Gamma Scintigraphy and Portosystemic Shunts

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General Principles
Nuclear medicine procedures are performed after administration of a radiopharmaceutical with special affinity to the organ or structure imaged. The radionuclide used is most commonly Technetium 99m (99mTc), which can be labeled with different molecules. 99mTechnetium-MethyleneDiPhosphonate (99mTc-MPD) is used for bone scan and transcolonic portal scintigraphy. The radiopharmaceutical injected accumulates in the target organ after a certain time 2 to 4 hours for a bone scan and immediately for transcolonic portal scintigraphy. Activity is then detected using the gamma camera and images displayed on the computer. Nuclear medicine images are usually of poor resolution, but provide unique information about specific organs.

What is a portosystemic shunt?
In animals with a portosystemic shunt (PSS) there is abnormal blood flow in the liver. Blood should flow from the digestive tract to the liver via the portal system into the blood vessels of the liver, and then to the caudal vena cava, which is the large blood vessel carrying blood back to the heart. In a portosystemic shunt, as the name implies, portal blood bypasses the liver and goes directly to the systemic venous circulation (caudal vena cava). One important function of the liver is to clear toxins, many of which are by-products of protein digestion, from the blood. In PSS, these toxins are not cleared, and circulate in the body. This causes the clinical signs associated with PSS, many of which are neurological. The complex of neurological and behavioural signs caused by liver dysfunction is called hepatic encephalopathy. Portosystemic shunts may be acquired secondary to another disease, or they may be congenital - that is the animal is born with a shunt. A congenital shunt usually occurs as a single abnormal blood vessel that is a remnant of normal embryonic development. These shunts are defined as intra-hepatic or extra-hepatic depending on the location of the blood vessel in relation to the liver. Most animals with congenital portosystemic shunts show clinical signs before 6 months of age. Where signs are subtle, the condition may not be diagnosed until much later.

Breeds affected by PSS | Protocols for PSS
Thames Valley Veterinary Services
London, Ontario
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