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Review Article: The Diagnosis and Management of Portosystemic Shunts

Here we review the options for diagnosis and management of congenital portosystemic shunting (PSS).

Signalment:

The clinical signs associated with congenital portosystemic shunting usually develop at a young age (often 6m-2y) but, in mild cases, signs can be delayed and the diagnosis should also be considered in slightly older dogs presenting with a consistent history.  Small breed dogs (eg.  Terriers and Miniature Schnauzers) and cats commonly have extrahepatic shunts while large breeds (eg. Golden Retrievers, Border Collies, Old English Sheepdogs) usually have intrahepatic shunting vessels.

History and Clinical Signs:

  • Failure to thrive (poor growth, poor body condition score)
  • CNS signs – primarily forebrain signs related to hepatic encephalopathy (e.g. lethargy, head pressing, blindness, seizures, coma)
  • Gastrointestinal – chronic vomiting/diarrhoea, pica
  • Urinary tract – polyuria/polydipsia, dysuria (secondary to ammonium urate urolithiasis causing cystitis and occasionally leading to urinary obstruction in males).
  • fig 1. Lateral radiograph of a normal IMPV, showing good opacification of hepatic portal vasculature with no contrast visible in either the PVC or azygous vein

    Poor metabolism of drugs (in particular, slow anaesthetic recovery)

Diagnosis:

Serum biochemistry – low urea, albumin, glucose and cholesterol levels are often noted as a result of reduced functional liver mass. Liver enzymes can be elevated but results within the normal range are not uncommon and ALT can sometimes be reduced.

Haematology – microcytosis is often present but this may be absent in postal samples as a result of post-sampling erythrocyte swelling

fig.3 - lateral view of a IMPV showing a portocaval shunt with post-shunt dilatation and opacification of PVC

Bile acid stimulation test – elevation of serum bile acid levels will be present in all cases. Pre-prandial levels are often markedly elevated but, if normal, a two-hour post-prandial sample is required to confirm hepatic dysfunction.

Ammonia – for a valid result blood ammonia levels must be assayed as soon as the blood is drawn (in-house testing kits are available).  Elevations confirm encephalopathy but levels within the normal range do not exclude a diagnosis of PSS.

Portal scintigraphy – although a sensitive test for PSS this is only available

fig.2 - ventrodorsal radiographic view of a IMPV showing a portocaval shunt with opacification of PVC

at specialist centres and does not define the shunt anatomically.

Ultrasonography – This is an extremely useful test for PSS but is highly operator dependant. Intrahepatic shunts tend to be easier to visualise than extrahepatic vessels.

fig.4 - Lateral view of a IMPV showing multiple shunt vessels which empty into the left renal vein

Intraoperative Mesenteric Portovenography (IMPV, figs. 1,2,3 &4 – click images to enlarge) – this usually results in good imaging of the shunt vessel but necessitates anaesthesia with coeliotomy to exteriorse and cannulate a mesenteric vessel and therefore carries with it the inherent morbidity and risks of surgery. It is usually performed to confirm shunt anatomy prior to surgical ligation.

Advanced imaging (CT and MRI) can give clear visualisation of shunt-vessel anatomy which aids surgical planning (G. Bertolini, et al., see figs. 5-7).

Therapeutic Plan:

fig.5 - dorsal T1W MRI scan of case in fig.3, inserting into PVC cranial to left renal vein

All cases presenting with neurological signs should be stabilised prior to considering surgical intervention. To this end, oral antibiotics and lactulose are used to reduce intestinal ammonia production and absorption respectively. Feeding of a ‘hepatic diet’ is also advocated.

fig.6 - transverse T1W MRI scan of case illustrated in figs.3 and 5

However, excessive protein restriction should be avoided, particularly in cases with significant hypoalbuminemia, and supplementation with high quality protein (eg. cottage cheese) should be used where required to prevent catabolism. In patients showing severe signs of hepatoencephalopathy more aggressive therapy should be instigated. Lactulose enemas can be used to quickly reduce gut ammonia absorption and anti-convulsant therapy should be instigated. Intravenous phenobarbitone loading is often used but, given the reduced hepatic function, conservative doses are usually advocated.  It is important that any

fig.7 - sagittal T1W MRI scan of unusual shunt vessel coursing dorsally to stomach which inserted into the PVC cranial to the diaphragm

dehydration, hypoglycaemia and electrolyte abnormalities are addressed concurrently. Once the patient has been stabilised and the shunting vessel has been characterised medical therapy can be continued long term or surgical treatment may be offered.

Surgical Management:

The surgical treatment options are dependant on the shunt vessel anatomy and on portal pressure implications. Most extrahepatic shunts can be identified and ligated using standard techniques (see below). Surgical management is possible for intrahepatic shunts but this is a highly involved procedure which is only offered at a small number of institutions. There have been several strategies advocated to attenuate solitary, congenital, aberrant vessels;

  • Direct ligation with non-absorbable ligature (usually silk) with concurrent portal pressure manometry.

Complete ligation carries a significant risk of acute portal venous hypertension which, if not immediately corrected, can result in the death of the patient.  Complete abandonment of the procedure or partial occlusion is necessary if this complication is encountered, however, the long-term results are not as good as with complete occlusion. Note that, in practice, direct assessment of portal pressure (using water column manometry) seems to be as subjective as assessing small intestinal congestion when the shunt vessel is occluded, and is subject to numerous variables (including the cardiovascular effects of anaesthesia).

  • Slow attenuation with cellophane-banding or ameroid constrictors (fig.8).

fig.8 - ameroid constrictor with 'key' in position

Both these techniques slowly occlude the shunting vessel over several weeks, thereby reducing the risk of portal hypertension.  Here at Down’s Veterinary Referrals we use ameroid constrictors (see fig.9 from case fig.7).  It is important that the constrictor fits snugly over the vessel and a range of sizes should therefore be kept in stock; too tight and it may reduce the luminal diameter too quickly, too loose and similarly it may flip over and occlude the vessel acutely. (It is also useful to keep a spare ‘key’ handy as these are very small and fiddly to fit, and can easily be lost, either inside or outside the patient!).

fig.9 - intraoperative photograph showing constrictor in place across the shunt vessel

The success rate for resolution of clinical signs following slow-occlusion devices on extrahepatic shunts is approximately 80%, with the remainder requiring some additional life-long medical support.  Since complete occlusion may not possible in a significant proportion of cases, it is likely that many patients subsequently develop multiple acquired shunts in response to the surgery.

Postoperative Management:

Due to the delayed closure of the shunting vessel, medical management is continued post-operatively as above. Where possible, treatment is then phased out in a staged manner over several months.  Intermittent monitoring of liver parameters is recommended to assess response to surgery but patients should be primarily assessed based on the presence or absence of ongoing clinical signs.

Reference:

G. Bertolini, et al.  Three-Dimensional Multislice Helical Computed Tomography Techniques for Canine Extra-hepatic Portosystemic Shunt Assessment. Veterinary Radiology & Ultrasound 47:439-443, 2006.