent long-term outcomes following surgery, often requiring no medication, and having a normal life-expectancy.