Taking a Muscle Biopsy for EPSM by Beth A. Valentine, DVM, PhD
Although a positive response to diet change is good evidence that a
horse has equine polysaccharide storage myopathy (EPSM), the best diagnostic
test is a muscle biopsy. You might want to have your horse biopsied for two good
reasons:
- To know for sure if your horse has EPSM and therefore needs daily fat
supplementation. Since some horses don't take to this diet right away, and since
EPSM diets can be messier than other feeds, it is useful to know if a diet
change is really necessary.
- To find out just how bad things are if EPSM is the diagnosis. Some horses
have mild changes that bode well for a positive response to diet modification,
while others have severe and irreversible changes. Although the latter may still
be helped with a fat supplemented diet, if the changes are severe you will need
to be aggressive about the diet modification, regular exercise, and turnout.
Some EPSM mares with severe changes have difficulty maintaining muscle strength
and mass after foaling, even with an EPSM diet, and therefore perhaps should not
be bred. The diagnostic possibilities for a horse with odd problems may include
something like a severe spinal cord problem that might not be treatable. In such
cases a muscle biopsy showing signs of EPSM offers at least some hope of
treatment. Of course, some horses have more than one problem, but knowing for
sure EPSM is one of them is a big help to both you as the horse's owner and to
your veterinarian.
Muscle Biopsy Muscle biopsy is not a difficult or dangerous
procedure, and may be performed by your veterinarian at your barn. The best
muscles to biopsy are the hamstring muscles, which are the muscles that run up
and down on the back of the thigh on both sides of the tail. Taking a biopsy
sample high up, just below the base of the tail and well above where a harness
brichen would lie, is the ideal site. This area generally heals without suture
breakdown and is one spot a horse cannot cover with mud when it rolls. Taking
the sample as close as possible to the midline of the rump (near the tail) means
the tail hairs will likely cover the resulting small scar.
In people muscle biopsies often involve the insertion of a wide-bore
needle. Unfortunately such biopsies are not as useful in horses as they are in
people. For one thing, most veterinarians don't have the specialized needle
needed to get a good muscle needle biopsy sample. Even more important, the
changes of EPSM are not evenly distributed within the muscle. Taking a small
sample of a large muscle in a horse runs the risk of having a false negative
biopsy result due to what is known as sampling error. The best biopsy procedure
is what is called an incisional biopsy to obtain a sample about 1" to 1.5"
long and about the diameter of a pencil. From such a sample several different
sections may be taken, which helps avoid the problem of sampling error. Horses
have a lot of muscle and this procedure does not seem to affect their muscle
function in the slightest.
Removing the Sample The surgery is performed with the horse
standing and sedated. The area is scrubbed, sterilized, and injected with a
local anesthetic to block the pain. An incision is made into the skin that is
about 2" to 2.5" long and runs up and down, paralleling the tail
hairs. Usually the muscle is just under the skin, but some horses have a fat
layer over the muscle the veterinarian will also need to cut through. Bleeding
is usually minimal and always easily controlled.
Another incision, about 1.5" to 2" long, is made in the muscle,
again running up and down the way the tail hairs run. A strip of muscle between
the two incisions is removed and placed in fixative to be shipped to the
laboratory. Our instructions are to lay out the muscle strip on a piece of
wooden tongue depressor to keep it flat during fixation. Once the muscle is in
the fixative, fixing takes at least a day before it may be processed by the
laboratory, and if it does not get to the laboratory for several days that's
okay. The fixative ³fixes² things in place, so nothing changes inside
the muscle fibers.
After Surgery Once the muscle sample has been removed, the
connective tissue fascia covering the muscle is stitched closed, followed by
suturing of the skin incision. I like to have a rolled-up surgical gauze pad
pressed against the incision and sutured in place. This gauze helps keep the
site clean for a few days, and puts some pressure on the incision to help keep
it from breaking down. The worst that can happen is the sutures break down, in
which case the site is treated as if the horse had a wire cut wound. It will
eventually heal, although the scar will be larger than if the sutures had stayed
in place.
Veterinary surgeons often recommend stall rest until the skin sutures are
removed at 10 to 14 days after surgery. I don't recommend stall rest for EPSM
suspect horses, as standing in a stall does their muscles no good at all. The
horse should be turned out immediately after surgery. At worst a few sutures may
break loose and the horse may have a slightly larger scar. Not only may an EPSM
horse in a stall rub the incision open just as easily, but will be susceptible
to muscle damage when it is finally turned out.
Costs & Report
Costs for the procedure are variable, but should generally be less than
$200. Costs for reading and reporting out the biopsy at the veterinary
pathology laboratory at Oregon State University are (as of 3-20-06)
$59. Muscle samples from EPSM suspect horses are processed by the laboratory
both for a routine stain and also with a special stain for glycogen.
Routine stain: Certain changes are obvious on the routine stain,
which is the stain on the first sections the laboratory prepares for microscopic
examination. The routinely stained sections are available on the first working
day after receiving the biopsy. If obvious changes of EPSM are present, I will
send a preliminary report to the veterinarian indicating that the horse
definitely has EPSM.
Glycogen stain: In some cases determining whether or not the horse
has EPSM is not possible until the glycogen stains are completed. Currently we
run the glycogen stain once a week, so the longest you will have to wait for a
final biopsy result is one week. The cases that turn out to be positive only
after the glycogen stains are less severe than those that become obvious on the
routine stains, so sometimes having to wait a few days for the report is a good
thing.
Beth A. Valentine, DVM, PhD,
is a veterinary pathologist at the Oregon State University College of Veterinary
Medicine in Corvallis, this site's virtual vet, and co-author of
Draft
Horses, an Owner's Manual. She will be happy to email, fax, or mail you or
your veterinarian a set of instructions on how to take a muscle biopsy and how
to package and send it to the OSU laboratory. If you would like to receive these
instructions, please contact
Beth.Valentine@oregonstate.edu.
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