Canine X-linked Muscular Dystrophy

Numerous examples of progressive muscle diseases (myopathies) have been described in animals. They may be inherited, and many resemble various types of muscular dystrophy in humans. Muscular dystrophy is a term that is broadly used to refer to any primary skeletal muscle disease that results in progressive degeneration and limited regeneration of muscles.

The human form of dystrophin deficiency is called Duchenne's muscular dystrophy.

Several Types of Muscular Dystrophy

Several types of muscular dystrophy are seen in dogs. Sex-linked muscular dystrophy associated with dystrophin deficiency has been reported in several breeds of dogs and is best characterized in the Golden Retriever. Dystrophin is a protein that functions to stabilize the muscle membrane during contraction. Since the dystrophin gene is so large, it is a relatively frequent target of mutations, and because males have only one X-chromosome, dystrophin deficiency predominantly affects males. Female dogs often show no clinical signs or have a very mild form of the disease. A similar type of X-linked muscular dystrophy has been reported in the cat.

Canine X-linked Muscular Dystrophy

Canine X-linked muscular dystrophy (CXMD) was first described in the Golden Retriever. Variations of this disease have since been documented in multiple breeds including the Samoyed, Brittany Spaniel, Irish terrier, Groenendaeler Shepherd, Miniature Schnauzers, Pembroke Welsh Corgi, German Shorthaired Pointer, Labrador Retriever and a Rat Terrier.

Signs of Muscular Dystrophy

Dogs with muscular dystrophy begin to show clinical signs at about eight weeks of age. Initially they have muscular weakness, exercise intolerance and a stiff gait. Within a few weeks muscle atrophy spreads and becomes apparent. These initial signs are followed by generalized weakness, enlargement of the tongue, and difficulty swallowing. An affected puppy or dog has problems moving his tongue, drools excessively, retains food in his mouth, has difficulty breathing, muscle spasms of the neck that last 10 to 30 seconds, and increased exercise intolerance. The dog is able to chew but drops food during swallowing due to inappropriate tongue movement. Likewise, he is not able to drink normally but has to extend his head and neck to swallow water.

Diagnosis of Muscular Dystrophy

Obtaining a diagnosis in canine muscular dystrophy is important for a number of reasons. Because it occurs most commonly in purebred animals it is important for breeders to be aware of a genetic defect in their animals, and furthermore to know the mode of inheritance. Diagnosis is also important to owners since the various muscular diseases call for different treatments and carry vastly different prognoses. Muscle biopsy evaluation is critical to the diagnosis of muscle diseases in young puppies. Progressive muscle diseases affecting young puppies can be noninflammatory and include dystrophies, metabolic and mitochondrial disorders; or they can be inflammatory secondary to immune-mediated disease or infection with Toxoplasma gondii or Neospora caninum. An accurate diagnosis is critical for prognosis, for determination of appropriate therapy if available, and for elimination of genetic diseases from breeding populations.

Treatment Unfortunately at this time there is neither a cure nor a specific therapy for muscular dystrophy of any genetic origin. Anabolic supplements and/or growth factors may help slow the progression of the disease, but their efficacy is unproven, and they are expensive and can quickly become cost prohibitive for some owners. A stem cell breakthrough could lead to a treatment for muscular dystrophy (MD), research has revealed. Scientists found transplanting stem cells into dogs with a version of the disease markedly improved their signs - they can be seen here before (inset) and after (main screen) treatment. However, the researchers need to find out why not all dogs respond positively.

Current research into therapies for muscular dystrophy proceeds on several fronts. One of the most widely studied is gene therapy.

Prognosis Life expectancy depends on the severity of clinical signs. Death may occur within the first few days in the case of severe complications, or dogs can survive for up to several years. Like people, dogs with muscular dystrophy often develop a heart muscle disease, and their heart disease is often what eventually leads to death.

References:
1. Canine Muscular Dystrophy: Case Presentation and Pathophysiology by Erica D. Morgan
2. Dystrophin-Deficient Muscular Dystrophy in a Labrador Retriever by Robert L. Bergman, Karen D. Inzana, William E. Monroe, Linda G. Shell, Ling A. Liu, Eva Engvall, and G. Diane Shelton