Page 895 - Adams and Stashak's Lameness in Horses, 7th Edition

Principles of Musculoskeletal Disease 861

or active myonecrosis; persistently elevated serum CK tissue. As muscle fibers regenerate, there is a progressive
indicates that myonecrosis is likely to be continuing. increase in echogenicity. Relatively hyperechoic regions
VetBooks.ir normal CK activity indicates that myonecrosis has causing shadowing artifacts reflect mineralization. 70
may develop due to fibrous scarring. Hyperechoic regions
Elevated AST activity accompanied by decreasing or
ceased. The degree of elevation of CK and AST does not
necessarily reflect the severity of clinical signs. Electromyography (EMG)
Elevations in plasma/serum myoglobin concentra­
tions indicate acute muscle damage because this small Electromyography (EMG) is particularly useful for
protein leaks into plasma immediately after muscle evaluating muscle atrophy, muscle fasciculations, and
damage and is rapidly cleared in the urine by the myotonia. Normal muscle shows a brief burst of electrical
kidney. 30 activity when the EMG needle is inserted and then quies­
cence unless motor units are recruited (motor unit action
potentials) or the needle is very close to a motor end plate
Exercise Response Test
(miniature end plate potentials). Spontaneous electrical
Diagnosing chronic exertional rhabdomyolysis may activity, fibrillation potentials, and positive sharp waves
be difficult in horses that have normal serum AST and are seen in horses with denervation and myotonic dis­
CK when examined between episodes. In such cases, an charges, or complex repetitive discharges are seen in
exercise challenge can be helpful in detecting subclinical horses with muscle conduction system abnormalities (e.g.
exertional rhabdomyolysis. In addition, quantifying the myotonia, hyperkalemic periodic paralysis [HYPP]). 80
extent of rhabdomyolysis during mild exercise is helpful
in deciding how rapidly to put a horse back into train­
ing. Blood samples should be taken before exercise and Muscle Biopsy
about 4–6 hours after exercise to evaluate peak changes A number of specific equine myopathies have been
in CK. Serum CK activity measured immediately post‐ identified through the routine examination of muscle
exercise does not reflect the amount of damage occur­ biopsies from horses with evidence of muscle dysfunc­
ring during the exercise test. Small fluctuations in serum tion. Muscle fiber sizes, shapes, and fiber type distribu­
CK activity may occur with exercise due to enhanced tion, mitochondrial distribution, polysaccharide staining
muscle membrane permeability, particularly if exercise pattern, neuromuscular junctions, nerve branches, con­
is prolonged or strenuous and the horse is untrained. 27,57 nective tissue, and blood vessels can be fully evaluated in
A submaximal exercise test is often valuable for frozen sections using a battery of tinctorial and histo­
detecting rhabdomyolysis because it provides more con­ chemical stains.
sistent evidence of subclinical rhabdomyolysis than Inflammation, muscle fiber necrosis, cellular infil­
maximal exercise tests. Fifteen minutes of trotting is trates, and proliferation of connective tissue are evident
67
often sufficient to produce subclinical muscle damage in in formalin and frozen sections. However, there are
horses prone to chronic exertional myopathies. If signs many pathologic alterations that cannot be detected in
36
of stiffness develop before this, exercise should be con­ formalin‐fixed tissue but can readily be seen in histo­
cluded. A normal response is less than a three‐ to four­ chemical stains of fresh‐frozen biopsy samples. These
14
fold increase from basal CK. In the author’s experience, include identification of muscle fiber types to differenti­
horses with type 1 polysaccharide storage myopathy ate between neurogenic and myogenic atrophy, charac­
(PSSM1) and active recurrent exertional rhabdomyoly­ terization of vacuolar storage material, characterization
sis (RER) are more likely to have a positive exercise of inclusion bodies, and mitochondrial density. In addi­
response than horses with type 2 polysaccharide storage tion, frozen samples may be used for biochemical analy­
myopathy (PSSM2) or well‐managed PSSM1. 38 sis of substrate concentrations and enzyme activities as
well as DNA isolation.
Muscle biopsies are preferably collected from abnor­
Ultrasonography
mal/diseased muscle. A 6‐mm outer diameter percutane­
Physical disruption of muscle fibers and fibrosis can ous needle biopsy technique can be used to obtain small
be detected by diagnostic ultrasonography. The typical muscle samples through a 1.5‐cm skin incision using a
striated echogenic pattern of skeletal muscle varies local anesthetic subcutaneously. If this technique is used,
according to the muscle group, and careful comparisons enough muscle should be obtained to form a 1.5‐cm
2
must be made between similar sites in contralateral sample at a minimum. These samples do not tolerate
limbs, in both transverse and longitudinal images. The shipment to an outside laboratory well. The optimum
appearance of muscle is also sensitive to how the horse biopsy for shipment of histopathologic tissues to a labo­
stands and whether the muscle is under tension, so it is ratory is collected using surgical or open techniques,
important that the horse stand squarely and bear weight performed under local anesthesia (Figure 7.65).
evenly. Muscle fascia appears as well defined, relatively Semimembranosus samples are often selected for open
echodense bands. Care must be taken in identifying biopsy in cases of exertional rhabdomyolysis. Care must
large vessels and artifacts created by them. 58 be exercised to infiltrate only the subcutaneous tissues,
Relatively hypoechoic areas within muscle that lack not the muscle, with local anesthetic agent. The objective
the normal muscle fiber striation indicate acute injury. is to obtain approximately a 2‐cm cube of tissue without
The jagged edge of the margin of the torn muscle may be crush artifact; hence, a suitably long skin incision is
increased in echogenicity. Tears in the muscle fascia may required. Subsequently, two parallel incisions 2 cm apart
be identified. A loculated hematoma may fill the injured should be made longitudinal to the muscle fibers with a
area and slowly be replaced by hypoechoic granulation scalpel. The muscle should only be handled in one corner