INTRODUCTION

There are several anomalous connections between the portal system and the systemic circulation (PSS). Persistent patent ductus venosus, portacaval, gastrocaval, portoazygos and portomesenteric shunts are the more common ones.

PSS is either intrahepatic or extrahepatic in location. Intrahepatic shunts are most commonly seen in the large breed dogs. Dogs with intrahepatic shunts are usually presented at an earlier age than dogs with extrahepatic shunts because of the greater volume of splanchnic blood bypassing the liver. Extrahepatic shunts are anomalous vessels that leave the portal system before it enters the liver and communicates with the vena cava or another systemic vessel.

Purebred dogs are at higher risk for portosystemic shunts. The most common breeds affected are the Yorkshire terrier, Miniature Schnauzers and other toy breeds.

The diversion of blood away from the liver results in its underdevelopment. This decreases hepatic function and the development of neurological signs due to hepatoencephalopathy (HE). HE develops from toxins entering the systemic circulation that are normally filtered or metabolized by the liver.

CLINICAL SIGNS

Most animals show signs within the first 6 months of life. It is not uncommon to see dogs with a single extrahepatic shunt present as young adults. Signs in dogs with PSS usually include some neurological deficits (temporary blindness, ataxia, head pressing, circling, depression, lethargy, and seizures). The severity of CNS signs varies and are often worse after eating especially if it is a protein-rich meal. The CNS signs are usually intermittent and progressive in nature.

The second most common signs are related to the gastrointestinal system. Animals are often presented underweight and have a history of intermittent diarrhea, vomiting, and nonspecific gastrointestinal signs.

Urinary tract signs include pollakiuria, polydypsia, polyuria, stranguria, and occasionally urolithiasis. On some occasions, urinary signs may be the only reason for owner's seeking veterinary care. Urolithiasis is seen in about 30% of dogs with PSS.

Physical examination findings usually include a small body size, nonpalpable liver, and prominent kidneys. Other anomalies that may coexist with PSS include cryptorchidism and a heart murmur. Neurological signs may be noticeable during the physical examination.

DIAGNOSIS

A complete blood count (CBC) usually demonstrates an anemia (microcytic, normochromic). Biochemical tests usually reveal a low blood urea nitrogen (BUN), hypoalbuminemia, hypoglycemia, and occasionally increased liver enzymes. Serum bile acids are elevated (resting and postprandial).

Abdominal radiography will show microhepatica, occasionally renomegaly, and possibly uroliths. Ultrasonography will demonstrate a small liver, confirm renal and/or cystic uroliths, and may reveal a shunt vessel. Finding the shunt vessel is very difficult and highly dependent on the skill of the sonographer.

In referral institutions, rectal portoscintigraphy is used as a screening test for PSS and can predict the approximate percentage of blood being shunted away from the liver. It cannot, however, differentiate single from multiple shunts or intrahepatic from extrahepatic shunts.

Occasionally, dogs will present with mild signs of liver disease that would be consistent with a PSS. However, the biochemical changes are mild (BUN, albumin, hematocrit levels are usually normal) and the serum bile acids are mildly elevated. This is consistent with a disease called hepatic microvascular dysplasia. Confirmation of this disease is by the negative findings of a single extrahepatic or intrahepatic shunt during surgery and the results of a liver biopsy.

Another differential diagnosis should include acquired shunts secondary to primary liver disease. This can be confirmed by an exploratory laparotomy, mesenteric portography, or possibly ultrasonography.

TREATMENT

Medical Management

Medical therapy is directed toward stabilizing the animal with neurological deficits prior to any surgical intervention. Both the production and absorption of CNS toxins are inhibited with various types of medications. A low protein/high carbohydrate diet reduces the source of ammonia. Medications directed toward ameliorating the sign of HE include lactulose given orally at a dose of 1-3 ml three times daily. Antibiotics (amoxicillin, metronidazole, neomycin) are given to reduce urease-producing intestinal bacteria, which hydrolyze intestinal urea to produce ammonia. Severely affected animals not eating should receive a balanced electrolyte solution intravenously spiked with glucose. Several days may be necessary before signs of HE are under control.

It is tempting to maintain animals on medical therapy indefinitely since they respond so nicely with a day or two. However, the shunting of blood continues and the liver continues to slowly deteriorate. Eventually, signs of HE will return.

Surgical Management

Surgery is the only long term "cure" because it corrects the underlying problem of decreased flow to the liver. Redirecting portal vein flow to the liver will reverse the liver pathology. However, even animals that go to surgery will continue to have medical therapy as described above for 2-4 weeks postoperatively.

A good knowledge of portal vein anatomy is necessary when attempting to find and close the shunt vessel. Exploration should include identification of the existing portal vein and its tributaries, the vena cava and finally, the shunt vessel. Most single extra hepatic shunts are either prominent straight vessels or tortuous vessels entering the pre-hepatic vena cava, or in some instances, (10% of the time), the azygous vein.

The experienced surgeon can readily recognize most single extrahepatic shunt vessels and venoportagraphy is seldom necessary. Contrast venoportagraphy is used when a single extrahepatic shunt is not identified and the possibility of an intrahepatic shunt exists.

The ideal situation in the surgical treatment of PSS would be to identify the anomalous vessel and ligate it completely without causing signs of portal hypertension (PH). Pre and post-ligation portal pressures can be done with the ligature method of partially or completely closing the shunt vessels. However, the author believes that clinical criteria (congestion of the pancreas, bluish-gray color of the bowel, and pulsating mesenteric vessels) are usually sufficient to prevent PH and portal pressures are usually unnecessary.

Portal hypertension is a major concern for surgeons when surgically treating PSS. A technique that gradually closes the shunt vessel slowly over weeks using an amaroid constrictor device (ACD) is the author's procedure of choice. The ACD was originally used in research models before we adapted it for use for PSS in dogs and cats. The ACD is made of hydroscopic casein material and has a stainless steel jacket. A small notch in the constrictor ring is made to allow its placement over the shunt vessel. A round "key" fits into the notch to keep it in place around the shunt vessel.

One of the major advantages using the ACD is to eliminate the need for portal vein pressure measurements. Having to decide between "partial" or "complete" ligations is no longer a factor in the surgical correction of PSS. Using an ACD also reduces the anesthesia time in these high-risk patients.

The ACD was originally thought to be of greatest value in patients that had portal pressure measurement that exceeded 20 cm of water. Following the successful use of the ACD in approximately 15 animals, we began to use it in all dogs with single extrahepatic shunts regardless of the portal pressure.

The ACD comes in various sizes with an inside diameter of between 2.0-6.0 mm. The ACD should be gas-sterilized. Over time, the author has noticed that the stainless steel jacket will loosen because the casein will dry out and shrink. The "notch" on the casein ring and the stainless steel jacket will have to be realigned and "spot-welded" with a drop of cyanoacrylate glue. The "key" that fits into the notch will sometimes fall out and will need to also be glued once the ring is in place around the shunt vessel.

Placing the ACD requires careful dissection of the shunt vessel adjacent to the systemic vessel into which it empties. In most cases, this is the vena cava. Dissection is kept to a minimum so that once the ACD is in place against the vena cava, side-to-side movement is limited. An assistant gently places a pair of hemostats or right angle forceps under the vessel and carefully spreads the jaws of the forceps while gently lifting the vessel. This flattens the vessel somewhat making it easier to slip the device over the vessel. The correct diameter of the ACD is determined by making sure there is a small space between the outside wall of the shunt vessel and the inside surface of the ACD. Total occlusion of the shunt vessel will gradually occur over 20-40 days. Some recent work by Hunt et al describes the use of a cellophane banding technique that may have the advantage of a slower occlusion time.

All animals postoperatively remain on a special low protein diet for 60 days. At this time, most dogs can return to a normal diet without need for any medication.

Postoperative care includes close monitoring for hypothermia, signs of portal hypertension (PH) (abdominal pain, ascites, ileus, shock, and diarrhea with blood and mucus). If PH is thought to be present the animal needs to be returned to surgery immediately and have the ACD removed. On rare occasions, some animals will have status ellipticus following shunt vessel ligation using any method of occlusion.

Two major advantages are associated with amaroid constrictor placement: a second surgery is not necessary in those animals where partial ligation may be indicated; portal hypertension is less likely to be a postoperative concern.

References

1.  Vogt JC, Krahwinkel DJ, Bright RM et al: Gradual occlusion of extrahepatic portosystemic shunts in dogs and cats using the amaroid constrictor Vet Surgery 25: 495-502 1996

2.  Kyles AE, Hardie EM, Mehl M et al: Evaluation of amaroid ring constrictors for the management of single extrahepatic portosystemic shunts in cats: 23 cases (1996-2001)

3.  Swalec KIM, Smeak DD: Partial versus complete attenuation of single portosystemic shunts Vet Surgery 19: 406-411, 1990

4.  Youmans KR, Hunt GB: Experimental evaluation of four methods of progressive venous attenuation in dogs Vet Surgery 28: 38047, 1999

5.  Youmans KR, Hunt GB: Cellophane banding for the gradual attenuation of single extrahepatic portosystemic shunts in eleven dogs Aust Vet Jour 76: 531-537