What is Myopathy (including Duchenne’s)

Myopathies are diseases that affect the skeletal muscles (those attached to bones). Myopathies can be caused by inherited genetic defects (e.g. muscular dystrophies), and endocrine, inflammatory (e.g. polymyositis), and metabolic disorders. Nearly all of the myopathies produce weakening and wasting of skeletal muscles.

Statistics on Myopathy (including Duchenne’s)

Duchenne’s muscular dystrophy occurs in 1 in 3000 male births and has a prevalence of 3 per 100,000. Becker’s dystrophy is approximately 5-10 times less frequent. The most common adult muscle disorder is myotonic dystrophy with an incidence of 15 per 100,000 and a prevalence of 5 per 100,000. Polymyositis is a relatively uncommon disorder with an incidence of approximately 1 in 100,000. Incidence and prevalence of endocrine and metabolic myopathies is unknown. Corticosteroid myopathy is the most common endocrine myopathy and endocrine disorders are more common in women. Metabolic myopathies tend to be rare overall.

Risk Factors for Myopathy (including Duchenne’s)

Inheritable myopathies are caused by a genetic defect . The most common muscular dystrophies, Duchenne and Becker MD, result from a genetic defect on the X chromosome. Some risk factors for other myopathies include:

Progression of Myopathy (including Duchenne’s)

Patients with DMD experience motor developmental delay, noticeable after the 1st year, and onset of walking may be delayed beyond 18 months. By 5 years of age, boys may have difficulty running and climbing stairs. Gradually, joint contractures and thoracic deformity develop. By the teenage years, boys are unable to walk and are usually wheel-chair bound. Patients may live into the second or third decade but ultimately succumb to complications of respiratory or cardiac failure. Patients with BMD usually have a later onset of the condition – around 12 years of age. Patients may demonstrate similar features to DMD but the condition is less severe and myocardial involvement less common. Patients may live for many decades, with only mild to moderate symptoms. Patients with other types of muscular dystrophy, such as metabolic myopathy, or myotonic, facioscapulohumeral or limb-girdle dystrophy may have a normal lifespan despite progressive weakness and variable disability.

How is Myopathy (including Duchenne’s) Diagnosed?

Following a complete medical history and physical examination, a number of tests may be performed to help establish a diagnosis. These include:

  • Blood tests: Creatine kinase may be markedly raised, due to the amount of muscle protein circulating in the blood in certain myopathies such as DMD or BMD. Levels of potassium and antibodies may also be raised, depending on the cause of the myopathy.
  • Urinalysis: May show protein in the urine
  • Gene studies: Can detect the genetic mutation present, and also identify carriers of the gene for DMD or BMD – thus facilitating genetic counselling.

Other Tests:

  • Electromyogram (EMG): measures the electrical activity of the muscle. Many conditions can be differentiated from each other on the basis of electrophysiologic changes.
  • Electrocardiogram (ECG): Abnormal heart activity may be seen in patients with DMD
  • Muscle biopsy: Involves surgically removing a very small amount of tissue to be examined under the microscope and analyzed for cellular and protein abnormalities.

Prognosis of Myopathy (including Duchenne’s)

DMD: Most patients wheelchair bound by 12 years of age; death usually occurs during 20’s.

BMD: Much more variable prognosis. Patients may or may not become wheelchair bound. Many patients have children and survive beyond middle age.

FSHD: Many patients are only mild affected. Rate of disease progression is generally slow, and most individuals are able to remain in employment.

LGMD: Variable, but generally slowly progressive.

How is Myopathy (including Duchenne’s) Treated?

All patients and families who have been diagnosed with a myopathy should receive information and counselling regarding their condition. There are extensive support associations in the community for DMD. Where appropriate, genetic counselling should also be sought.

Muscular Dystrophies

The goals of treatment are to slow progression of disease and relieve symptoms. Genetic counselling and education is important for all families affected. Passive physiotherapy, night splints and bracing help maintain mobility and prevent stiffness in the later stages of the disease. Surgery may be needed in severe disease. Duchenne MD and Becker MD are the subjects of current medical research and clinical trials may be available for patients with either disease.

Endocrine Myopathies

Treating the underlying condition helps relieve muscle weakness and associated pain.

Inflammatory Myopathies

The inflammatory myopathies, such as polymyositis and dermatomyositis, are usually treated with drugs that suppress the action of the immune system. prednisolone is most commonly used; however, immunsuppressant drugs such as azathioprine, methotrexate and cyclophosphamide may be used.

Metabolic Myopathies

The primary aim in treating metabolic myopathies is to avoid situations, like strenuous exercise, that tax the muscles and promote muscle pain and weakness.

Myopathy (including Duchenne’s) References

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  2. Cotran, Kumar, Collins 6th edition. Robbins Pathologic Basis of Disease. WB Saunders Company. 1999.
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