Rural Heritage Vet Clinic

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:

  1. 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.

  2. 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

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26 October 2011 last revision