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

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Chapter | 15 Skeletal Muscle Function
inflammatory muscle disorders in the dog ( Shelton and
Cardinet, 1987 ).
VII . MUSCLE BIOPSY AND
HISTOCHEMISTRY IN THE DIAGNOSIS OF
NEUROMUSCULAR DISORDERS
Muscle biopsies provide an excellent opportunity to evaluate
the integrity of myofibers, neuromuscular junctions,
intramuscular nerve branches, connective tissues, and blood
vessels in various neuromuscular disorders. As discussed
previously, histochemical examination of skeletal muscle
provides information relative to the morphological, biochemical,
and metabolic properties of myofibers. Therefore,
the application of histochemical techniques in conjunction
with routine light and electron microscopic examination of
muscle biopsies offers the potential to evaluate and integrate
the pathoanatomical, biochemical, and physiological manifestations
of neuromuscular disorders. Further, the advent of
immunocytochemistry has extended our ability to recognize
immunopathological mechanisms and disorders as well.
The application of histochemical techniques has become
an essential diagnostic procedure for the evaluation of neuromuscular
disorders in humans, dogs, cats, and horses
( Cardinet and Holliday, 1979 ; Dubowitz and Brooke, 1973 ;
Engel and Franzini-Armstrong, 2004 ; Schatzberg and
Shelton, 2004 ; Valberg, 1999 ). Their application has been
most helpful in determining which portion of the motor unit
(neuron, myofiber, or both) is involved in the disease process
or providing profiles specific for selected neuromuscular
disorders ( Dubowitz and Brooke, 1973 ). Neuropathic disorders
frequently produce angular atrophy of type 1 and type 2
muscle fibers. In contrast, myopathic disorders may be characterized
by angular atrophy of type 2 fibers, muscle fiber
necrosis, inflammatory infiltrates, or abnormal storage products.
Common histochemical stains used with frozen sections
include myosin ATPase stains for fiber typing, trichrome and
NADH staining of mitochondria, oil-red-O and periodicacid
Schiff’s staining for lipid and glycogen, respectively,
acid phosphatase stains for lysosomal storage products, and
a variety of stains for enzymatic activity of phosphorylase,
PFK, and cytochrome oxidase. Appropriate immunostains
can be used to identify deficiencies in interstitial components
such as dystrophin, to adherent immunoglobulin to end plates
or myofibers, as well as to subtype lymphocytic infiltrates.
A detailed consideration of muscle biopsy techniques in the
evaluation of neuromuscular diseases is beyond the scope of
this chapter. For details, refer to Dubowitz and Brooke (1973)
and Engel and Franzini-Armstrong (2004) .
VIII . MOLECULAR DIAGNOSTIC TESTING
Advances in the understanding of the genetic basis for a number
of myopathies in dogs, horses, swine, and cattle have led
to the development of new molecular diagnostic tests. These
tests are often performed on hair roots or blood samples and
distinguish the presence of a mutation (homozygous or heterozygous)
in a specific portion of a gene that has previously
been shown to be associated with a disease. A selection
of these genetic diseases is reviewed in Section IX.
IX . SELECTED NEUROMUSCULAR
DISORDERS OF DOMESTIC ANIMALS
Neuromuscular disorders in animals are associated with
spontaneous and inherited endocrine, immune-mediated,
infectious, toxic, metabolic, and neoplastic diseases. This
section presents a selection of neuromuscular disorders to
highlight the spectrum of acquired and genetic myopathies
found in domestic animals. With continued application of
advanced histochemical, biochemical, and molecular techniques
and sequencing of animal genomes, undoubtedly
many heretofore unrecognized neuromuscular disorders in
animals will be recognized.
A . Ion Channelopathies
1 . Acetylcholine Receptor Ion Channels and Myasthenia
Gravis
Myasthenia gravis is a disorder of neuromuscular transmission
in which there is a reduction in the number of ligandgated
AChR ion channels on the postsynaptic sarcolemmal
membrane (PSM). This condition results in weakness
because of the reduced sensitivity of the PSM to the transmitter,
ACh. Two basic forms of MG exist: (1) acquired
autoimmune MG and (2) congenital MG. Different mechanisms
are responsible for the reduction of AChRs in these
two disorders.
a . Acquired Autoimmune Myasthenia Gravis
Acquired MG is an immune-mediated disorder of humans
( Engel and Hohfield, 2004 ), dogs, and cats ( Dewey et al. ,
1997 ; Shelton, 2002 ; Shelton et al. , 2000 ) in which autoantibodies
are produced against AChRs. The density of AChRs
is reduced by the complement-mediated destruction, accelerated
internalization, and degradation of AChRs by crosslinking
of the receptors by antibody. In humans and dogs,
the autoantibody response is heterogeneous. Most antibodies
are IgG and directed against the main immunogenic region
(MIR), a specific external portion of the α-subunit that is
distinct from the ACh-binding site; however, autoantibodies
are also produced against all of the other subunits ( Engel
and Hohfield, 2004 ; Shelton, 1999 ). Only a small percentage
of antibodies is directed against the ACh-binding sites
on the α-subunits.
Dogs with MG usually exhibit some form of muscular
weakness; however, this can be quite variable and may