Muscular Dystrophy (X-Linked)

Liaison: Jan Young

Muscular dystrophy is an inherited type of muscle disease (myopathy) that is progressive and nonregenerative (permanent loss of muscle tissue and muscle use). X-linked refers to a specific type of muscular dystrophy that is transmitted by mothers to sons through the X chromosome. The disease shows up predominantly in males because they have only one X chromosome. Since females have two X chromosomes (and only one is typically affected) they may be carriers, but show little or no signs of disease.

Signs and Symptoms

Signs and symptoms begin in early puppy hood and are almost always present by six to nine weeks of age. Muscle weakness and exercise intolerance are the first signs. Gait may be awkward or stiff, and the head may hang low because of weak neck muscles. As the disease progresses, the tongue may become enlarged and the dog has decreased ability to move the tongue resulting in excessive drooling, difficulty swallowing, and difficulty breathing. There is also a gradual increase in generalized weakness and exercise intolerance as the muscle disease progresses, and there begins to be obvious muscle wasting (atrophy). Dogs are able to chew normally but often drop food when swallowing and are not able to drink normally.

Depending on the severity of the disease, dogs may die very young, or, on occasion, live several years. Eventually the muscles of the heart become involved and this is often the cause of death.

Causes

X-linked Muscular Dystrophy (XLMD) is caused by a mutation in the dystrophin gene on the X chromosome. This gene is responsible for making a protein called dystrophin. The function of this protein is to stabilize the muscle membrane during contraction, and when this protein is deficient, muscle disease results. It is more common in some breeds than in others, and the Samoyed is one of the breeds in which it is more common. It is very similar to X-linked Duchenne’s muscular dystrophy in humans.

Risk Factors

This is a genetically transmitted disease and animals with this defect in their line should not be bred. As noted above, male dogs are at much greater risk for exhibiting the disorder, but females are at risk for being carriers and having affected male offspring.

Diagnostic Tests

Muscle biopsy is the definitive diagnostic tool when a myopathy or dystrophy is suspected. It is extremely important that an accurate diagnosis be made because there are many types of canine myopathy, many of them genetic, and there are different treatments and prognosis based on the specific type of myopathy. Because of the genetic etiology (cause) of this disease XLMD, it is extremely important to make an accurate diagnosis so appropriate breeding decisions can be made.

Because certain infections such as toxoplasmosis can mimic the signs and symptoms of muscular dystrophy, blood tests may also be obtained to rule these out.

Creatine kinase (CK) levels are abnormally high in affected pups as young as one week of age, making it possible to identify affected dogs when XLMD is present in a line or is otherwise suspected. Creatine kinase is a measure of muscle injury or breakdown, as seen in progressive myopathies, including XLMD.

Treatment Guidelines

Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.

There is no cure and no specific treatment for XLMD. Steroid treatment (prednisone) and/or growth factors may help slow the progression but their efficacy is not proven and they can be prohibitively expensive.

In 2006 stem cell therapy appeared promising although it was not effective in all dogs and it was not understood why. Since 2009 gene therapy research has been very promising. Unfortunately both of these potential therapies are still at the experimental stage and not available outside the laboratory.

References

Dystrophy-like myopathies in the Merck Veterinary Manual four bonesrating

X-linked muscular dystrophies. In: The Merck Veterinary Manual 10th Edition. Ed Kahn, Cynthia M. Pub Merck and Co, Inc. pp 950, 1128. four bonesrating

Canine X-linked Muscular Dystrophy at www.gopetsamerica.com two bonesrating