Theresa W. Fossum, DVM, PhD, DACVS
Tom and Joan Read Chair in Veterinary Surgery, Director, Clinical Programs and Biomedical Devices, Michael E. DeBakey Institute Professor of Surgery, Texas A&M University College of Veterinary Medicine
College Station, TX, USA
When portal blood bypasses the liver, many substances that are normally metabolized or excreted in the liver enter the systemic circulation. Also, important hepatotrophic substances from the pancreas (e.g., insulin) and intestines do not reach the liver, resulting in hepatic atrophy or failure of the liver to attain normal size. Hepatic insufficiency or hepatic encephalopathy frequently occur. Hepatic encephalopathy is a clinical syndrome of altered CNS function resulting from hepatic insufficiency. A variety of substances (i.e., ammonia, methionine/mercaptans, short-chain fatty acids, alterations in the ratio between circulating levels of branched-chain and aromatic amino acids, and γ-aminobutyric acid) have been incriminated in the resulting elaboration of false neurotransmitters.
Portosystemic shunts (PSS) may be broadly categorized as intrahepatic or extrahepatic. Congenital extrahepatic shunts usually are single anomalous vessels that allow abnormal blood flow from the portal vein to the systemic circulation. Intrahepatic shunts usually are congenital, singular shunts that occur because the ductus venosus fails to close after birth, or they may arise when other portal to hepatic vein or caudal vena cava anastomoses exist. Congenital intrahepatic portosystemic shunts (IHPSS) constitute about 35% of single shunts in dogs and approximately 10% in cats.
Purebred dogs are at increased risk for PSS. Domestic shorthair cats are most commonly affected, although these aberrations also occur in purebred cats (i.e., Himalayan). Single PSS usually are congenital and are most commonly diagnosed in animals under 1 year of age although dogs as old as 13 years have been diagnosed. Intrahepatic PSS are more commonly diagnosed in large-breed dogs (e.g., German shepherds, golden retrievers, Doberman pinschers, Labrador retrievers, Irish setters, Samoyeds, and Irish wolfhounds). Small breed dogs most likely to have IHPSS are toy and miniature poodles. There may be a hereditary basis for IHPSS in Irish wolfhounds. Congenital extrahepatic and IHPSS have been reported in cats. There is no convincing gender predisposition for these anomalies in either species.
Note: In general, small-breed dogs are more likely to have extrahepatic shunts, and large-breed dogs are more likely to have IHPSS.
The presenting history for animals with PSS varies considerably. Affected animals usually are evaluated because of failure to grow, small body stature, or weight loss. Other common abnormalities include intermittent anorexia, depression, vomiting, polydipsia or polyuria, ptyalism (especially in cats), pica, amaurosis, and behavioral changes. Some animals are presented for evaluation of urinary dysfunction (i.e., hematuria, dysuria, pollakiuria, stranguria, urethral obstruction) associated with urate urolithiasis. Signs of hepatic encephalopathy can vary tremendously from those that are extremely mild and hard to identify as a significant abnormality (e.g., lethargy, being "tired", being "slow") to severe changes (e.g., ataxia, weakness, stupor, head pressing, circling, amaurosis, pacing, seizures, or coma). These signs may be constant or intermittent and sometimes, but not invariably, worsen after eating (especially a high-protein diet composed of animal protein). Hepatic encephalopathy may also worsen after gastrointestinal hemorrhage (e.g., caused by parasites or ulceration). In a recent study, 82% of dogs had CNS signs, 76% had gastrointestinal signs, and 39% had urinary signs. In addition to ptyalism, affected cats typically show episodic central blindness.
Physical Examination Findings
Most animals with PSS have microhepatica, and the kidneys may feel prominent or plump. A golden or copper color to the iris has been observed in many cats with PSS. Neurologic abnormalities may be noted (see above). Ptyalism is a common finding in cats but rare in dogs. Animals with hepatic A-V fistulae may have a palpably enlarged liver (rare) or ascites. An audible bruit sometimes can be auscultated in the cranial abdomen of affected animals.
Survey abdominal radiographs are an important part of screening for congenital PSS. It is extremely rare to find a dog with PSS that does not have some degree of microhepatica. Plain abdominal radiographs are more sensitive in finding microhepatica than abdominal ultrasound.
Definitive diagnosis of PSS is made by surgical identification of the shunt, intraoperative positive contrast portography, ultrasound identification of the shunt, or nuclear hepatic scintigraphy. Jejunal vein portography is the simplest and most effective portographic technique. Ultrasound guided splenoportography can also be performed in large breed dogs. The most consistent finding on survey abdominal radiographs is microhepatica.
Ultrasound has become the diagnostic tool of choice for imaging PSS. Both intrahepatic and extrahepatic shunts have been identified with this technique; however, an inconclusive ultrasound examination does not rule out PSS. Occasionally a dilated intrahepatic vessel or the communication of an intrahepatic shunt with the caudal vena cava is noted. Nuclear scintigraphy is a rapid, noninvasive method of documenting abnormal hepatic blood flow. Sodium pertechnetate technetium 99m (99mTc) is typically used in scintigraphic studies to detect PSS. A new scintigraphic technique has recently been reported--trans-splenic portal scintigraphy. This technique is unique in that it utilizes ultrasound guidance to inject a small amount of 99mTc into the parenchyma of the spleen.
Hematologic, serum biochemical, and urine analysis of animals with PSS may disclose various abnormalities, but dogs can have a congenital PSS without any abnormalities on CBC or serum biochemistry panel. Low serum albumin is a common finding in dogs; however, some dogs (and most cats) with PSS have normal albumin levels. Low BUN results from reduced conversion of ammonia to urea in the hepatic urea cycle, but the polyuria-polydipsia seen in many patients may contribute. Other abnormalities occasionally include mild to marked increases in serum alanine aminotransferase, aspartate aminotransferase, and alkaline phosphatase
Hepatic function tests are important in screening for congenital PSS. Serum bile acids have been the standard hepatic function test in dogs and cats for years, but it is now recognized that they have some major limitations. First, it is critical to measure both pre- and post-prandial serum bile acid concentrations; approximately 20% of dogs have a higher pre-prandial value. Second, some dogs with very high serum bile acid concentrations (> 150 umol/L) do not have clinically significant hepatic disease while some dogs with congenital PSS have serum bile acid concentrations that are only moderately increased (e.g., 50-60 umol/L, normal < 30 umol/L). Third, unlike what is expected for most biochemical determinations, there can be substantial variation in bile acid concentrations from day to day (as much as or greater than 100%). Currently, urinary bile acids appear to be about as useful as serum bile acids but may have the advantage of being easier to collect, especially in cats.
Ligation or Attenuation of Intrahepatic Shunts
Both intravascular and extravascular methods have been described for ligation of IHPSS. Ligation of IHPSS can be extremely challenging because the vessel often is difficult to locate. Occasionally the shunt can be identified as a palpable depression or soft spot in a liver lobe, or it may be seen entering the caudal vena cava if it is not completely encircled by hepatic parenchymal tissue. Intraoperative ultrasound scans have been used to help identify the shunt in hepatic tissue, but this technique is not always successful. Intrahepatic shunts are classified as left, central, or right sided. Left and central divisional shunts account for a majority of shunts. Left sided IHPSS (patent ductus venosus) are typically located in the left lateral or medial hepatic lobes. Ligation or attenuation of the left hepatic vein may be performed in these animals. Central shunts are generally found in the right medial lobe, while right shunts are typically located in the right lateral or caudate lobes. An intravascular technique involving temporary hepatic vascular occlusion in conjunction with caudal caval venotomy was described by Breznock for intrahepatic shunt occlusion; however, because this procedure is technically difficult and surgery time is prolonged, many surgeons prefer extravascular techniques. Isolation and obstruction of the specific branch of the portal vein supplying the IHPSS have been described. Indirect passage of suture for ligation of right-sided intrahepatic PSS was recently reported (Tobias et al, 2004). The ligature should encircle the right portal branch approximately 4 mm lateral to it bifurcation from the parent vein (see below).
Note: Warn owners that ligation of IHPSS is difficult because the shunts are often hard to identify at surgery.
Isolation and Ligation of IHPSS Involving the Left Medial or Lateral Liver Lobes
Many shunts can be found cranial to the liver. Extend the abdominal incision proximally into the caudal sternebrae. Incise the diaphragm if necessary. Incise the left triangular ligament and free the left lateral liver lobe so that it can be retracted to the right. Use a combination of sharp and blunt dissection to isolate the anomalous vessel at its junction with the hepatic vein. Place a single silk ligature around the vessel and attenuate flow while measuring portal pressures. Alternately, ligate or attenuate the left hepatic artery as it enters the liver while measuring portal pressures.
Isolation and Ligation of Right-sided IHPSS
If necessary, ligate the right hepatic duct. Pass a Carmalt forceps from the dog's right to left over the dorsal surface of the main portal vein, just caudal to its bifurcation, but cranial to the termination of the gastroduodenal vein. Grasp one end of a 2-0 silk suture and pull it back across the dorsal aspect of the portal vein. Then, pass the forceps from the dog's right to left, dorsal to the left portal vein and within 5 mm of its bifurcation from the main portal vein. Grasp the opposite end of the suture and pull it back over the vein.