This disorder occurs when the liver is unable to purify the blood properly. In particular, the liver serves to remove noxious substances from blood that has just passed through intestinal blood vessels. As it passes through the intestine the blood absorbs a number of potentially harmful substances along with nutrients, vitamins, and minerals. When the liver (i.e. hepatic) fails to remove these harmful substances they are liberated into the general circulation and can have a toxic effect on the brain (i.e. encephalopathy). Liver dysfunction can occur mainly as one of two types.
The first type is unique to animals and occurs where the animal is born with an abnormal blood vessel that takes blood from the intestines and delivers it directly into the general circulation without allowing it to go through the liver. This is often called a congenital shunt as the blood is shunted around the liver from birth onwards. In this case, the liver is healthy otherwise although it is small because it is deprived of much of its normal blood supply. Clinical signs may come on in the first months of life although in some cases the animal can be normal until later in life. This form of shunt can be corrected when a surgeon ties off the abnormal blood vessel. Some animals with congenital shunts have a less severe variant where there is actually no single large blood vessel shunting blood around the liver but rather the blood is hurried through the liver in a number of smaller highways that nevertheless prevent the liver cells from purifying it properly. This is called hepatic (i.e. liver) microvascular (i.e. the blood vessels are small compared to the more typical shunt) dysplasia (i.e. these blood vessels are not formed normally or are dysplastic). These types are normally managed using dietary modification.
The second type of shunt is when the liver is diseased and the resultant scarring squeezes the normal blood vessels so that they are unable to accommodate a normal amount of blood. This forces the blood to find other routes to travel and eventually much of the blood has to bypass the liver entirely. The end result is the same as in the first type of congenital shunting and blood is again delivered to the general circulation without being purified by the liver. Not surprisingly this is called an acquired shunt.
Clinical signs of hepatic encephalopathy can be quite variable but usually the animal shows signs of forebrain disease such as seizures, disorientation, marked depression and in some cases head pressing, pacing and coma. These are often precipitated by eating a meal that is high in protein although this is by no means always the case. Diagnosis is either by a bile acids test, identifying ammonium biurate crystals in the urine, or by an abdominal ultrasound examination. The condition must be differentiated from other causes of forebrain disease such as encephalitis in younger animals or a brain tumor in older animals. Treatment depends on whether the problem is congenital or acquired, the latter are generally much harder to treat. Congenital shunts are best treated by surgery that uses a device called an ameroid constrictor that slowly shuts down the shunting blood vessel thereby forcing the blood to take a normal path through the liver. Occasionally this will precipitate seizures and so the animal may be started on potassium bromide before surgery as a precaution against this. Otherwise, the prognosis is very good and the animal should be able to lead a normal life after surgery. Animals with hepatic microvascular dyplasia, and those with shunts that are acquired secondary to liver disease are usually managed by diet and medications.
The prognosis is good for most dogs with congenital shunts but is more guarded for those with acquired shunts. The main complication after surgery for a congenital shunt is the development of seizures in a small proportion of dogs and these may be hard to control. The prognosis is more guarded for cats with shunts.