Muscular Dystrophy

By Malcolm Weir, DVM, MSc, MPH; Catherine Barnette, DVM.

What is muscular dystrophy?

Muscular dystrophy is an inherited disease that affects the muscles. Signs of muscular dystrophy are typically seen early in life (at one to three months of age) and they progress as the dog ages. There are multiple forms of muscular dystrophy noted in animals and humans. The most common form of muscular dystrophy in dogs is seen in Golden Retrievers, but has been reported in the Labrador retriever, German shorthaired pointer, rottweiler, Cavalier King Charles spaniel, Pembroke Welsh corgi, cocker spaniel, Tibetan terrier, Norfolk terrier, and miniature poodle.

What causes muscular dystrophy?

Most cases of muscular dystrophy in dogs are associated with a defect of a gene on the X chromosome. The X chromosome is a sex chromosome; male dogs have one copy of the X chromosome, while female dogs have two copies of the X chromosome. Because male dogs only receive one copy of the X chromosome, they are more likely to show signs of muscular dystrophy than female dogs. Females receive two copies of the chromosome and therefore more likely to receive at least one functional copy of the gene.

"In dogs with muscular dystrophy, the body produces a defective version of a protein called dystrophin."

In dogs with this form of muscular dystrophy, the body produces a defective version of a protein called dystrophin. Without functional dystrophin, the membranes of muscle cells are leaky. Abnormal dystrophin allows substances that should remain inside the cell to leak out, and allows foreign substances to enter the cell. Leaky cell membranes lead to abnormalities in muscle structure and function.

Other, less common forms of muscular dystrophy also exist, in both dogs and cats. What they all have in common is an abnormality in, or a deficiency of, cytoskeletal proteins that affect muscle health and function.

What are the signs of muscular dystrophy?

The first signs of muscular dystrophy are a stiff gait or a dog that “bunny hops” awkwardly with his back legs. These dogs also have decreased agility and exercise tolerance, compared to normal dogs. Affected dogs often lose muscle mass. This may make the bones along the dog’s spine, ribs, and skull appear more prominent than in a normal dog.

"The first signs of muscular dystrophy are a stiff gait or a dog that “bunny hops” awkwardly with his back legs."

Muscular dystrophy also affects muscles used for eating and swallowing. Dogs with muscular dystrophy may regurgitate (bring swallowed food back up into their mouth), due to a condition known as megaesophagus. In megaesophagus, the esophagus does not move food to the stomach normally, resulting in food and water sitting in the esophagus.

Dogs with muscular dystrophy often have abnormally large tongues, making it difficult to eat and drink. They may drool excessively, because it is difficult for them to swallow their saliva.

How is muscular dystrophy diagnosed?

Your veterinarian may suspect muscular dystrophy based on your dog’s clinical appearance. Routine bloodwork is often normal, although some dogs will have elevated levels of a muscle enzyme called creatine kinase (CK). A specialized test called electromyography (EMG) may be performed as well to establish that the signs of weakness are muscular in origin rather than being due to a neurological problem.

"Diagnosis of muscular dystrophy requires a muscle biopsy."

Diagnosis of muscular dystrophy requires a muscle biopsy. For this test, your dog will be placed under general anesthesia. Once he is anesthetized, your veterinarian will create an incision in the skin directly over a muscle. Several small samples of muscle tissue will be removed through this incision. The incision will then be sutured closed. Depending on the suture material used, your veterinarian may need to schedule your dog for a follow-up visit for suture removal. In some cases, the veterinarian may use dissolving suture that will break down in the coming weeks.

After surgery, the muscle samples will be sent to a diagnostic laboratory for special processing and evaluation. Examination of the muscle cells under the microscope is typically adequate to diagnose muscular dystrophy, but further specialized testing will be needed to determine the exact type of muscular dystrophy that is present.

Is there a genetic test available for muscular dystrophy?

Yes, there are genetic tests available that screen for muscular dystrophy in certain breeds. This testing can be used to identify whether a dog is affected with muscular dystrophy, an asymptomatic carrier of muscular dystrophy, or clear of the mutation.

"There are genetic tests available that screen for muscular dystrophy."

Only females can be asymptomatic carriers of the dystrophin-deficient form of muscle dystrophy. Asymptomatic carriers have one X chromosome carrying a normal dystrophin gene and one X chromosome carrying an abnormal dystrophin gene. Male dogs only carry a single X chromosome, and therefore any male dog carrying an abnormal dystrophin gene will show signs of muscular dystrophy.

If my dog is diagnosed with muscular dystrophy, what can be done?

There is no treatment for muscular dystrophy. A number of experimental therapies have been studied, including stem cell injections and gene therapy, but these therapies have not been approved for clinical use. Steroids may slow the progression of the disease, but the disease remains fatal in most cases. As the muscles deteriorate, dogs have increased difficulty meeting their nutritional needs and experience reduced heart function.

"There is no treatment for muscular dystrophy."

When a dog is diagnosed with muscular dystrophy, it is important to take measures to avoid breeding more dogs with muscular dystrophy in the future. A dog that is diagnosed with muscular dystrophy should not be bred, even if he does survive to adulthood. The mother of an affected dog also should not be bred, as she is likely a carrier of the trait. Her carrier status can be confirmed with genetic testing.

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