Barbara M. Kirby, BS, RN, DVM, MS, DACVS, DECVS, Australasian Registered Specialist in Small Animal Surgery
Section of Veterinary Surgery, School of Agriculture, Food Science and Veterinary Medicine, University College Dublin
Belfield, Dublin, Ireland
In just over three decades since the first major report of portosystemic shunts (PSS) in small animals, there has been a huge amount of clinical and research interest in these challenging and fascinating cases, with a resultant mountain of veterinary literature. Despite advances in diagnostic capabilities, ever-advancing diagnostic imaging, advances in surgical techniques, advances in anesthetic management, and advances in critical care management, the morbidity and mortality rates remain disappointingly high, particularly for intrahepatic shunts. The diversity of surgical options available for management of portosystemic shunts is a reflection of the lack of superior outcome with any single technique.
This lecture provides an update on aspects of the veterinary literature regarding portosystemic shunts, concentrating on the past 5 years. It also reviews the currently available surgical techniques for management of intrahepatic and extrahepatic portosystemic shunts, with comparison of reported outcomes between techniques.
What's the Current State of Knowledge of Breed Distributions and Heritability?
Single congenital portosystemic shunts have been identified as hereditary in Irish wolfhounds, cairn terriers, and humans. In addition, single congenital PSS are considered most likely to be hereditary in Yorkshire terriers. The breed predisposition to congenital PSS in a large number of purebred dogs suggests a hereditary etiology, but remains to be proven.
In the cairn terrier, an autosomal mode of inheritance that is most likely polygenic or monogenic with variable expression has been proposed. In the Irish wolfhound, an autosomal recessive mode of inheritance with incomplete penetrance has been proposed.
There is a tendency for single extrahepatic portosystemic shunts (EHPSS) in small breeds of dogs and single intrahepatic portosystemic shunts (IHPSS) in large and giant breeds, but essentially any shunt type can be encountered in any breed in any geographic location. The breed distribution of shunt type and specific shunt location, particularly in relation to divisional location of IHPSS, cannot be predicted with any accuracy. Variations in shunt distributions in differing geographic locations likely reflects the popularity of various breeds, geographic isolation, degree of inbreeding, and frequency of use of specific sires within a population of dogs.
What's New in Diagnostics in Suspected PSS?
Suspicion of a diagnosis of PSS rests on history and clinical signs. Routine diagnostic investigation including complete blood count, platelet count, biochemistry profile, coagulation profile and urinalysis provide additional clues in most cases. In practices without immediate access to a diagnostic laboratory or capabilities for in-house ammonia measurement, 12-hour fasted and 2-hour post-prandial measurement of serum bile acids are recommended. A single 12-hour fasted serum ammonia concentration that is above reference range is reported 100% sensitive and 89% specific for portal vascular anomaly (PVA) compared to a single fasted bile acid concentration being 92% sensitive and 68% specific for PVA.
A recent prospective study of coagulation factors in dogs with congenital PSS documented prolonged APTT and normal PT in all affected dogs, but no clear diagnostic specificity or prognostic value of specific coagulation factor abnormalities.
What's New in Diagnostic Imaging?
Sonographic diagnosis of PSS is now routine in many universities and private practices. Sonographic diagnosis is reported 100% sensitive and specific in IHPSS and slightly less in EHPSS depending on the expertise of the sonographer and other factors. Intraoperative ultrasound guidance can be particularly useful in difficult intrahepatic shunt identification and dissection. Intraoperative sonographic determination of reversal direction of portal flow has recently been reported as a means to determine end-point of partial attenuation of EHPSS.
Other advances in diagnostic imaging include trans-splenic scintigraphy, although scintigraphy is not useful for surgical planning. Trans-splenic ultrasound-guided portography provides surgically useful contrast angiography by a relatively non-invasive technique. The gold standard of imaging of the portal system in veterinary medicine is generally considered angiography (percutaneous or operative mesenteric portography), however in humans it has been replaced by CT angiography. There are recent reports of 2D and 3D helical CT angiography providing detailed images of the entire portal system and beautiful images of shunt vessels, including specific origin and insertion of the shunt vessel as well as its entire course. CT angiography provides rapid, minimally invasive, specific information on shunt morphology allowing accurate preoperative planning by the surgeon. 3D reconstructions are especially useful for complex shunt morphologies.
Pre-operative Medical Therapy
Pre-operative medical therapy for PSS generally includes a low-protein diet, antibiosis to decrease ammonia-producing bacteria in the gut, and lactulose. Medical therapy for pre-operative seizures and post-ligation neurologic dysfunction remain controversial.
There is general agreement that animals with shunt vessels that will tolerate acute complete occlusion have a favorable prognosis and the vessels should be fully ligated. The difficulty comes in determining which patients will tolerate complete ligation without severe acute portal hypertension or subsequent development of acquired multiple extrahepatic PSS as a consequence of chronic portal hypertension. Objective criteria (portal pressure measurements, change in mean arterial pressure, change in central venous pressure, change in heart rate, etc) cannot discriminate between shunts that can be safely completely occluded and shunts that require partial attenuation. For this reason, many surgeons have abandoned these objective measurements in an attempt to decrease operative time and perioperative morbidity.
Techniques for partial attenuation of EHPSS and IHPSS include silk suture attenuation, amaroid constrictor placement, cellophane banding, and hydraulic occluders. Transvenous coil embolization has previously been reported for IHPSS and the technique has recently been modified with deployment of a vena caval wall stent to prevent migration of the coils.
Laparoscopic shunt attenuation has also recently been reported.
How I Manage Extrahepatic PSS Cases
As I have not yet had the opportunity to work in a facility with 3D CT angiographic imaging, I am most often exploring EHPSS cases on the basis of ultrasound imaging. I find being present for the ultrasound examination useful. My goal for EHPSS cases is to minimize anesthesia and surgical time. Invasive monitoring is usually limited to a central venous catheter for blood sampling. I identify the portal vein and rule out portal vein aplasia. The shunt vessel is identified at its point of insertion into the systemic circulation, isolated, and temporarily fully occluded. I do not attempt to definitively identify the origin of the shunt. I make the assessment of whether or not the shunt can be fully ligated on the basis of subjective signs of portal hypertension. If there is no or minor clinical evidence of portal hypertension, I fully ligate the shunt. If there are signs of portal hypertension, I partially attenuate the shunt. I have successfully used various methods over the years. When it is available, I feel most comfortable with the amaroid constrictor, generally striving to attenuate the vessel acutely by about 50%.
How I Manage Intrahepatic PSS Cases
For shunts that have been diagnosed by ultrasound as intrahepatic, I prefer to have a preoperative portogram for planning purposes. This will generally be an operative mesenteric portogram via small celiotomy incision or percutaneous splenoportography.
For intrahepatic shunts, a central venous catheter and arterial catheter are placed for anesthesia monitoring. I use a ventral midline celiotomy and to date, have not required a median sternotomy approach. On rare occasions, I have used a diaphragm myotomy to improve visualization of the post-hepatic anatomy. I use subjective signs of portal hypertension to decide whether or not the shunt can be fully ligated. For left divisional shunts, particularly patent ductus venosus, post-hepatic dissection and attenuation of the left hepatic vein is usually most expedient. For right divisional shunts, about 50% have been isolated by pre-hepatic dissection and about 50% by post-hepatic dissection. In all cases, a trial complete occlusion of the shunt vessel is performed initially and intraoperative mesenteric portography performed with the vessel temporarily occluded to confirm correct identification of the shunt vessel and rule out additional extrahepatic or intrahepatic shunts. For shunts that are difficult to identify intraoperatively, a catheter or nylon leader line can be useful to pass through the portal vein, shunt, and into the caudal vena cava. Intraoperative ultrasound can also be useful. I look forward to the day 3D CT angiography is available to me for these difficult shunts. I use amaroid constrictor or cellophane banding for shunts that cannot be fully ligated.
The animal is monitored post-operatively in ICU for the first 24 hours. IV fluid therapy, IV colloid therapy, and IV antibiotics are routinely administered. Blood glucose is monitored and supplemented as required. Acid-base and electrolyte status is monitored and IV fluids supplemented with potassium as required. Analgesia is usually provided by morphine constant rate infusion (CRI) or morphine-ketamine CRI. Non-steroidal antiinflammatories are not routinely administered. Daily body weight is monitored during hospitalization.
Medical therapy (hepatic support diet, ampicillin or neomycin p.o. and lactulose p.o.) is continued for a variable period after surgery. Hepatic diet is continued until at least 4-6 weeks after surgery, at which time fasting and post-prandial bile acids are repeated. If bile acids normalize, medical therapy is discontinued.
The prognosis for surgical management of PSS varies widely, most likely the result of multiple factors. Mortality rates are generally lower for EHPSS than IHPSS. Recently reported mortality rates vary from 3% to 27% for EHPSS and from 27% to 90% for IHPSS. Recurrence of clinical signs and/or persistence of biochemical abnormalities are reported in approximately 10% of cases, although follow-up with scintigraphy or angiography is uncommon.
How Can We Improve Management and Outcomes?
Clearly, the ideal method to manage intrahepatic and extrahepatic shunts remains to be found. A means to predict preoperatively which animals will tolerate complete acute occlusion of their shunt, without immediate complications or long-term recurrence of signs associated with continued shunting or acquired multiple extrahepatic PSS, is needed. A more reliable, and possibly slower, method of gradual attenuation is needed for those animals that cannot tolerate complete occlusion. The maximum rate of shunt occlusion possible without development of chronic portal hypertension needs to be elucidated, but is probably not possible due to individual patient differences. A means to predict which individuals will develop post-ligation neurologic dysfunction and prevent its occurrence is needed.
Where Do We Go From Here?
Identification of the heritability and genetic defect in PSS is critically important. Breeders and breed organizations need to be recruited to screen puppies in more of the affected breeds, to participate in research into the genetic basis of PSS and to eliminate the disease through selective breeding.
References are available upon request.