Definition

• Congenital PSVA-are venous malformations connecting the portal and systemic circulations permit portal blood to circumvent the liver (hepatofugal circulation, away from the liver).
• May be extrahepatic (more common in small breed dogs and cats) or intrahepatic (more common in large-breed dogs).
• Most are single vessels.
• Many small-breed dogs with PSVA also have vascular abnormalities involving tiny branches of the intrahepatic microvasculature (see Hepatoportal Microvascular Dysplasia).
• Acquired portosystemic shunts (APSS)-develop subsequent to portal hypertension (PH) and result if the patient is intolerant to shunt attenuation (surgical); see Portosystemic Shunting, Acquired.
• Genetically transmitted in high-risk small breeds; complex polygenic trait.

Pathophysiology

• Clinical signs-caused by hepatofugal circulation; prohibits hepatic cleansing of enteric toxins and food derived nitrogenous toxins substances from portal blood.
• Microhepatia-deprivation of enteric hepatotrophic factors causes hepatic atrophy.
• Episodic HE-associated with high-protein food, gastrointestinal bleeding, dehydration, azotemia, alkalosis, electrolyte disturbances, blood transfusion, hemolysis, infections, constipation, catabolism, and certain drugs.
• Ammonium biurate crystalluria/urolithiasis-caused by hyperammonemia and impaired transformation of uric acid to water-soluble allantoin; may be presenting problem.
• Clinical signs reflect the magnitude of shunting through anomalous vasculature.

Systems Affected

• Nervous-episodic HE in many but not all.
• Gastrointestinal-intermittent inappetence; vomiting; diarrhea; pica; ptyalism (cats).
• Urogenital-“plump” kidneys; ammonium urate urolithiasis; 50% of male dogs cryptorchid (one report).
• Asymptomatic-up to 20% of dogs.

Genetics

• Breeds affected-e.g., most small-breed dogs: esp., Yorkshire terrier, cairn terrier, Maltese, Tibetan spaniel, miniature schnauzer, Norfolk terrier, pugs, Shih Tzu, Havanese, Papillon; large breeds: Irish wolfhound; Labrador retriever, Old English sheepdog.
• Autosomal dominant polygenic complex trait causes PSVA/MVD trait in small breeds; MVD most common phenotype.

Incidence/Prevalence

0.2–0.6% of large referral clinic population

Geographic Distribution

Reported worldwide

Signalment

Signs

Causes

• Congenital malformations.
• APSS in congenital PSVA-develop subsequent to portal atresia (cats > dogs) or surgically induced PH (from shunt attenuation).

Risk Factors

PSVA-purebred dogs, especially small terrier-type breeds; Irish wolfhound have slow ductus venosus closure, early puppy screening in this breed may erroneously identify apparently “affected” dogs.

Differential Diagnoses

• CNS signs-infectious disorders (e.g., FIP, canine distemper, toxoplasmosis, FeLV- or FIV-related infections); toxicities (e.g., lead, mushrooms, recreational drugs); hydrocephalus; idiopathic epilepsy; metabolic disorders (e.g., severe hypoglycemia, hypokalemia or hyperkalemia, hypocalcemia).
• Gastrointestinal signs-bowel obstruction; dietary indiscretion; foreign body ingestion; inflammatory bowel disease.
• Urinary tract signs-bacterial urinary tract infection; urolithiasis.
• PU/PD-disorders of urine concentration (e.g., diabetes insipidus, abnormal adrenal function, hypercalcemia, primary polydipsia); associated with high GFR in PSVA.
• Primary liver disease-distinguished by diagnostic imaging and liver biopsy.
• Abnormal liver function (high TSBA) suggests PSVA, without clinical signs or clinicopathologic features or macroscopic shunt-most likely hepatoportal MVD.
• APSS-many differentials; see Portosystemic Shunting, Acquired.

CBC/Biochemistry/Urinalysis

• CBC-microcytosis; mild nonregenerative anemia; poikilocytosis (cats); target cells (dogs).
• Biochemistry-low BUN, creatinine, and cholesterol common but inconsistent; hypoglycemia-least common, liver enzyme activity variable (ALP high in young patients [bone isoenzyme]); bilirubin normal, hypoalbuminemia inconsistent and mild.
• Urinalysis-dilute urine (PU/PD); ammonium biurate crystalluria, hematuria, pyuria, and proteinuria: mechanical inflammation and infection secondary to metabolic calculi.

Other Laboratory Tests

• TSBA-sensitive indicator of PSS; random fasting values may be within reference intervals; 2 h postprandial values usually markedly high (>100 µmol/L); always use paired samples around meal ingestion, no need to fast.
• Blood ammonia-sensitive indicator of HE and shunting but less reliable than TSBA in practice (analytic/methodologic issues [samples cannot be frozen or mailed for analysis]); ammonia tolerance testing-oral or rectal NHCl₄ administration demonstrates ammonia intolerance. Note: may cause transient HE.
• Coagulation tests-prolonged clotting times compared to healthy dogs but not clinically significant; not associated with bleeding.
• Protein C-low values help differentiate PSVA from MVD; protein C <70% in PSVA, usually >70% in MVD; reflects shunting magnitude. Protein C may be normal in asymptomatic PSVA dogs; used to estimate improved portal flow postoperatively. TSBA often remains abnormal due to concurrent MVD. Not similarly used in cats.
• Abdominal effusion-as post-surgical complication reflects iatrogenic portal hypertension; cytologic/physicochemical analysis: pure or modified transudate

Imaging

Diagnostic Procedures

• Fine-needle aspiration cytology-cannot differentiate PSVA; binucleate small hepatocytes common.
• Liver biopsy-open surgical wedge or laparoscopic cup samples necessary, biopsy of several liver lobes (avoid caudate lobe: best perfused lobe receives circulation from first portal vein branch; fewest lesions); needle core samples may be inadequate for definitive diagnosis of portal hypoperfusion.

Pathologic Findings

• Gross-small, smooth-surfaced liver; PSVA may be difficult to verify at autopsy; young animals with portal atresia may have APSS.
• Portal hypoplasia: invalid microscopic diagnosis; lesion best termed “portal hypoperfusion” as portal thrombi, portal vein ligation, surgically created PSS-have histologic features identical to PSVA.
• Microscopic-small non-perfused or absent portal venules; multiple portal arteriole cross-sections; juvenile portal triads (tiny, too many), lobular atrophy, variable scattered lipogranulomas (coalesced foamy macrophages contain hemosiderin-iron); thick prominent throttling musculature of hepatic venules; unusual longitudinal hepatic venule profiles; engorged lymphatics; some dogs with severe zone 3 lipid vacuolation, lipogranulomas, and nonsuppurative inflammation shrouding hepatic venules, appear intolerant to complete PSVA ligation.
• Note: MVD has histologic features identical to PSVA but lack macroscopic shunt. MVD diagnosis requires clinical information (lack of signs, absence of clinicopathologic features typical of PSVA), and scintigraphy or vascular imaging (usually not justified).

Appropriate Health Care

Inpatient-severe signs of HE; medical intervention prior to liver biopsy and ligation.

Nursing Care

See Hepatic Encephalopathy

Diet

• Nutritional support-essential to maintain body condition as muscle functions as a site of temporary ammonia detoxification.
• Balanced protein-restricted diet-use commercial canine liver diet for dogs and feline liver diet for cats as baseline diet. To this diet, additional protein allocations are titrated based on patient response) in combination with treatments ameliorating HE.
• Dogs-as tolerated, add 0.5 g protein/kg body weight using dairy quality protein (e.g., cottage cheese, cheddar cheese, yogurt): observe over 5–7 day intervals to adjust (see Hepatic Encephalopathy).
• Asymptomatic or minimally symptomatic animals can survive well with dietary and medical interventions.

Client Education

• Explain therapeutic options: medical vs. surgical.
• Surgical ligation-has potential to cure; expect improvement in all symptomatic dogs that tolerate some degree of shunt attenuation.
• Postoperative clinical signs-may persist requiring chronic nutritional and medical management; some dogs can have medical and dietary interventions withdrawn.
• Clinical improvement after surgical ligation despite persistent high TSBA values.
• Surgical/anesthetic risks-5–25% mortality; depending on surgeon, type of PSVA, hepatic microscopic lesions, supportive critical care.
• Monitoring protein C-documents change in hepatic portal perfusion; values increase and may normalize if successful surgery; re-test 2–6 months after surgery.
• If surgery not pursued or ligation not tolerated: remain vigilant for ammonium biurate obstructive uropathies (all levels of urinary system); urethral obstruction (males) may require permanent urethrostomy.

Surgical Considerations

• HE-should be mitigated with medical management before anesthesia and surgery.
• ICU monitoring-recommended postoperatively for 72–96 h.
• Surgical PSVA ligation-optimal goal is total ligation; may not be tolerated leading to APSS (esp. ameroid or cellophane banding).
• Partial ligation: improves patient health.
• Degree of tolerated PSVA closure judged at surgery: physiologic responses to temporary shunt occlusion (change in portal/systemic blood pressure, splanchnic circulation, pulse rate); but intraoperative assessments can be inaccurate.
• Ameroid constrictor-reduces immediate surgical risks of ligation; caution: may later result in APSS in some patients.
• Intrahepatic PSVA-most difficult to ligate; coil embolectomy is alternative procedure, may require multiple procedures, expensive.
• Intraoperative portography advised for all patients-verifies correct vascular ligation.
• Postoperative complications-acute severe PH; portal venous thrombi; mesenteric ischemia; endotoxemia; seizures; sepsis; acute pancreatitis; enteric hemorrhage.
• Intra- and postoperative hypothermia-risk for very small patients; prolongs recovery.
• Emergency surgery-rarely required for ligature removal; ameroid constrictors difficult to remove if complications develop.
• Abdominal effusion-transient after shunt ligation; alone, does not indicate intolerable PH if resolves within 7 days; serious PH indicated by signs of mesenteric ischemia (bloody diarrhea, abdominal pain, failure to recover from surgery/anesthesia, unexplained tachycardia, hyper- or hypothermia); monitor girth and body weight postoperatively.
• Hetastarch can increase bleeding risks.
• Blood component therapy containing ACD may provoke hypocalcemia and coagulopathy (hypercitratemia) in patients <5 kg.
• Intractable seizures in <5% dogs post-ligation; cause unclarified, treatment for cerebral edema and sedation required; no rigorous clinical evidence that prophylactic KBr or Keppra is beneficial; CRI propofol unreliable; flumazenil (5–10 µg/kg IV bolus) anecdotally suggested but is unreliable for resolution of neurologic signs.

Drug(s)

See Hepatic Encephalopathy

Patient Monitoring

• Reevaluate-patient's at-home behavior; body condition, weight, girth circumference (postoperatively); CBC (resolution of microcytosis), biochemistry (resolution of low cholesterol, BUN, creatinine), urinalysis (resolution of ammonium biurate crystalluria).
• TSBA (small-breed dogs)-persistent high values do not substantiate surgical failure because of common coexistent MVD.

Possible Complications

See “Surgical Considerations”

Prevention/Avoidance

Multiple portosystemic shunts-should be detected by US; if identified indicate need for liver biopsy but contraindicates pursuit of shunt ligation; implicates portal atresia or another underlying disorder causing PH; PSVA is not associated with PH and APSS.

Expected Course and Prognosis

• Cannot predict individual response to surgery.
• Dogs-ligation improves signs in ∼80% symptomatic patients.
• Cats-many develop APSS with ligation.
• Post-surgery-continue HE management (especially diet) until reevaluation and observation confirm improvement.
• Some patients-require indefinite nutritional and medical support after surgical ligation.
• Partial ligation-may result in full shunt attenuation (granulation response to ligature).
• Ameroid constrictor-may result in full ligation within a few days (twisting after placement, thrombi); may cause APSS.
• Increased risk for poor surgical outcome-in small dogs with zone 3 lipogranulomas/lipid vacuolation shrouding hepatic venule; and cats.
• Despite initial good response, recrudescent shunting may develop 3 years post-ligation.
• Asymptomatic non-ligated PSVA dogs can have a full life expectancy with medical management.

Associated Conditions

• Ammonium urate urolithiasis
• Copper-colored iris (cats)
• Cryptorchidism (dogs)
• Hepatic encephalopathy

Age-Related Factors

• PSVA-surgical outcome may be good in young and old patients, especially those with minimal signs of HE.
• Dogs with PSVA identified at older ages are usually asymptomatic for HE and may have easily attenuated shunts (portoazygous).

Pregnancy/Fertility/Breeding

• Asymptomatic bitches can carry litters to term.
• Asymptomatic dogs have been used for stud.
• Breeding PSVA dogs not advised (genetic).

Synonym

Portacaval shunt

See Also

• Hepatic Encephalopathy
• Hepatoportal Microvascular Dysplasia
• Juvenile Fibrosing Liver Disease
• Portosystemic Shunting, Acquired

Abbreviations

• APSS = acquired portosystemic shunt
• CRS = colorectal scintigraphy
• HE = hepatic encephalopathy
• MVD = microvascular dysplasia
• PSVA = portosystemic vascular anomaly
• SPS = splenoportal scintigraphy
• TSBA = total serum bile acids
• US = ultrasound

Author Sharon A. Center

Consulting Editor Sharon A. Center