Canine portosystemic shunts:an overview of diagnosis and treatment options- Part 2
Ultrasonography can identify the presence of shunt vessels, and also allows evaluation of liver size, parenchymal texture and arborization of blood vessels. In addition, the bladder and kidneys can be evaluated for renal architecture, crystalluria and uroliths. Color flow Doppler, if available, allows identification of turbulence patterns within the vena cava and portal vein typically associated with (a) shunting vessel(s). Accuracy of ultrasound for identification of PSS varies widely, and it is generally recognized that the diagnostic usefulness of ultrasonography is heavily dependent on the skill and experience of the operator (15).
This is still widely considered to be the ‘gold standard’ for diagnosis. Operative portography with injection into a mesenteric vein provides excellent imaging of the portal system and vascular anomalies, but does of course require general anesthesia and a minor surgical procedure. Where advanced imaging is available then either CT or MRI can be used with peripheral contrast injection via an intravenous catheter to obtain excellent images. Depending on the preference of the individual surgeon, the facilities available and the stability of the patient under anesthesia, it is often possible to follow CT or MRI angiography with definitive surgery. If operative portography is preferred then availability of radiographic facilities in theater allows both imaging and surgical correction under the same anesthetic.
Where surgery is contraindicated (e.g. portal vein atresia) medical management is the only appropriate treatment option. Medical management can significantly improve survival times, but periodic assessment is essential, a normal life-span is very unlikely and quality of life can be variable.
It is generally considered beneficial for animals with PSS due to undergo surgery to have 2-3 weeks of medical therapy initially, unless a complicating factor creates a surgical emergency e.g. urinary tract obstruction with urate calculi. There is currently no information available to provide an evidence-based figure for the period of time that is most beneficial, or even to confirm whether it is essential to have this period of stabilization. Intuitively most clinicians would agree that anesthetizing and operating on a patient with overt signs of HE is not to be recommended. If the only presenting signs are related to the urinary tract it is less clear whether a period of medical stabilization is necessary.
Medical therapy involves correction of fluid, glucose and electrolyte imbalances, and prevention of HE. Either a prescription hepatic support diet, or home-made diet based on high biological value protein can be fed. Although excessive protein should be avoided, over-enthusiastic protein restriction is also unhelpful as animals with abnormal liver function have a fundamental requirement for protein. Gastric inflammation or gastritis should be treated with sucralfate and a suitable antacid such as omeprazole. Oral antibiotics are administered to reduce colonic bacterial populations based on individual clinician preference; common choices include ampicillin, amoxycillin, and metronidazole. Lactulose can be administered to reduce colonic pH and thereby trap ammonium ions, inhibit protein and amino acid metabolism, increase fecal nitrogen excretion and decrease overall colonic transit time.
Some clinicians argue that medical management offers similar results to surgery in terms of survival times and quality of life, and that it should be considered an equally valid method of treatment. So far, however, the evidence to support this is lacking and it remains an opinion-based viewpoint rather than evidence-based medicine.
Surgical options and issues
Initially surgical intervention for PSS produced poorer success rates than are now routinely achieved in specialist referral centers. Intrahepatic shunts in particular had morbidity and mortality that was significantly higher than what can be achieved nowadays (2,11,16). Improvement in
anesthetic drugs and protocols, better intensive care provision, and changes in the surgical approach to management of PSS have brought about marked improvements in surgical outcomes in the last decade and now the general expectation is that extra-hepatic PSS should have a success rate in the region of 90-95%, and intrahepatic PSS 75-80% (2,10,15). There are various options available. Surgical ligation was the initial method of choice but techniques allowing gradual progressive attenuation of PSS are now widely considered to be the preferred approach. Advantages of such techniques include a reduction in the incidence of postoperative portal hypertension, and increased time for the cardiovascular and central nervous systems to adapt to changing hepatic metabolism. The two main techniques in current use are ameroid constrictors and cellophane banding, although there are isolated reports of the use of extra-vascular hydraulic occluders as well as more recent reports of minimally invasive techniques (e.g. percutaneous coil embolization).
This involves ligating the shunt either partially or completely with silk suture, depending on changes in the portal and systemic pressures in response to ligation. A particular issue with suture ligation is that in many cases full ligation induces unacceptable portal hypertension, but up to 50% of cases with only partial shunt ligation can have recurrence of clinical signs (17). The degree of occlusion achieved at surgery does not subsequently change significantly; therefore cases with recurrence of clinical signs due to persistent shunting require repeat surgery to achieve resolution. This of course both increases the risks and the expense. Where suture ligation is used, it is recommended that portal pressures be monitored during occlusion, to ensure that the degree of ligation will not induce life-threatening portal hypertension, whether full or partial. The surgeon can use subjective measures such as the color of the pancreas and intestines, stagnation of blood flow through mesenteric vessels, and development of random disordered motility of intestinal segments together with objective measurements such as changes in central venous pressure (decrease of >1 cm H2O is a cause for concern) and arterial pressure (changes in excess of 5 mm Hg are again a cause for concern). Provided the surgeon has the appropriate level of experience and surgical judgment these measurements are very reliable. Manometric measurement of portal pressures via a mesenteric vein, although theoretically providing direct values for portal pressures and therefore preferable, is in the author’s experience time-consuming, inaccurate, frustrating and unreliable.
These have been used clinically since 1996 (18) although they were employed as long ago as the 1950’s to induce experimental models of vascular stenosis. Ameroid constrictors consist of hygroscopic casein clay contained in a stainless steel ring. When implanted into the peritoneal cavity (Figure 3) the clay absorbs fluid and expands. The presence of the rigid stainless steel casing forces expansion inwards, occluding the vessel. In addition to physical occlusion the ameroid stimulates a fibrous tissue reaction that completes vessel closure. This means there is an initial rapid phase of vessel attenuation (3-14 days after implantation) followed by a slower phase (probably 2-3 weeks). Concerns have been raised regarding the initial fast phase of closure that the rate may in fact be too rapid, leading to development of multiple secondary acquired ‘pressure relief’ shunt vessels, but there is little clinical evidence to support this concern. Ameroid constrictors come in various diameters, and although there are no definitive guidelines regarding selection, the author’s preference is to choose a constrictor that produces minimal reduction in vessel diameter after placement.
The use of cellophane (Figure 4) was first reported in a clinical case in 1990 (19). As with ameroid constrictors, cellophane had previously been used experimentally as a model of portal hypertension. When placed around blood vessels cellophane incites an initial acute inflammatory reaction followed by a chronic low grade foreign body type reaction. This response is still ongoing as long as 6 weeks after surgery. The cellophane is cut in a rectangle, and then folded into 3-4 layers to form a band approximately 1 cm wide. Tapering the end that is to be advanced around the shunt vessel facilitates placement, and once the band is in place it can then be secured with ligaclips or sutures, although sutures are extremely difficult to place without damaging the cellophane whilst ensuring that the band is held securely in place (Figure 5). Again there are no definitive guidelines available regarding the degree of occlusion one should aim for. Most surgeons will place a band around the vessel to produce little immediate reduction in diameter, as there is some evidence that placing cellophane to actively induce 40-50% attenuation in diameter is associated with a poorer long-term outcome. The advantages of cellophane over ameroid constrictors include ease of availability, low cost, low weight and bulk, and greater flexibility. The disadvantage is that cellophane cannot be steam sterilized so an alternative sterilization method such as ethylene oxide is required.
Percutaneous embolization with intravascular coils has been used to occlude both extra-hepatic and intrahepatic PSS. This technique has the advantage of being minimally invasive, but this does not mean that the procedure is risk-free. Problems can include loss of coils to the systemic and pulmonary circulation, with resulting complications varying from mild to fatal. Recent adaptations have significantly reduced the risk of coil embolization, particularly the use of intracaval expandable stents which act as ‘traps’ to prevent coils moving out of the shunt. Success rates similar to those achieved with surgery are reported (20) although availability of this technique is limited by a requirement for specialized facilities and expertise, the costs involved and the fact that more than one procedure may be required; limited follow-up is available as yet.
• Acute portal hypertension has been recognized as a risk within the first 12 hours after surgery, although with ameroid constrictors and cellophane bands this is really only likely to be a problem if there has been a surgical error in terms of the size of implant chosen or inappropriate implant placement. Typical signs of portal hypertension include abdominal pain and distension, endotoxic or hypovolemic shock, hemorrhagic diarrhea, hypothermia or severe systemic hypotension. It is a complication that requires immediate surgical intervention to relieve the obstruction to portal flow.
• Ascites may develop secondary to chronic mild portal hypertension present pre-operatively. It is generally not life-threatening, and will spontaneously resolve with time.
• Post-ligation seizure activity is of unknown etiology; it can occur up to 4-5 days after surgery and can sometimes be difficult to control. Treatment generally consists of supportive care and administration of anti-seizure medications such as diazepam or (preferably) levetiracetam. Severely affected cases may require propofol by bolus or continuous rate infusion to control their seizures.
• Severe hypoglycemia can occasionally be seen, especially in toy breeds. Careful monitoring of blood glucose levels through surgery and recovery is essential.
• Portal vein thrombosis is a rare complication (better described in humans) but the author has seen this occasionally after partial suture ligation resulting in mild-moderate portal hypertension with development of multiple acquired shunts. Acute portal vein obstruction could potentially be fatal.
• Uncontrollable intra- or post-operative hemorrhage. This complication is heavily influenced by surgeon experience and ability, and is more likely with intrahepatic shunts, reflecting the typical location of such shunts and the degree of surgical complexity involved.
Appropriate case selection and stabilization, the pre-operative presence of hepatic encephalopathy and/or other complicating factors and the location/morphology of the shunt all influence outcome; however the experience of the anesthetic and surgical team are also paramount. Where surgery is performed by a team of clinicians and nurses with the appropriate experience and the facilities for adequate postoperative care, the outcome for dogs with single extra-hepatic shunts is excellent. Intrahepatic shunt surgery is significantly more challenging, and this is reflected in the somewhat lower success rate.
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