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==Description==
==Description==
'''Hepatic encephalopathy''' (HE) is characterised by a complex of neurological abnormalities that occur due to congenital or acquired abnormalities in hepatic structure or function. HE is usually associated with some form of [[Portosystemic Shunt|'''portosystemic shunt''']] (PSS) in dogs and cats, although it may also be caused by a marked reduction in functional hepatic mass. Other causes of HE, such as [[Hepatic Microvascular Dysplasia|'''microvascular dysplasia''']], '''congenital defects of enzymes of the urea cycle''', '''arginine deficiency''' or '''organic acidaemias''', are very rare or of only experimental interest. The clinical signs of HE are associated with increases in the blood concentration of several metabolites:
'''Hepatic encephalopathy''' (HE) is characterised by a complex of neurological abnormalities that occur due to congenital or acquired abnormalities in hepatic structure or function. HE is usually associated with some form of [[Portosystemic Shunt|'''portosystemic shunt''']] (PSS) in dogs and cats, although it may also be caused by a marked reduction in functional hepatic mass. Other causes of HE, such as [[Hepatic Microvascular Dysplasia|'''microvascular dysplasia''']], '''congenital defects of enzymes of the urea cycle''', '''arginine deficiency''' or '''organic acidaemias''', are very rare or of only experimental interest. The clinical signs of HE are associated with increases in the blood concentration of several metabolites:
*'''Ammonia''' is a by-product of protein degradation which is transported in the portal vein and metabolised to urea in the liver via the urea cycle. In animals with PSS, ammonia is not removed from the portal blood as this bypasses the liver, leading to an increase in its blood concentration. Ammonia is generated in normal animals through the following physiological processes:
*'''Ammonia''' is a by-product of protein degradation which is transported in the portal vein and metabolised to urea in the liver via the urea cycle. In animals with PSS, ammonia is not removed from the portal blood as this bypasses the liver, leading to an increase in its blood concentration. For ammonia generation in normal animals [[Ammonia|see here]].
*'''Mercaptans''' are products of the bacterial degradation of the amino acid methionine in the colon which are usually removed from the portal blood in the liver.
*'''Mercaptans''' are products of the bacterial degradation of the amino acid methionine in the colon which are usually removed from the portal blood in the liver.
*'''Aromatic amino acids''' may be involved especially when the ratio of aromatic to branched chain amino acids is very high. This is considered to be a significant component in the pathogenesis of HE in horses.
*'''Aromatic amino acids''' may be involved especially when the ratio of aromatic to branched chain amino acids is very high. This is considered to be a significant component in the pathogenesis of HE in horses.