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VETERINARY VIEWS
ADDITIONAL DIAGNOSTIC TECHNIQUES YIELD IMPORTANT INFORMATION ABOUT MUSCLE DAMAGE:
• SERUM CHEMISTRY PANEL for biochemical analysis of muscle enzymes often provides telling information about the health of muscles, as well as the duration of muscle injury.
• THERMOGRAPHY is useful to detect heat from inflammation, particularly when comparing both sides of the horse.
• NUCLEAR SCINTIGRAPHY
zeroes in on a subtle complaint of poor performance possibly related
to rhabdomyolysis. Release of large amounts of calcium from damaged muscle is an attractant for radioactive material injected into the horse for the nuclear scan.
• DIAGNOSTIC ULTRASOUND
examines for traumatic fiber disruption and/or scar tissue. To avoid confusing artifact with pathology in ultrasound scans, it is important that a horse stand squarely with full weight equally dispersed between both legs and the same muscle compared on left and right sides.
• ELECTROMYOGRAPHY(EMG; needle electrodes placed in the muscles) is practical to use when there are
signs of muscle atrophy, tremors, or dimpling. EMG evaluates conduction of the electrical system of muscle and nerve units. Abnormalities are seen with hyperkalemic periodic paralysis (HYPP), myotonia, or atrophy created by nerve injury.
• GENETIC TESTING uses
DNA markers to identify genetic mutations in muscle diseases – hair root samples are evaluated for hyperkalemic periodic paralysis (HYPP); hair root or whole blood samples identify Type 1 polysaccharide storage myopathy (PSSM).
• MUSCLE BIOPSY to diagnose immune-mediated myopathies and Type 2 PSSM.
EQUINE POLYSACCHARIDE STORAGE MYOPATHY (PSSM)
Equine polysaccharide storage myopathy (PSSM) is a glycogen disorder that causes muscle pain, stiffness and cramping due
to abnormal storage of sugar as well as accumulation of an abnormal form of sugar (polysaccharide) in muscle tissues. Some horses demonstrate exercise intolerance, poor performance, muscle weakness and/or atrophy, or related behavioral changes.
Type I PSSM is caused by an inherited enzyme defect (mutation in the GYS1 gene) that allows accumulations of glycogen in the muscle – it occurs in 90% of cases. The cause of type
2 PSSM has not yet been identified but does
not have the gene mutation nor is it necessarily heritable. Gene expression of Type 1 is markedly influenced by the horse’s environment, particularly related to diet and exercise. While no breed is immune, the most prevalent breeds affected include Quarter Horse-related breeds, Draft-related and Warmblood horses.
The typical horse with PSSM is calm,
even sedate, and well muscled, but not very
fit. Signs usually appear at the beginning of a training session, within the first 10-20 minutes, although a horse can exhibit signs in the absence of exercise. Many PSSM horses appear sluggish when asked to work. An affected horse may stop moving and posture as if to urinate
– this may relieve cramping of abdominal
Free exercise is a key component to keeping a Polysaccharide Storage Myopathy horse comfortable over the long term.
muscles. Muscle tremors may occur in the flank area during an episode; hindquarter muscles may feel firm to touch. Occasionally, a horse demonstrates colic signs of pawing or rolling in response to the discomfort.
In Draft-related breeds, it is typical to
see muscle atrophy, abnormal gait, and high-stepping action in the rear limbs. Progressive weakness and muscle atrophy eventually develop. Warmblood horses typically demonstrate signs of a sore back, sore muscles, reluctance to collect, and overall poor performance issues. Testing for a genetic marker for the classic Type 1 PSSM mutation is available at the University of Minnesota: (http://www.cvm.umn.edu/umec/lab/ Advances_in_PSSM/home.html).
For Type 2 PSSM, a definitive diagnosis relies on a muscle biopsy sent to an
appropriate lab for analysis, with grading as mild, moderate, or severe. Once abnormal polysaccharide has accumulated in the muscles, its presence remains with no decrease over time.
Not all horses with PSSM demonstrate clinical signs nor do they all have elevations in muscle enzymes. The problem is generally recognized in
112 SPEEDHORSE June 2022
Equine polysaccharide storage myopathy (PSSM) is generally recognized in Quarter Horses aged 4-5 years.
© Getty Images
© Getty Images