Portosystemic shunts are a type of vascular abnormality that causes blood to be emptied into the circulation, without passing through the liver. This prevents the liver from detoxifying the blood. The condition may be either congenital or acquired.
Portosystemic shunts are either congenital or acquired and some will be categorised as being either extraheptic or intraheptic. [1]
Intrahepatic shunts are caused by patent ductus venosus. The aetiology of extrahepatic shunts is complex and poorly understood. [2]
All forms of portosystemic shunts produce various neurological, gastrointestinal, and urinary symptoms. [3]
Symptoms of congenital PSS usually appear by six months of age [4] and include failure to gain weight, vomiting, and signs of hepatic encephalopathy (a condition where toxins normally removed by the liver accumulate in the blood and impair the function of brain cells) such as seizures, depression, tremors, drooling, and head pressing. Urate bladder stones may form because of increased amounts of uric acid in circulation and excreted by the kidneys.
Neurological symptoms include ataxia and seizure. [2]
Irish Wolfhounds are predisposed to the condition with a digenic inheritance. [5]
Extrahepatic shunts are more common in toy breeds whilst intrahepatic shunts are more common in large breeds. [2]
Congenital PSS is caused by an anomaly in fetal liver circulatory system development. Normally, 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. On the other hand extrahepatic shunts usually result from a developmental abnormality of the vitelline veins, which connect the portal vein to the caudal vena cava. [6] 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. [7]
Initial diagnosis of PSS is through laboratory bloodwork showing either elevated serum bile acids 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. [8] Various diagnostic imaging techniques are used to demonstrate PSS. Ultrasonography is a rapid, convenient, non-invasive, and accurate method for diagnosis of PSS. [9] Ultrasonographic diagnosis of congenital PSS depends on finding an anomalous vessel either in the liver or just caudal to the liver in the dorsal abdomen, usually draining into the caudal vena cava. Ultrasonography can also be used to estimate hepatic volume and vascularity, and to identify related lesions affecting other abdominal structures, such as urinary calculi. Computed tomography (CT) may be considered when ultrasound expertise is lacking or ultrasonography is considered sub-optimal (e.g. because of the conformation of the patient). Control of respiration and careful timing of CT acquisition after contrast injection is necessary for optimal depiction of PSS. Rectal portal scintigraphy using 99mtechnetium pertechnetate, a technique of imaging involving detection of gamma rays emitted by radionuclides absorbed through the rectum and into the bloodstream, demonstrates the blood vessel bypassing the liver. In certain institutions, scintigraphy is the preferred diagnostic technique, but this leaves the patient radioactive for 24h, which may be inconvenient depending on nursing needs. Portal venography is the definitive method for demonstrating PSS, but is invasive, hence it is best reserved for animals with a known shunt or those considered highly likely to have a shunt that was not detectable by ultrasonography. [10]
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. [4] Methods of shunt attenuation should aim to slowly occlude the vessel over several weeks to months in order to avoid complications associated with portal hypertension. These methods include ameroid ring constrictors, cellophane banding, intravascular or percutaneous silicone hydraulic occluders. The most common methods of attenuation used by veterinarians are ameroid ring constrictors [11] and cellophane banding. [12] Both methods have reportedly good outcomes in both cats and dogs, although the true composition of readily sourced cellophane has been found to be made from plastics (inert) and not cellulose (stimulates a fibrous reaction). Recently, a commercial supplier of regenerated cellulose based cellophane for veterinarians has been established for use of cellophane banding for portosystemic shunts in dogs and cats. 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). [13] 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. [14] When surgery is not an option, PSS is treated as are other forms of liver failure. Antibiotics 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.
The intrahepatic shunts found in large dog breeds are passed on in a simple autosomal recessive way. [15] Extrahepatic shunts are believed to be a polygenic trait. It is not sex linked. [2]
The Pomeranian is a breed of dog of the Spitz type that is named for the Pomerania region in north-west Poland and north-east Germany in Central Europe. Classed as a toy dog breed because of its small size, the Pomeranian is descended from larger Spitz-type dogs, specifically the German Spitz.
In medicine, a shunt is a hole or a small passage that moves, or allows movement of, fluid from one part of the body to another. The term may describe either congenital or acquired shunts; acquired shunts may be either biological or mechanical.
Gastric dilatation volvulus (GDV), also known as gastric dilation, twisted stomach, or gastric torsion, is a medical condition that affects dogs and rarely cats and guinea pigs, in which the stomach becomes overstretched and rotated by excessive gas content. The condition also involves compression of the diaphragm and caudal vena cavae. The word bloat is often used as a general term to mean gas distension without stomach torsion, or to refer to GDV.
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 may have been developed from the smallest specimens of the Standard Schnauzer, or crosses between the standard and one or more smaller breeds such as the Affenpinscher, Miniature Pinscher, and Poodles, as farmers bred a small dog that was an efficient ratting dog. They are described as "spunky" but aloof dogs, with good guarding tendencies without some guard dogs' predisposition to bite. Miniature Schnauzers are recognized in four colors internationally: solid black, black and silver, salt and pepper, and white.
Budd–Chiari syndrome is a very rare condition, affecting one in a million adults. The condition is caused by occlusion of the hepatic veins that drain the liver.
Atrial septal defect (ASD) is a congenital heart defect in which blood flows between the atria of the heart. Some flow is a normal condition both pre-birth and immediately post-birth via the foramen ovale; however, when this does not naturally close after birth it is referred to as a patent (open) foramen ovale (PFO). It is common in patients with a congenital atrial septal aneurysm (ASA).
Portal hypertension is defined as increased portal venous pressure, with a hepatic venous pressure gradient greater than 5 mmHg. Normal portal pressure is 1–4 mmHg; clinically insignificant portal hypertension is present at portal pressures 5–9 mmHg; clinically significant portal hypertension is present at portal pressures greater than 10 mmHg. The portal vein and its branches supply most of the blood and nutrients from the intestine to the liver.
Gastric varices are dilated submucosal veins in the lining of the stomach, which can be a life-threatening cause of bleeding in the upper gastrointestinal tract. They are most commonly found in patients with portal hypertension, or elevated pressure in the portal vein system, which may be a complication of cirrhosis. Gastric varices may also be found in patients with thrombosis of the splenic vein, into which the short gastric veins that drain the fundus of the stomach flow. The latter may be a complication of acute pancreatitis, pancreatic cancer, or other abdominal tumours, as well as hepatitis C. Gastric varices and associated bleeding are a potential complication of schistosomiasis resulting from portal hypertension.
Caput medusae is the appearance of distended and engorged superficial epigastric veins, which are seen radiating from the umbilicus across the abdomen. The name caput medusae originates from the apparent similarity to Medusa's head, which had venomous snakes in place of hair. It is also a sign of portal hypertension. When the portal vein, that transfers the blood from the gastrointestinal tract to the liver, is blocked, the blood volume increases in the peripheral blood vessels making them appear engorged. It is caused by dilation of the paraumbilical veins, which carry oxygenated blood from mother to fetus in utero and normally close within one week of birth, becoming re-canalised due to portal hypertension caused by liver failure.The appearance is due to cutanous portosystemic collateral formation between distended and engorged paraumbilical veins that radiate from the umbilicus across the abdomen to join systemic veins.
In the fetus, the ductus venosus shunts a portion of umbilical vein blood flow directly to the inferior vena cava. Thus, it allows oxygenated blood from the placenta to bypass the liver. Compared to the 50% shunting of umbilical blood through the ductus venosus found in animal experiments, the degree of shunting in the human fetus under physiological conditions is considerably less, 30% at 20 weeks, which decreases to 18% at 32 weeks, suggesting a higher priority of the fetal liver than previously realized. In conjunction with the other fetal shunts, the foramen ovale and ductus arteriosus, it plays a critical role in preferentially shunting oxygenated blood to the fetal brain. It is a part of fetal circulation.
Transjugular intrahepatic portosystemic shunt is an artificial channel within the liver that establishes communication between the inflow portal vein and the outflow hepatic vein. It is used to treat portal hypertension which frequently leads to intestinal bleeding, life-threatening esophageal bleeding and the buildup of fluid within the abdomen (ascites).
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A portosystemic shunt or portasystemic 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 or acquired condition and occurs in humans as well as in other species of animals. Congenital PSS are extremely rare in humans but are relatively common in dogs. Improvements in imaging and awareness have contributed to an increase in cases.Thus a large part of medical and scientific literature on the subject is grounded in veterinary medicine.
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Anorectal varices are collateral submucosal blood vessels dilated by backflow in the veins of the rectum. Typically this occurs due to portal hypertension which shunts venous blood from the portal system through the portosystemic anastomosis present at this site into the systemic venous system. This can also occur in the esophagus, causing esophageal varices, and at the level of the umbilicus, causing caput medusae. Between 44% and 78% of patients with portal hypertension get anorectal varices.
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