Polysaccharide Storage Myopathy
July, 2007
SIDEBAR:
Equine Polysaccharide Storage Myopathy
A separate glycogen storage disorder has been identified in draft horses and warmbloods
and is called Equine Polysaccharide Storage Myopathy (EPSM). While some characteristics
of this condition are similar to the PSSM seen in western-type riding horses, there
are differences. For instance, signs of this disorder include difficulty backing,
difficulty holding up limbs for the farrier, a "shivers"-like gait, and
loss of muscle mass. Fortunately, the same dietary changes--reducing NSC and adding
fat—seem to help these horses too.
Most horse owners are familiar with the term "tying up," which describes
the pain and cramping of a horse’s muscles with exercise. Veterinarians use the
term "chronic exertional rhabdomyolysis" to describe repeated episodes
of the condition, and now differentiate between two types, according to the fifth
edition of Current Therapy in Equine Medicine.
Recurrent Exertional Rhabdomyolysis typically occurs in the thoroughbred
racehorse that "ties up" when stressed and is most likely due to a calcium
regulation problem. Polysaccharide Storage Myopathy (PSSM) is an inherited
condition that occurs in western-type breeds such as quarter horses, paints and
appaloosas. It is due to a dysfunction in the way muscles store glycogen (glucose
or sugar).
The classic PSSM horse has a calm demeanor, good body condition, and may have been
off work for a few days before showing signs of "tying up" after only
a few minutes of exercise. These signs include:
- Stiffness, reluctance to move
- Posturing to urinate, stretching-out
- Muscle twitches
- Pawing, rolling
Horses that "tie up" may also have firm and painful muscles, increased
heart and respiratory rates, and sweat excessively. Exercise should be stopped and
a veterinarian called immediately if any of these signs occur. Diagnosis is confirmed
with bloodwork and muscle biopsy.
Changing the diet and scheduling regular exercise and turnout are the main ways
to control the condition. The best diet for a horse with PSSM contains less non-structural
carbohydrates and more fat. Non-structural carbohydrates (NSC) are the easily digestible
sugar and starch portion of the feed and forage. Structural carbohydrates are the
fiber portion. Since grains are highest in NSC, they should be removed from the
diet and replaced with a multi-vitamin/mineral supplement or ration balancer. It
is especially important to meet or exceed the vitamin E/Se and electrolyte requirements
of any horse that "ties up."
Because forage makes up most of a horse’s diet, significant amounts of NSC can be
eaten if hay and pasture are overlooked as source of sugars and starches. Typically,
grass pasture is higher in NSC than grass hay, and grass hay is higher in NSC than
alfalfa hay. However, the NSC content of forage cannot be guessed just by looking
at it, so it is best to have the forage analyzed for NSC content and feed “low carb”
hay (under 10% NSC) or "moderate carb" hay (under 20% NSC).
Reducing simple sugars and starches in the diet is just one step. Adding fat to
the diet as a replacement energy source is also important. Up to one pound of fat
per 1000 lbs of body weight per day should be gradually added to the diet, as long
as this concentrated source of calories doesn’t cause too much weight gain or GI
disturbance. There are a number of powdered fat supplements available that are accepted
by horses, are nearly 100% fat, and contain no sugars or starches.
Finally, turnout and exercise must be regularly scheduled for PSSM horses. Ideally,
these horses should be kept outside 24 hours per day. Because pasture grass may
at times contain high NSC levels though, this turnout may need to be on a dry lot.
If stalling is necessary, it should be limited to no more than 12 hours per day.
Controlled exercise should also be provided on a daily basis, whether lunging, riding,
driving or other activity. Horses that have suffered a bout of "tying up"
should return to light exercise upon the advice of a veterinarian.
Polysaccharide Storage Myopathy
By: Lydia F. Gray, DVM, MA
SmartPak Staff Veterinarian and Medical Director
July 2007
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