Page 905 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

880 CHAPTER 5

VetBooks.ir relate to the remaining effective functional cellular concentrations of liver enzymes may be associ-
ated with a poorer prognosis, this association is not
mass, rather than to those cells that have been dam-
aged. Thus, serum enzymes generally offer an indi-
centrations and perhaps, more consistently, with cer-
cation of ongoing hepatic insult whereas functional strong. It may only apply to markedly increased con-
indicators reflect the impact that the hepatic damage tain enzymes such as GGT and ALP, rather than
has had. GLDH and AST. There may be additional useful
The potential restriction or diversity of tissue differential information in the pattern of enzyme
sources of serum enzymes clearly impacts greatly abnormalities, in that GGT and ALP are generally
on diagnostic usefulness. Enzymes such as aspar- regarded as reflecting damage primarily to the biliary
tate aminotransferase (AST) and lactate dehydroge- epithelium, whereas enzymes such as GLDH, SDH,
nase (LDH) may come from multiple tissue types, LDH and AST are generally regarded as reflecting
although liver and muscle appear to be the main parenchymal hepatocellular damage. Liver disease
origin of increased serum concentrations. Although in horses will almost invariably be associated with
glutamate dehydrogenase (GLDH) and sorbitol insult to both the hepatocellular and biliary popula-
dehydrogenase (SDH) are widely regarded as liver tions, but where there are clearly disproportionate
specific, this assumption lacks an evidence basis in effects on the two enzyme groups then diagnostic
horses and other tissues such as the gastrointesti- inferences can be made.
nal (GI) tract appear to be the source of increases A common clinical dilemma facing equine prac-
in serum concentrations in some cases. The liver is titioners is when serum biochemical indicators of
also widely regarded as the specific source of high liver disease are found in apparently healthy horses.
serum gamma-glutamyltransferase (GGT) concen- Under such circumstances there are two possible
trations, although it is also accepted that rare cases fundamental explanations: firstly, the laboratory
of pancreatic disease might contribute high serum results are correct and the horse does indeed have
GGT levels. Renal tubules are also known to be a liver disease, but that it is subclinical; and secondly,
rich source of GGT, alkaline phosphatase (ALP) and the laboratory results incorrectly imply hepatic
GLDH, although damage to such cells is assumed disease and the horse is either healthy or has non-
to result in enzyme appearance in urine rather than hepatic disease. As previously mentioned, subclinical
serum. There is a widely-recognised association liver disease is not a surprising or unusual concept,
between mild to moderately increased liver enzymes given the large reserve capacity of the liver and this
and primary GI diseases such as colon impaction, is certainly a common and plausible explanation for
colon displacement and gastric ulceration. Clinical apparently healthy horses to have increased serum
experiences support these observations, although concentrations of liver-derived enzymes. On the
it is unclear whether this might reflect primary GI other hand, given that reference intervals are gen-
sources of such enzymes, or whether a primary GI erally calculated to include approximately 95%
disease may secondarily cause hepatic insults via of a normal population, it should be expected that
physical pressure, or via injurious GI-derived sub- around 2.5% (1 in 40) of normal horses will show a
stances carried to the liver via the hepatic portal vein. value greater than the reference interval when a sin-
Although there is no disputing the diagnostic gle analyte is measured, with about 10% of normal
value of liver-derived enzymes in the evaluation of horses demonstrating at least one increased value
suspected hepatic insult, it is important to remem- out of a panel of four serum enzymes. However,
ber that no enzyme is entirely specific for liver dis- this statistical certainty can only be used to explain
ease, nor will it be found to be abnormal in every mild increases above the reference interval, as more
case of liver disease. Thus, such enzymes are useful marked increases will be extremely uncommon
alongside other diagnostic data to raise the suspicion in entirely normal horses. When higher values of
of liver disease in an individual horse, or group of serum enzymes (e.g. greater than 3 or 4 standard
horses, but clinicians should be constantly aware of deviations above the mean) are found in horses with
their diagnostic limitations. Although high serum no corroborative evidence of hepatic disease found in