A Comparison of the Ameroid Constrictor Versus

A Comparison of the Ameroid Constrictor Versus Ligation in the Surgical Management of Single Extrahepatic Portosystemic Shunts Twenty-two dogs were managed surgically for a single extrahepatic portosystemic shunt; 12 with surgical ligation and 10 with an Ameroid constrictor. Utilization of the Ameroid constrictor significantly decreased surgery time to approximately half that of the ligation technique. A decreased intraoperative and postoperative complication rate was noted with the Ameroid constrictor group. Follow-up evaluation demonstrated comparable efficacy when comparing surgical techniques. The Ameroid constrictor offered a surgical occlusion technique for management of a single extrahepatic portosystemic shunt that was equally effective to ligation while shortening surgical time and minimizing the risks that are commonly associated with ligation of the shunting vessel. J Am Anim Hosp Assoc 2001;37:390–396.

Scott T. Murphy, DVM, Diplomate ACVS Gary W. Ellison, DVM, MS, Diplomate ACVS Melissa Long, DVM Jim Van Gilder, BS

RS

From the Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida 32610-0126. Doctor Murphy’s current address is Veterinary Surgical Specialists, 7131 Cahill Avenue, Inver Grove Heights, Minnesota 55076. 390

Introduction A portosystemic shunt (PSS) is a congenital defect in the development of the portal circulation where there is an abnormal communication between the portal venous and systemic circulation. The shunt diverts portal blood away from the liver into the systemic circulation, allowing toxins that are normally removed and metabolized by the liver to remain in circulation.1 Portosystemic shunts are classified as congenital intrahepatic, congenital extrahepatic, or acquired extrahepatic. Single extrahepatic PSS are the most frequently diagnosed and are recognized in young, small, purebred dogs.2 Yorkshire terriers and miniature schnauzers may be predisposed to single extrahepatic PSS.2 The PSS clinically manifests in abnormalities of hepatic function, neurological signs, and gastrointestinal and urinary tract disease. Surgical management of a single extrahepatic PSS has traditionally been achieved by complete (or partial) attenuation of the shunting vessel. Previous reports indicate excellent clinical results for dogs that have had complete ligation of a congenital PSS.3,4 However, in 48% to 68% of dogs with a single extrahepatic PSS, only partial attenuation of the shunting vessel is achieved.5-7 In these dogs, complete attenuation of the shunting vessel at the time of surgery results in life-threatening portal hypertension. Portal hypertension is caused by resistance of blood flow through the poorly developed portal or hepatic vascular system, or both systems, and results in splanchnic venous pooling. A recent advance in the surgical management of PSS in dogs has involved the gradual occlusion of the shunting vessel with an Ameroid constrictora (AC).8 An AC is a hygroscopic, compressed casein band with an outer titanium ring and casein key mechanism that allows placement around a vessel. The casein material expands inward when surrounded with tissue fluid, thereby allowing a gradual occlusion of the PSS. The unit comes in multiple sizes (diameters of 3.5, 5.0, 6.5, 7.0, 8.0, and 9.0 mm), with the 3.5- or 5.0-mm diameter ring utilized most JOURNAL of the American Animal Hospital Association

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commonly with the single extrahepatic PSS. The AC is reported to have a rapid expansion phase during the first 14 days after implantation and then a slow expansion phase for 60 days.8 However, the AC has been reported to experimentally produce complete vascular occlusion within 7 days.9 In the initial report describing this technique, excellent results were achieved in 10 of 12 dogs with a single extrahepatic PSS that were determined to be inappropriate candidates for complete attenuation of the shunting vessel.8 No direct comparison has been reported between use of the AC and surgical ligation (partial or complete) in the management of a single extrahepatic PSS in dogs. The purposes of this investigation are to compare the effectiveness of the surgical management of single extrahepatic PSS when utilizing ligation versus application of the AC band, and to compare the incidence and severity of complications.

Materials and Methods A retrospective review of the medical database was conducted. Records of all PSS surgeries performed from 1988 to 1997 were examined. The medical records of 22 dogs with a single extrahepatic PSS were selected. All PSS cases were verified by intraoperative visualization, portovenogram, or both. The following information was obtained from the medical record: signalment, body weight, clinical signs, fasting and 2-hour postprandial serum bile acid results, technetium 99 (Tc99) transcolonic scintigraphy shunt fraction,10 venous portography results when available, operative location of the PSS, portal pressures, ligation versus AC correction procedure, 3.5- or 5.0mm diameter AC ring size, duration of surgery, complications, and the number of days from surgery to time of discharge. All cases in which surgery was performed were evaluated to compare data up to the time of discharge. Criteria evaluated to compare the postoperative efficacy of each procedure included serum bile acid results; Tc99 transcolonic shunt fraction; follow-up interview with referral veterinarians, clients, or both; and a clinical rating score. A clinical rating score was assigned to each dog to assess the efficacy of the PSS surgical procedure (1=absence of signs; 2=improvement with occasional signs, but a low-protein diet, lactulose, or antibiotics still necessary; 3=no improvement or relapse). The safety of each procedure was compared for both intraoperative and postoperative complications, perioperative mortality, and survival time. Intraoperative complications included cardiovascular (e.g., hypotension, hemorrhage), gastrointestinal (e.g., altered motility), anesthetic, and death. Postoperative complications included seizures, portal hypertension (e.g., decreased systemic blood pressure, sepsis, bloody diarrhea), hypoglycemia, and ascites. Perioperative mortality was defined as death that occurred during surgery or prior to discharge. Survival time was from discharge to the last follow-up evaluation. The independent sample t-test was used to compare mean levels of the serum bile acids, transcolonic scintigra-

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phy shunt fractions, survival time, duration of surgery, and length of hospital stay between the AC and ligation groups. Clinical rating scores between the two groups were compared using the Mann-Whitney rank sum test. Intraoperative and postoperative complication rates and perioperative mortality rates were compared using the Fisher’s exact test.

Results Twenty-two dogs with single extrahepatic PSS managed surgically were evaluated in this study. Ten dogs had an AC applied [Table 1], while 12 dogs had partial or complete surgical ligation of the shunting vessel [Table 2]. The mean age at surgery for all dogs in the study was 14.8 months (range, 4 to 68 months). Dogs managed with surgical ligation were older (mean age, 18.3 months) than those managed with an AC (mean age, 10.1 months). There was an equal distribution of males to females (11 males, 11 females). The mean body weight at the time of surgery for all dogs was 3.5 kg. There was a tendency for dogs in the surgical ligation group to weigh more (mean body weight, 4.1 kg) than dogs in the AC group (mean body weight, 2.8 kg). The breed most commonly represented was the Yorkshire terrier (9/22 cases). All dogs in the study had preoperative clinical signs suggestive of PSS. The most commonly recognized clinical manifestation originated in the central nervous system (15/22); gastrointestinal signs were noted in 10/22 dogs; and urinary signs were seen in 8/22 dogs. Half of the dogs had more than one clinical sign suggestive of PSS at the time of diagnosis. Preoperative bile acids were obtained in 18 cases (10 ligation; eight AC). The mean bile acid results for dogs in the ligation group were 84.5 µmol/L preprandial and 272.2 µmol/L 2 hours postprandial (reference range for preprandial bile acid, 0 to 9 µmol/L; 2 hours’ postprandial bile acid, 0 to 30 µmol/L). For the AC group, the mean preprandial bile acid was 119.6 µmol/L, and the mean 2 hours’ postprandial bile acid was 272.8 µmol/L. There were no significant differences in preoperative bile acid results when comparing surgical groups. Preoperative Tc99 transcolonic scintigraphy shunt fraction results were obtained for eight dogs in the ligation group and 10 dogs in the AC group. The mean shunt fraction for the ligation group was 76% (range, 60% to 88%), and the mean shunt fraction for the AC group was 72.8% (range, 60% to 85%) (reference range, 2× resting portal pressure levels.3,4 When attenuation of the shunting vessel dangerously elevates portal pressures, there are many postoperative complications that arise. These complications can be severe and potentially life threatening. When portal pressure following attenuation of the shunting vessel exceeds recommended levels and/or complications associated with complete ligation of the shunting vessel are evident at the time of surgery or are anticipated following surgery, then partial attenuation of the shunting vessel is recommended.12 Unfortunately,

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attenuation of the shunting vessel to the correct diameter can be a difficult and somewhat imprecise procedure.13 Partial ligation of the shunting vessel can still result in complications following surgery, but these complications may be less severe than if the shunting vessel is completely ligated.3,6 Partial ligation of the shunting vessel reportedly alleviates clinical signs associated with PSS for the short term; but long-term complications can arise in the central nervous system (e.g., seizures, dementia), gastrointestinal tract (e.g., diarrhea, vomiting), urinary tract (e.g., urate calculi, crystals, or both), or as general complications (e.g., depression, lethargy, weight loss).12,13 To improve longterm prognosis, it has been recommended that if a partial ligation is performed, then a second surgery should be done to eventually achieve total occlusion of the shunting vessel.12 All dogs had evidence of a single extrahepatic PSS that was verified at the time of surgery. However, portovenograms were not performed in all dogs to rule out the possibility of a double congenital extrahepatic shunt. The increased surgical time, morbidity, and expense associated with a portovenogram made routine utilization of this diagnostic procedure infrequent at this institution. In this study, signalment, body weight, preoperative clinical signs, bile acid results, and Tc99 transcolonic scintigraphy shunt fraction were not significantly different between groups at the time of initial PSS diagnosis. Ten dogs were managed with an AC, while 12 dogs had surgical ligation of the shunting vessel. None of the dogs managed via either technique died during the perioperative period. A significant finding in this study was that use of the AC for PSS surgery decreased the time spent in the operating room by approximately half in comparison to the ligation technique. The most obvious benefit of this decreased surgical time is the potential to decrease the duration of time in which a small, metabolically fragile dog must be anesthetized. Patients requiring a portovenogram to locate the shunting vessel were usually awakened from anesthesia following the diagnostic procedure, and the shunt attenuation procedure was performed at a later date to minimize anesthetic duration. Dogs in the AC group had fewer intraoperative and postoperative complications when compared to dogs in the ligation group, especially in those cases where partial ligation of the shunting vessels was performed. A higher incidence of postoperative complications has been previously recognized in dogs with partial shunt ligation compared to those with complete ligation.3,6 The increased complication rate following partial ligation of the PSS may be due to excessive elevation of the portal pressure from baseline values following partial attenuation of the shunting vessel. It is likely that the hepatic vasculature in these animals is unable to rapidly accommodate to the increased blood flow to the liver.6 Vessels of the hepatic parenchyma may be atretic, so immediate perfusion of intrahepatic portal vessels may not be possible after complete or partial attenuation of the shunting vessel. Subjective and objective intraoperative

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assessment of portal pressure also may not accurately reflect the true hemodynamic capabilities of the hepatic vascular system and have well-recognized inaccuracies inherent with its measurement.6,14,15 The decreased complication rate associated with utilization of the AC is likely the result of the benefits achieved via a gradual occlusion of the shunting vessel and gradual increase in hepatic blood flow and accommodation of the hepatic vasculature.8 Utilization of an AC also eliminates the necessity of and potential pitfalls associated with portal pressure measurement and assessment. Follow-up mean bile acid results were decreased in both groups compared to the mean preoperative values, but they were not within the normal reference range for many dogs. Liver function may still remain abnormal for many dogs regardless of the type of surgery performed. This agrees with results of earlier studies that determined clinical performance is independent of postoperative bile acid values at follow-up evaluation.5,8 Follow-up evaluation via transcolonic scintigraphy, when available, did appear to correlate well with the clinical rating score. This may be because many dogs remained on low protein diets after the initial diagnosis of the PSS, thereby ameliorating some of the clinical signs seen prior to diagnosis. The follow-up clinical rating results demonstrated a trend toward decreased clinical function for the ligation group compared to the AC group. This recognized difference may be due in large part to the longer duration of follow-up available for the ligation group. For dogs in which either complete or partial attenuation of a single extrahepatic PSS has been performed, longterm deterioration of clinical and clinicopathological signs does occur.12,13 Secondary collateral shunts develop due to inadequate development of hepatic portal vasculature as the PSS becomes totally occluded.8 The AC may produce complete vascular occlusion as early as 7 days following surgery.9 This early shunt occlusion might not allow sufficient time for the hepatic vasculature to adapt to increased portal flow, potentially predisposing the animals to the development of early collateral shunt formation. Collateral shunt development was documented in two of the AC dogs upon reevaluation during a second exploratory laparotomy. Perhaps a procedure with a slower rate of vein attenuation, such as cellophane banding,9 would allow the hepatic and portal vasculature more time to accommodate. Cellophane banding produces slow, progressive (but not complete) attenuation of a vessel over a 6-week period.9 If this study was carried out for a greater duration, the authors suspect that the AC group would have similar long-term follow-up results as the ligation group. Future evaluation of the AC should include critical assessment of liver function and hepatic blood flow in all dogs to determine the long-term efficacy of this procedure.

Conclusion Utilization of the AC allows gradual occlusion of a single extrahepatic PSS vessel. This method may allow the hepatic

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vasculature time to slowly accommodate to the increased portal blood flow, allow the development of collateral shunting vessels prior to the appearance of portal hypertension, or both. The significantly shorter surgical duration associated with the AC technique will also decrease the amount of time these small, metabolically fragile patients must be anesthetized. In this study, the AC offered a surgical occlusion technique for the management of a single extrahepatic PSS that was equally effective to ligation while shortening surgery time and avoiding many of the risks and complications that are classically associated with ligation of the shunting vessel. a Ameroid constrictor; Research Instruments and Mfg., Corvallis, OR

References 11. Payne JT, Martin RA, Constantinescu GM. The anatomy and embryology of portosystemic shunts in dogs and cats. Sem Vet Med Surg (Sm Anim) 1990;5:76-82. 12. Whiting PG, Peterson SL. Portosystemic shunts. In: Slatter DH, ed. Textbook of small animal surgery. 2nd ed. Philadelphia: WB Saunders, 1993:660-677. 13. Johnson CA, Armstrong PJ, Hauptman JG. Congenital portosystemic shunts in dogs: 46 cases (1979-1986). J Am Vet Med Assoc 1987;191:1478-1483. 14. Butler LM, Fossum TW, Boothe HW. Surgical management of extrahepatic portosystemic shunts in the dog and cat. Sem Vet Med Surg (Sm Anim) 1990;5:127-133.

July/August 2001, Vol. 37 15. Lawrence D, Bellah JR, Diaz R. Results of surgical management of portosystemic shunts in dogs: 20 cases. J Am Vet Med Assoc 1992;201:1750-1753. 16. Swalec KM, Smeak DD. Partial versus complete attenuation of single portosystemic shunts. Vet Surg 1990;19:406-411. 17. Matthews K, Gofton N. Congenital extrahepatic portosystemic shunts occlusion in the dog: gross observations during surgical correction. J Am Anim Hosp Assoc 1998;24:387-394. 18. Vogt JC, Krahwinkel DJ, Bright RM, Daniel GB, Toal RL, Rohrbach B. Gradual occlusion of extrahepatic portosystemic shunts in dogs and cats using the ameroid constrictor. Vet Surg 1996;25:495-502. 19. Youmans KR, Hunt GB. Experimental evaluation of four methods of progressive venous attenuation in dogs. Vet Surg 1999;28:38-47. 10. Van Vechten BJ, Komtebedde J, Koblik PD. Potential for spontaneous, progressive, postoperative attenuation of blood flow through partially ligated single extrahepatic portosystemic shunts as assessed by transcolonic portal scintigraphy. J Am Vet Med Assoc 1994;204:17701774. 11. Daniel GB, Bright R, Ollis P, et al. Per rectal portal scintigraphy using Tc99 pertechnetate to diagnose portosystemic shunts in dogs and cats. J Vet Intern Med 1991;5:23-27. 12. Komtebedde J, Koblik PD, Breznock EM, Harb M, Garrow LA. Long-term clinical outcome after partial ligation of single extrahepatic vascular anomalies in 20 dogs. Vet Surg 1995;24:379-383. 13. Hottinger HA, Walshaw R, Hauptman JG. Long-term results of complete and partial ligation of congenital portosystemic shunts in dogs. Vet Surg 1995;24:331-336. 14. Johnson SE. Portal hypertension. Part I. Pathophysiology and clinical consequences. Comp Cont Ed Sm Anim Pract 1987;9:741-748. 15. Richardson PDI, Withrington PG. Liver blood flow: intrinsic and nervous control of liver blood flow. Gastroenterol 1981;81:356-375.