Understanding Portosystemic Shunts in Animals: A Comprehensive Guide

Portosystemic shunts are abnormal connections between the portal vein and systemic circulation, bypassing the liver's detoxification process. This can lead to a buildup of toxins in the bloodstream.

The liver plays a crucial role in filtering toxins from the blood, but in animals with portosystemic shunts, this process is disrupted. As a result, toxins can accumulate in the body, causing a range of health problems.

Portosystemic shunts can be congenital or acquired, with congenital shunts present at birth and acquired shunts developing over time due to liver disease or other factors.

Explore further: Yorkshire Terrier Liver Shunt

A congenital portosystemic shunt develops in two main ways. The ductus venosus, a large shunt that carries blood through the fetal liver, fails to collapse at birth and remains open.

This can happen if the ductus venosus doesn't close properly after birth, or if a blood vessel outside the liver develops abnormally and remains open. I've seen cases where this has happened due to genetic predisposition or other factors.

There are two main types of portosystemic shunts: congenital and acquired. Congenital shunts are present at birth and are often caused by the ductus venosus failing to close. Acquired shunts, on the other hand, develop later in life due to liver disease.

Here are the key differences between congenital and acquired shunts:

It's worth noting that acquired shunts are generally seen in older dogs with liver disease, such as cirrhosis or hepatitis.

A congenital portosystemic shunt develops if the ductus venosus fails to collapse at birth and remains intact and open after the fetus no longer needs it.

Congenital shunts occur more commonly in purebred dogs than in mixed breeds, with certain breeds at increased risk.

Miniature schnauzers, Yorkshire Terriers, and Irish Wolfhounds appear to be at increased risk of developing congenital shunts.

The prevalence of portosystemic shunts in certain breeds suggests an inherited predisposition, which has been proven only in Irish Wolfhounds.

Intrahepatic shunts represent between 6% and 40% of congenital shunts and are more common in large and giant breeds of dogs.

The majority of intrahepatic shunts are a result of the embryonic connection between the umbilical vein and the caudal vena cava remaining open.

This connection typically closes 3 days after birth, but remains open in dogs with intrahepatic congenital shunts.

Here are the common breeds affected by intrahepatic shunts:

Acquired shunts occur in older dogs due to liver conditions such as cirrhosis, hepatitis, or neoplasia.

These conditions cause excessive and sustained pressure within the portal vein, leading to the formation of embryonic, nonfunctional vascular communications.

In contrast to congenital portosystemic shunts, a number of vessels are usually affected by acquired shunts.

This is because the liver disease causes widespread damage, leading to multiple vascular communications opening up.

Portosystemic shunts are abnormal vascular connections between the hepatic portal vein and the systemic circulation. This diversion of blood past the liver limits its vital functions in metabolism and detoxification.

Such anomalies can be present at birth or acquired later in life. Congenital shunts are more common, making up approximately 75% of all canine cases.

These shunts can result from anatomic abnormalities of the portal vasculature or the persistence of fetal vessels, often involving one or two vessels.

The liver is a vital organ that plays a crucial role in removing toxins and other byproducts from the blood. It collects blood from the gastrointestinal system, pancreas, and spleen through the portal vein.

The liver is responsible for filtering the blood and removing waste products, which is essential for maintaining overall health. This process helps to keep the blood clean and free of toxins.

A liver shunt occurs when an abnormal connection persists or forms between the portal vein or one of its branches, and another vein, allowing blood to bypass the liver. This can lead to a buildup of toxins in the blood.

In most cases, a liver shunt is caused by a birth defect called a congenital portosystemic shunt.

Portosystemic shunts are abnormal vascular connections between the hepatic portal vein and the systemic circulation, diverting blood past the liver and limiting its vital functions.

This can expose the body to toxic by-products of digestion, including toxins and bacteria, and can mimic the effects of liver failure.

Congenital shunts are more common, representing approximately 75% of all canine cases, and often result from anatomic abnormalities of the portal vasculature or persistence of fetal vessels.

One or occasionally two vessels are involved in these shunts, and they can be classified according to their location as either outside (extrahepatic) or within (intrahepatic) the liver.

Portosystemic shunts can be present at birth or acquired later in life due to another disease process, and understanding the difference between these two types is crucial for diagnosis and treatment.

Diagnosis of a portosystemic shunt typically starts with a complete blood count and biochemical evaluation, which may reveal a classic "footprint" of anaemia, mild-moderate elevations of liver enzymes, low urea, and low protein. This can prompt further investigation.

Specific diagnostic testing for portosystemic shunting includes fasting and postprandial bile acid measurement, abdominal ultrasound evaluation, and transcolonic nuclear scintigraphy. Bile acid measurement is a relatively straightforward test that detects circulating levels of bile acid pre- and two hours post-feeding.

A Complete Blood Count (CBC) and Serum Chemistries are common diagnostic tests, which may show mild anemia or smaller than normal red blood cells (microcytosis), low blood urea nitrogen (BUN) and albumin, and increases in liver enzymes (AST, ALT).

Certain breeds such as Yorkshire Terriers, Old English Sheepdogs, Irish Wolfhounds, Cairn Terriers, and Beagles have an increased incidence of portosystemic shunts, with small breed dogs usually having extrahepatic shunts and larger breeds having intrahepatic shunts.

Diagnosing a portosystemic shunt or liver shunt requires a combination of medical history, clinical signs, and diagnostic tests. A complete blood count and biochemical evaluation are usually the first steps in determining if a shunt is present.

Young animals may be an important clue to the potential diagnosis, but animals with acquired shunts or microvascular dysplasia can present at any age. Clinical investigation is often commenced with a complete blood count and biochemical evaluation.

Many shunts will demonstrate a classic 'footprint' on routine blood screening, which includes anaemia, mild-moderate elevations of liver enzymes, low urea, and low protein. Dogs with intrahepatic PSS often have significantly higher ALP levels than those with extrahepatic shunts.

Bile acid measurement is a relatively straightforward diagnostic test that detects circulating levels of bile acid pre- and two hours post-feeding. It's essential to obtain both pre-prandial and post-prandial samples, as fasting levels may be normal in some animals with a portosystemic shunt.

A 'fatty' meal is necessary to promote adequate gall-bladder contraction, but care must be taken not to precipitate an encephalopathic crisis by feeding too generous a meal. Ideally, feed a protein-restricted meal laced with a small quantity of corn oil.

Diagnostic tests for portosystemic shunting include fasting and postprandial bile acid measurement, abdominal ultrasound evaluation, and transcolonic nuclear scintigraphy. Certain breeds such as Yorkshire Terriers, Old English Sheepdogs, Irish Wolfhounds, Cairn Terriers, and Beagles have an increased incidence of portosystemic shunts.

Here are some common diagnostic tests for liver shunts:

Small breed dogs usually have extrahepatic shunts (blood vessels outside of the liver), while larger breeds have intrahepatic shunts (abnormal blood vessels inside the liver). Extrahepatic shunts are less challenging to surgically repair than intrahepatic shunts.

X-rays are a crucial tool in diagnosis, allowing doctors to see inside the body without surgery. They can detect broken bones, lung problems, and even some types of cancer.

An X-ray image is created by passing a beam of radiation through the body, which then hits a detector and produces an image. This process is quick and relatively painless.

Doctors can use X-rays to diagnose a range of conditions, from fractures and sprains to lung disease and heart problems.

Portosystemic shunts in animals can manifest in various ways, depending on the individual case. Clinical presentation is often the most common way to identify the condition.

The central nervous system signs are the most common, occurring in over three-quarters of all cases, and may be vague and subtle such as anorexia, depression, and lethargy. More specific signs include episodes of hyperactivity, head pressing and circling, disorientation, temporary blindness, weakness, excess salivation, seizures, and occasionally coma.

The clinical signs of portosystemic shunts are frequently intermittent, and can be less dramatic in cats than in dogs. Neurological signs include mild disorientation, bizarre behavior, seizures, blindness, or coma, while gastrointestinal signs include vomiting, diarrhea, ptyalism, abdominal pain, intense borborygmus, or anorexia.

Certain breeds, such as Yorkshire Terriers, Old English Sheepdogs, Irish Wolfhounds, Cairn Terriers, and Beagles, have an increased incidence of portosystemic shunts. Small breed dogs usually have extrahepatic shunts, while larger breeds have intrahepatic shunts.

Common Clinical Signs:

The central nervous system signs are the most common, occurring in over three-quarters of all cases, and may be vague and subtle such as anorexia, depression, and lethargy.

These signs tend to wax and wane, and their onset may be connected with the recent ingestion of a protein-rich meal that resulted in increased production of neurotoxins within the large intestine.

Vomiting and/or diarrhea are present in about two-thirds of cases, and evidence of lower urinary tract disease is present in approximately one-half of cases.

Some dogs, particularly those that develop signs later in life, have polydipsia and polyuria (excessive drinking and urination).

The most common clinical signs include stunted growth, poor muscle development, abnormal behaviors such as disorientation, staring into space, circling or head pressing, and seizures.

Dogs with a liver shunt often take a long time recovering from anesthesia, and behavioral clinical signs may only occur after eating high protein meals.

Clinical signs of PSS are the result of hepatoencephalopathy and/or functional impairment of the liver, and are frequently intermittent.

Neurologic signs include mild disorientation, bizarre behavior, seizures, blindness, or coma, and gastrointestinal signs include vomiting, diarrhea, ptyalism, abdominal pain, intense borborygmus or anorexia.

Common neurologic signs in cats include abnormal mentation, behavioral changes, aggression, ptyalism, seizures, and blindness.

Here are some common clinical signs of a liver shunt in dogs:

Note: This list is not exhaustive, and not all dogs with a liver shunt will exhibit all of these symptoms.

Dogs with congenital portosystemic shunts are typically purebred dogs less than 1 year old.

The severity of clinical signs varies and is related to the anatomic position of the shunt and the fraction of portal blood that is shunted past the liver.

Affected animals are often in poor body condition and of small body stature, especially when compared to their littermates.

Owners may complain that the animals fail to thrive or grow and that the skin and coat condition are poor.

A significant number of dogs may develop signs later in life, up to 25% of cases of portosystemic shunts.

Treatment and management of portosystemic shunts in animals involves a multi-faceted approach to reduce toxin production and absorption in the large intestines.

Dietary changes are a crucial part of management, with the goal of reducing protein intake and feeding high-quality, highly digestible protein diets. Lactulose is often used to change the pH in the large intestines, decreasing the absorption of ammonia and other toxins.

Antibiotics may be prescribed to alter the bacterial population in the intestines and reduce intestinal bacterial overgrowth.

Medical management is indicated for all dogs with acquired shunts and all dogs with microvascular shunts. It should also be used for a period in those dogs that are about to undergo surgical ligation.

Dietary manipulations aim to limit neurotoxin production by reducing the amount of protein that reaches the large intestine. Feeding smaller meals more frequently can help maximize the digestive capacity of the small intestine.

Supplementation with zinc salts improves the detoxification of ammonia and the control of hepatic encephalopathy.

Surgery provides the best chance for a long, healthy life in most dogs with extrahepatic shunts, with a survival rate of over 95% if ameroid constrictor placement is performed.

After surgery, a protein-restricted diet is typically fed for at least six to eight weeks, after which the dog can return to a high-quality maintenance diet. Lactulose is usually given for several weeks after surgery.

Blood tests will be repeated at regular intervals to evaluate liver function, and the liver will begin to grow as the shunt closes, often returning to a normal size and function within two to four months.

The prognosis for animals with a portosystemic shunt is generally good if they survive the immediate postoperative period. With complete ligation, a good long-term outcome should be expected.

The overall success rate for surgical correction is about 85%, with most pets feeling better within 10 to 14 days after surgery. However, recurrence of clinical signs can occur in 40-50% of animals with partial shunt ligation.

Acute mortality rates for shunt ligation range from 0-25%, depending on the experience of the surgeon and other factors. Causes of death include peritonitis, thrombosis of the portal vein, and other complications.

Dogs with a single shunt, especially one that is extrahepatic, have an excellent prognosis if surgical correction is performed. Unfortunately, over half of the dogs treated medically are euthanized within ten months of diagnosis due to uncontrollable neurological signs.

In dogs, about one-third of those treated medically will live a relatively long life, but it's essential to note that higher mortality rates are expected with intrahepatic shunt ligation. Neurologic signs can often persist in many cats, and continued medical therapy is required.