Page 710 - Clinical Small Animal Internal Medicine

678 Section 7 Diseases of the Liver, Gallbladder, and Bile Ducts

on the cause of the glucocorticoid excess. In cases in Hepatic Lipidosis (Steatosis) in Cats
VetBooks.ir which it is not possible to identify the cause of glucocor- Etiology/Pathophysiology
ticoid excess, over time fibrosis may develop, leading to
One important difference between dogs and cats is the
worsening liver disease.
relatively low capacity of the liver enzymes responsible
for conjugation of cholephilic substances, used to make
Hepatic Lipidosis Secondary to Diabetes them more polar prior to biliary or urinary excretion, in
Mellitus
the cat. The low hepatic glucuronyl transferase capacity
Etiology/Pathophysiology is important for the metabolism of endogenous products
The liver plays a central role in fat metabolism. In diabe- such as bilirubin and exogenous compounds, either
tes mellitus, there is increased lipolysis in adipose tissue drugs or toxins. The elimination of drugs like chloram-
and therefore increased delivery of fatty acids to the liver, phenicol, nonsteroidal antiinflammatory drugs and anti-
while the production of triglycerides is also increased. convulsants depends on this function. Hence, toxic
Fat accumulates in the liver cells and in the dog it is side‐effects develop much more easily in cats in cases of
microvesicular, mainly caused by accumulation of tri- overdosage or reduced liver function.
glycerides. This fatty change begins around the central Cats also have different amino acid requirements from
veins and progresses toward the periphery of the lobule. dogs, as they cannot synthesize a sufficient number of
This results in the development of hepatomegaly. the essential amino acids (arginine, taurine, and carni-
Sometimes there is cholestasis, which can be quite tine). Arginine is an important intermediate metabolite
severe. There is much more severe liver dysfunction with in the hepatic urea cycle in which ammonia is being con-
fatty change than with glycogen accumulation. In verted into urea. This causes a relatively high susceptibility
humans, the chronic fatty change that occurs in diabetes to develop hyperammonemia and hepatic encephalopa-
mellitus can lead to hypertrophic liver cirrhosis, but in thy in cases of inadequate protein intake, for example
dogs and cats this is very rare. anorexia. Cats, unlike dogs, may have hepatic encepha-
lopathy with parenchymal liver disease without porto-
History and Clinical Signs systemic collateral circulation. Arginine and taurine are
Fatty change in the liver is usually present in diabetes involved in the hepatic production of apoproteins, which
mellitus and it can occur without causing clinical signs. are used to form the very low‐density lipoproteins. These
It can also cause mild to severe intrahepatic cholestasis. lipoproteins are the form in which the liver can secrete
In this case, physical examination reveals hepatomegaly triglycerides into the circulation for further metabolism.
and icterus. All of the liver enzymes can be mildly to Insufficient availability of arginine and taurine may lead
severely increased. The clinical signs are those related to to accumulation of triglycerides in the liver, for example
diabetes mellitus, namely polyuria, polydipsia, weight in the case of fasting. Taurine also plays a role in detoxi-
loss, and an increased appetite. fication, as it is used by the liver as a conjugating group to
make substances such as bile acids more hydrophilic.
Diagnosis Taurine is also involved in glutathione metabolism,
Liver histology demonstrates the fatty change, which is which is one of the major routes of hepatic detoxifica-
confirmed in combination with the diagnosis of diabetes tion. Carnitine is an intermediate enabling beta‐oxida-
mellitus (persistent hyperglycemia, glucosuria).
tion of long chain fatty acids in mitochondria, and this
function may be impaired in anorexia.
Therapy Fat accumulation in the liver occurs in different spe-
Therapy of fatty change in the liver of dogs is not neces- cies, as a result of metabolic conditions in which there is
sary. Regulation of the diabetes mellitus leads to a grad- increased deposition or impaired removal of triglycer-
ual disappearance of the fatty change.
ides in the liver. Clinically, this may be of importance in
diabetes mellitus as discussed for dogs. Subclinical fat
The Liver in Hyperthyroidism
accumulation, only detectable with histologic examina-
Hyperthyroidism is associated with fatty infiltration of tion of the liver, occurs in many other conditions such as
the hepatocytes. This does not cause clinical signs of high intake of calories, hypoxia of the liver and induction
liver dysfunction, but in many cases the liver enzymes by a number of toxic substances. Cats, however, display
and bile acid levels are abnormally high. The fatty an idiopathic form of hepatic lipidosis that may become
changes are not severe and there is no liver enlargement. life‐threatening. Accumulation of triglycerides is the
A number of clinical signs associated with hyperthyroid- result of a disturbed balance between triglyceride uptake
ism mimic those of primary liver diseases: polyuria, by the liver and their removal. The latter is normally
vomiting, diarrhea, and weight loss. achieved by formation of lipoproteins (especially very