Portosystemic shunt
Encyclopedia
A portosystemic shunt, also known as a liver shunt, is a bypass of the liver
by the body's circulatory system
. It can be either a congenital
(present at birth) or acquired condition.
Congenital PSS is a hereditary condition in dog
s and cat
s, its frequency varying depending on the breed. The shunts found mainly in small dog breeds such as Shih Tzu
s, Tibetan Spaniel
s, Miniature Schnauzer
s and Yorkshire Terrier
s, and in cats such as Persians, Himalayan
s, and mix breeds are usually extrahepatic (outside the liver), while the shunts found in large dog breeds such as Irish Wolfhound
s and Labrador Retriever
s tend to be intrahepatic (inside the liver).
Acquired PSS is uncommon and is found in dogs and cats with liver disease such as cirrhosis
causing portal hypertension
, which is high blood pressure in the portal vein.
circulatory system of the liver to change. Normally, the blood from the placenta
bypasses the liver and goes into circulation via the ductus venosus
, a blood vessel found in the fetus. A failure of the ductus venosus to close causes an intrahepatic shunt, while extrahepatic shunts are usually a developmental abnormality of the vitelline veins
, which connect the portal vein to the caudal vena cava
. Thus in the juvenile and adult animal with PSS, blood from the intestines only partly goes through the liver, and the rest mixes into general circulation. Toxins such as ammonia
are not cleared by the liver. Most commonly, extrahepatic shunts are found connecting the portal vein or left gastric vein
to the caudal vena cava.
Congenital shunts are usually solitary. Acquired shunts are usually multiple, and are caused by portal hypertension in dogs with liver disease. This is most commonly seen in older dogs with cirrhosis, but may also be seen in younger dogs with liver fibrosis
caused by lobular dissecting hepatitis
.
(a condition where toxins normally removed by the liver accumulate in the blood and impair the function of brain cells) such as seizure
s, depression, tremors, drooling, and head pressing. Urate bladder stone
s may form because of increased amounts of uric acid
in circulation and excreted by the kidney
s. Initial diagnosis of PSS is through laboratory bloodwork showing either elevated serum bile acid
s after eating or elevation of fasting blood ammonia levels, which has been shown to have a higher sensitivity and specificity than the bile acids test. Rectal portal scintigraphy
using 99mtechnetium pertechnetate
, a technique of imaging involving detection of gamma ray
s emitted by radionuclide
s absorbed through the rectum and into the bloodstream, demonstrates the blood vessel bypassing the liver. Surgery definitively shows the shunt if it is extrahepatic.
. Dietary protein restriction is helpful to lessen signs of hepatic encephalopathy, and antibiotic
s such as neomycin
or metronidazole
and other medicines such as lactulose
can reduce ammonia production and absorption in the intestines. The prognosis
is guarded for any form of PSS.
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
by the body's circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
. It can be either a congenital
Congenital disorder
A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life , regardless of causation...
(present at birth) or acquired condition.
Congenital PSS is a hereditary condition in dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
s and cat
Cat
The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...
s, its frequency varying depending on the breed. The shunts found mainly in small dog breeds such as Shih Tzu
Shih Tzu
The Shih Tzu is a breed of dog weighing with long silky hair. The breed originated in China and is among the earliest breeds. Shih Tzu were officially recognized by the American Kennel Club in 1969...
s, Tibetan Spaniel
Tibetan Spaniel
The Tibetan Spaniel is a breed of assertive, small, intelligent dogs originating in the Himalayan mountains of Tibet. They share ancestry with the Pekingese, Japanese Chin, Shih Tzu, Lhasa Apso, and Pug...
s, Miniature Schnauzer
Miniature Schnauzer
The Miniature Schnauzer is a breed of small dog of the Schnauzer type that originated in Germany in the mid-to-late 19th century. Miniature Schnauzers developed from crosses between the Standard Schnauzer and one or more smaller breeds such as the Poodle and Affenpinscher.The breed remains one of...
s and Yorkshire Terrier
Yorkshire Terrier
The Yorkshire Terrier is a small dog breed of terrier type, developed in the 19th century in the county of Yorkshire, England to catch rats in clothing mills. The defining features of the breed are its size, to , and its silky blue and tan coat...
s, and in cats such as Persians, Himalayan
Himalayan (cat)
The Himalayan cat is a breed of long-haired cat identical in type to the Persian, with the exception of its blue eyes and its point coloration, which were derived from the crossing of the Persian with the Siamese...
s, and mix breeds are usually extrahepatic (outside the liver), while the shunts found in large dog breeds such as Irish Wolfhound
Irish Wolfhound
The Irish wolfhound is a breed of domestic dog , specifically a sighthound. The name originates from its purpose rather than from its appearance...
s and Labrador Retriever
Labrador Retriever
The Labrador Retriever is one of several kinds of retriever, a type of gun dog. A breed characteristic is webbed paws for swimming, useful for the breed's original purpose of retrieving fishing nets. The Labrador is the most popular breed of dog by registered ownership in Canada, the United...
s tend to be intrahepatic (inside the liver).
Acquired PSS is uncommon and is found in dogs and cats with liver disease such as cirrhosis
Cirrhosis
Cirrhosis is a consequence of chronic liver disease characterized by replacement of liver tissue by fibrosis, scar tissue and regenerative nodules , leading to loss of liver function...
causing portal hypertension
Portal hypertension
In medicine, portal hypertension is hypertension in the portal vein and its tributaries.It is often defined as a portal pressure gradient of 10 mmHg or greater.-Causes:Causes can be divided into prehepatic, intrahepatic, and posthepatic...
, which is high blood pressure in the portal vein.
Pathology
Congenital PSS is caused by the failure of the fetalFetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
circulatory system of the liver to change. Normally, the blood from the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
bypasses the liver and goes into circulation via the ductus venosus
Ductus venosus
In the fetus, the ductus venosus shunts approximately half of the blood flow of the umbilical vein directly to the inferior vena cava. Thus, it allows oxygenated blood from the placenta to bypass the liver. In conjunction with the other fetal shunts, the foramen ovale and ductus arteriosus, it...
, a blood vessel found in the fetus. A failure of the ductus venosus to close causes an intrahepatic shunt, while extrahepatic shunts are usually a developmental abnormality of the vitelline veins
Vitelline veins
-Path:They run upward at first in front, and subsequently on either side of the intestinal canal.They unite on the ventral aspect of the canal, and beyond this are connected to one another by two anastomotic branches, one on the dorsal, and the other on the ventral aspect of the duodenal portion of...
, which connect the portal vein to the caudal vena cava
Inferior vena cava
The inferior vena cava , also known as the posterior vena cava, is the large vein that carries de-oxygenated blood from the lower half of the body into the right atrium of the heart....
. Thus in the juvenile and adult animal with PSS, blood from the intestines only partly goes through the liver, and the rest mixes into general circulation. Toxins such as ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
are not cleared by the liver. Most commonly, extrahepatic shunts are found connecting the portal vein or left gastric vein
Left gastric vein
The left gastric vein carries blood low in oxygen, tributaries derived from both surfaces of the stomach; it runs from right to left along the lesser curvature of the stomach, between the two layers of the lesser omentum, to the esophageal opening of the stomach, where it receives some esophageal...
to the caudal vena cava.
Congenital shunts are usually solitary. Acquired shunts are usually multiple, and are caused by portal hypertension in dogs with liver disease. This is most commonly seen in older dogs with cirrhosis, but may also be seen in younger dogs with liver fibrosis
Fibrosis
Fibrosis is the formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive process. This is as opposed to formation of fibrous tissue as a normal constituent of an organ or tissue...
caused by lobular dissecting hepatitis
Hepatitis
Hepatitis is a medical condition defined by the inflammation of the liver and characterized by the presence of inflammatory cells in the tissue of the organ. The name is from the Greek hepar , the root being hepat- , meaning liver, and suffix -itis, meaning "inflammation"...
.
Symptoms and diagnosis
Symptoms of congenital PSS usually appear by six months of age and include failure to gain weight, vomiting, and signs of hepatic encephalopathyHepatic encephalopathy
Hepatic encephalopathy is the occurrence of confusion, altered level of consciousness and coma as a result of liver failure. In the advanced stages it is called hepatic coma or coma hepaticum...
(a condition where toxins normally removed by the liver accumulate in the blood and impair the function of brain cells) such as seizure
Seizure
An epileptic seizure, occasionally referred to as a fit, is defined as a transient symptom of "abnormal excessive or synchronous neuronal activity in the brain". The outward effect can be as dramatic as a wild thrashing movement or as mild as a brief loss of awareness...
s, depression, tremors, drooling, and head pressing. Urate bladder stone
Bladder stone (animal)
Bladder stones or uroliths are a common occurrence in animals, especially in domestic animals such as dogs and cats. Occurrence in other species, including tortoises, have been reported as well. The stones form in the urinary bladder in varying size and numbers secondary to infection, dietary...
s may form because of increased amounts of uric acid
Uric acid
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates such as ammonium acid urate. Uric acid is created when the body breaks down purine nucleotides. High blood concentrations of uric acid...
in circulation and excreted by the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
s. Initial diagnosis of PSS is through laboratory bloodwork showing either elevated serum bile acid
Bile acid
Bile acids are steroid acids found predominantly in the bile of mammals. Bile salts are bile acids compounded with a cation, usually sodium. In humans, the salts of taurocholic acid and glycocholic acid represent approximately eighty percent of all bile salts. The two major bile acids are cholic...
s after eating or elevation of fasting blood ammonia levels, which has been shown to have a higher sensitivity and specificity than the bile acids test. Rectal portal scintigraphy
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...
using 99mtechnetium pertechnetate
Technetium
Technetium is the chemical element with atomic number 43 and symbol Tc. It is the lowest atomic number element without any stable isotopes; every form of it is radioactive. Nearly all technetium is produced synthetically and only minute amounts are found in nature...
, a technique of imaging involving detection of gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s emitted by radionuclide
Radionuclide
A radionuclide is an atom with an unstable nucleus, which is a nucleus characterized by excess energy available to be imparted either to a newly created radiation particle within the nucleus or to an atomic electron. The radionuclide, in this process, undergoes radioactive decay, and emits gamma...
s absorbed through the rectum and into the bloodstream, demonstrates the blood vessel bypassing the liver. Surgery definitively shows the shunt if it is extrahepatic.
Treatment
Surgical treatment is best, when it can be performed. Pressure within the portal vein is measured as the shunt is closed, and it must be kept below 20 cm H2O or else portal hypertension will ensue. Complete closure of extrahepatic shunts results in a very low recurrence rate, while incomplete closure results in a recurrence rate of about 50 percent. However, not all dogs with extrahepatic shunts tolerate complete closure (16 to 68 percent). Intrahepatic shunts are much more difficult to surgically correct than extrahepatic shunts due to their hidden nature, large vessel size, and greater tendency toward portal hypertension when completely closed. When surgery is not an option, PSS is treated as are other forms of liver failureLiver failure
Acute liver failure is the appearance of severe complications rapidly after the first signs of liver disease , and indicates that the liver has sustained severe damage . The complications are hepatic encephalopathy and impaired protein synthesis...
. Dietary protein restriction is helpful to lessen signs of hepatic encephalopathy, and antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...
s such as neomycin
Neomycin
Neomycin is an aminoglycoside antibiotic that is found in many topical medications such as creams, ointments, and eyedrops. The discovery of Neomycin dates back to 1949. It was discovered in the lab of Selman Waksman, who was later awarded the Nobel Prize in Physiology and medicine in 1951...
or metronidazole
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....
and other medicines such as lactulose
Lactulose
Lactulose is a synthetic, non-digestible sugar used in the treatment of chronic constipation and hepatic encephalopathy, a complication of liver disease. It is a disaccharide formed from one molecule each of the simple sugars fructose and galactose...
can reduce ammonia production and absorption in the intestines. The prognosis
Prognosis
Prognosis is a medical term to describe the likely outcome of an illness.When applied to large statistical populations, prognostic estimates can be very accurate: for example the statement "45% of patients with severe septic shock will die within 28 days" can be made with some confidence, because...
is guarded for any form of PSS.
Heredity
The intrahepatic shunts found in large dog breeds are passed on in a simple autosomal recessive way, while the extrahepatic shunts of the small breeds are inherited on a polygenic basis.External links
- Portosystemic liver shunt article by Dr. Thomas K. Graves
- http://pets.groups.yahoo.com/group/Liver_Shunt_And_MVD_Support/
- http://www.livershunt.net
- http://www.yorkielane.com/Health/livershunt2.htm