Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

VI. Diagnostic Laboratory Methods for the Evaluation of Neuromuscular Disorders
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Combined determination of CK and AST can be of
value in assessing the course of rhabdomyolysis ( Fig. 15-8 ).
Elevations in AST activities are present for weeks after the
onset of clinical disease, whereas CK activities remain elevated
for only a few days after myonecrosis. The course of
elevations of these enzymes in this disease can be directly
attributed to different half-lives and disappearance rates of
their activity in the plasma. Although CK is more specific for
myonecrosis than AST, the simultaneous determinations of
AST and CK in the horse are potentially valuable diagnostic
and prognostic aids owing to the different disappearance
rates of their serum or plasma activities: (1) elevated CK
activities indicate that myonecrosis is active or has recently
occurred; (2) persistent elevations of CK indicate that myonecrosis
continues to be active; and (3) elevated AST resulting
from myonecrosis accompanied by decreasing or normal
CK activities indicates that myonecrosis is no longer active.
3 . Lactate Dehydrogenase
Lactate dehydrogenase (LDH) activities are high in various
tissues of the body. Therefore, measurements of LDH are not
organ specific. Molecules of LDH are tetrameric, made up of
four subunits of the two parent molecules, M (muscle) and
H (heart). Various combinations of those subunits result in
five isoenzymes of LDH, which can be separated by electrophoresis.
The M monomer is found in purest form in skeletal
muscle as the isoenzyme M4 (or LDH5), whereas the H
monomer is found predominantly in the heart muscle as the
isoenzyme H4 (or LDH1). The other three forms are molecular
hybrids forming the isoenzymes M3H (or LDH4), M2H2
(or LDH3), and MH3 (or LDH2), and they are found in various
amounts in different organs. The H4 isoenzyme is maximally
active at low concentrations of pyruvate and strongly
inhibited by excess pyruvate, which favors the oxidation of
lactate ( Dawson and Romanul, 1964 ). The M form, on the
other hand, maintains activity at relatively high pyruvate concentrations,
which favors anaerobic reduction of pyruvate
( Dawson and Fine, 1967 ). Thus, tissues with essentially aerobic
metabolism, such as heart muscle, contain mostly heartspecific
isoenzymes, whereas tissues with more viable or
flexible metabolic properties, such as skeletal muscle, contain
predominantly the muscle-specific isoenzyme. Elevated LDH
activities have been reported in numerous muscle disorders
characterized by myonecrosis as well as a variety of hepatic
disorders. Therefore, unless isoenzyme analysis is utilized,
the measurements of LDH elevations are not organ specific.
B . Muscle-Specific Serum Proteins and
Antibodies
With the advent of immunochemical procedures such
as enzyme-linked immunosorbent assay (ELISA), radial
immunodiffusion assays, radioimmunoassays, and
immunocytochemical assays, sensitive tests for the detection
of tissue-specific proteins and antibodies in serum are
being applied to the diagnosis of neuromuscular disorders.
1 . Myoglobin
Myoglobin is a 17, 500-Da heme protein that stores and
transports oxygen in myofibers. Elevated levels of myoglobin
have been found in myopathies of humans ( Sieb and
Penn, 2004 ). The specificity of myoglobin for skeletal and
cardiac muscle and its plasma clearance make myoglobin
determinations a potentially effective method for monitoring
myonecrosis ( Holmgren and Valberg, 1992 ). In horses,
myoglobin concentrations peak shortly after myonecrosis
occurs, and clearance from the blood stream is more rapid
than CK activity ( Valberg et al. , 1993b ).
2 . Troponin
Troponin I assays have been used to identify myocardial
degeneration in several species ( Burgener et al. , 2006 ; Fuchs
et al. , 1999 ; Slack et al. , 2005 ). These assays offer improved
specificity and sensitivity for differentiating myocardial versus
skeletal myofiber necrosis. However, a false-positive
rate of 17% may still occur when using troponin I to assess
myocardial necrosis in patients with marked rhabdomyolysis
(Li et al. , 2005 ).
3 . Carbonic Anhydrase III
Serum carbonic anhydrase III has been proposed as a marker
for rhabdomyolysis. It increases and decreases in concentration
more rapidly than creatine kinase ( Nishita et al. , 1995 ).
4 . Acetylcholine Receptor Antibodies
Detection of circulating autoantibodies to acetylcholine
receptors (AChR) is a valuable adjunct to the diagnosis
of immune-mediated myasthenia gravis (MG) in humans
( Engel and Hohfield, 2004 ) and dogs ( Dewey et al. , 1997 ;
Palmer, 1977 ; Pflugfelder et al. , 1981 ). It has been estimated
that approximately 80% of human patients with MG
have detectable AChR antibodies ( Engel and Hohfield,
2004 ). Positive serum antibody titers to acetylcholine receptors
measured by immunoprecipitation radioimmunoassay
are normally less than 0.6 nmol/l ( Shelton et al. , 1990 ). A
valuable immunocytochemical screening test for circulating
AChR antibodies in canine MG employs staphylococcal
protein A conjugated to horseradish peroxidase (SPA-HRP),
a reagent that localizes IgG. Control sections of muscle containing
neuromuscular junctions when incubated with canine
MG patient sera and subsequently with SPA-HRP localizes
IgG at neuromuscular junctions ( Pflugfelder et al. , 1981 ).
This immunoreagent has also served to detect antinuclear
antibodies, antistrial antibodies, and sarcolemmal-associated
antibodies in immune-mediated myasthenia gravis and