Muscular dystrophies

The muscular dystrophies (MD) are a group of inherited genetic diseases that gradually cause the muscles to waste away and weaken, leading to an increasing difficulty walking.

There is unfortunately no cure for MD as the disease will progressively get worse over time leading to loss of quality of life or life-threatening complications.

©Douglas McCallum

MD are genetic diseases. Genes are made from a biological material called DNA. The genes control the chemicals (proteins) that the cell makes.  In MD, ‘faulty’ genes (mutations) interfere with the production of proteins needed to form healthy muscle and to protect muscle fibers from damage. The lack of these muscle proteins lead to the progressive degeneration of skeletal and sometimes cardiac muscles. Muscular dystrophy occurs when one of these genes is defective.

There are many kinds of MD. Each kind of MD is caused by a genetic mutation particular to that type of the disease. Many of these mutations are inherited but rarely some occur spontaneously.

Dystrophin deficiency is the most common form of muscular dystrophy and is associated with genetic mutations of the dystrophin gene. This type of MD is known in human as Duchenne MD. The dystrophin gene is located on the X-chromosome. A male has one X and one Y sex chromosome while a female has two X chromosomes. As males only have one copy of each gene on the X chromosome, they will be affected if one of those genes is mutated as with dystrophin-deficient MD. As females have two copies of the X chromosome, they are less likely to develop the disease as the normal copy of the chromosome can usually ‘cover’ for the defective version.

Signs usually develop in the first 6 months of life and are most often rapidly progressive. Affected cats will exhibit muscle weakness/stiffness, difficulty standing or walking, difficulty swallowing and excessive drooling. Most muscles rapidly become wasted. As a result, shortening of muscles (contractures) can further reduce mobility by limiting the range of movement in the limb joints. Some muscles may appear bulkier than normal and this is particularly true for the tongue, neck and thigh muscles. As a result, some cats may be unable to close their mouth and unable to groom.

Once the heart and breathing muscles are affected, MD becomes life threatening. Difficulty swallowing may also lead to inhalation of food and life-threatening pneumonia.

A number of tests may be performed to reach a diagnosis of MD but also rule-out other muscle conditions which may present with similar signs (eg infectious or immune disease causing muscle inflammation, other inherited muscle diseases, hormonal disease causing abnormal salt levels which affect muscle function).

• Blood enzyme tests: Damaged muscles release enzymes into the blood. One of these enzymes is creatine kinase (CK). There are many causes for elevated blood levels of CK including traumatic injury such as road traffic accidents. However, CK blood levels are often very high in animals with MD reflecting deterioration of muscle fibers.
• Electromyography: An electrode needle is inserted into the muscle to test muscle function and integrity. This test requires general anesthesia. Although this test can confirm the presence of a muscle disease, it cannot determine the nature of the disease present.
• Muscle biopsy: The removal and examination of a small sample of muscle tissue. Analysis of this muscle tissue allows confirmation of the diagnosis of MD. Characterization of the type of MD can be done by a specialized neuromuscular laboratory.

There is yet no known cure for MD but there are ways of helping to manage the effects of the condition.

Novel therapies are being investigated in human and dogs in the research setting (stem cells, gene therapy, pharmaceutical agents aiming at preventing muscle degeneration and promoting muscle regeneration).

The long-term prognosis is unfortunately very poor since most affected animals will progressively get worse and lose the ability to walk. Other life-threatening complications include difficulty breathing, heart failure and pneumonia due to inhalation of food.