Muscular Dystrophy

Introduction and Definitions

·         It is a hereditary condition marked by progressive weakening and wasting of the muscles.

·         The particular group of muscles which is predominantly affected or the distribution of muscle weakness is different in the different types of dystrophies

·         In some types muscle weakness remains localised (e.g. to the muscles of the eye and face) but in others the major muscle groups for moving the limbs are affected and so later other muscles may also be affected.

·         The essential feature in all muscular dystrophies is muscle wasting and weakness

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Epidemiology

·         Since there are many different types of muscular dystrophies it is not uncommon

·         More than 70,000 children and adults in the UK have MD or a related condition.

·         Duchenne MD is the most common type of MD and affects around one boy in every 4,000 in the UK

·         There is no specific gender, race or particular group of people that are affected by muscular dystrophy

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Types of MD

·         Over 30 different types of MD have been recognised so far

·         The main types are shown on the diagram

·         Duchenne muscular dystrophy is the most common form of dystrophy and it is due to a mutation on the X chromosome and therefore is a hereditary disease; and it is usually boys that are affected.

·         The muscle weakness is mainly in the 'proximal' muscles, which are those near the trunk of the body, around the hips and the shoulders. This means that fine movements, such as those using the hands and fingers, are less affected than movements like walking.

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Aetiology

·         MD is caused by mutations (alterations) in the genes responsible for healthy muscle structure and function. The mutations mean that the cells that should maintain your muscles can no longer fulfil this role, leading to muscle weakness and progressive disability.

·         If one or both of the parents has a mutated gene that causes MD, it can be passed on to the individual. Depending on the specific type of MD, the condition can be a:

  • recessive inherited disorder
  • dominant inherited disorder
  • sex-linked (X-linked) disorder

·         In a few cases, the genetic mutation causing MD can develop as a new event in the family and this is known as a spontaneous mutation.

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Pathophysiology

·         Multiple proteins are involved in the complex interactions of the muscle membrane and extracellular environment. For stability, dystrophin and the dystrophin-associated glycoproteins (DAGs) are important elements

·         Dystrophin is distributed in skeletal muscle and also in smooth and cardiac muscles and in the brain and DAGs stabalise the plasma membrane

·         Defects that interfere with the translation or with the promoter sequence that initiates synthesis of dystrophin can lead to an unstable, ineffective protein

·         Mechanically induced damage puts a high stress on fragile membranes and provokes micro-lesions that could eventually lead to loss of calcium homeostasis, and cell death. Muscle fibers undergo necrosis and are ultimately replaced with adipose and connective tissue.

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Diagnosis

·         Doctors can often be fairly certain of diagnosis after taking a detailed medical history and a careful physical examination but further tests are necessary to confirm.

·         There are essentially three diagnostic tests which are often used to determine MD. These are:

  • Blood tests
  • Electromyography
  • Muscle biopsy

·         There are two blood tests: one is the CPK test carried out using a venepuncture and level of the enzyme creatine kinase is measured; levels of elevation of CK can indicate the type of MD and the other is for DNA studies

·         EMG measures the electrical activity when muscles contract by placing a small needle into the muscle and amplifying the activity produced

·         Muscle biopsies are the second most important diagnostic test after the CPK test to diagnose MD as it can show if muscle fibres are replaced by adipose cells

·         The picture shows a cross section of a muscle with extensive replacement of muscle fibers by adipose cells.

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Treatments

·         There is no cure for MD as the defective gene is active in every muscle cell of the body

·         However a great deal can be achieved by adopting a positive approach, which involves the three Ps:

  • Promotion and maintenance of good health in general
  • Prevention of deformities through exercise, physiotherapy and surgery: As MD progresses, it weakens muscles and gradually mobility and strength are lost, so these physical problems can be helped with low-impact exercise, or physiotherapy can be useful for maintaining muscle strength, preserving flexibility and preventing stiff joints, also physical aids, such as a wheelchair, leg braces or crutches can help. Surgery is usually to treat scoliosis (curving of the spine to one side)
  • Preservation of respiratory function - Once the chest muscles become too weak to control breathing properly, ventilation machines may be necessary to assist with breathing and coughing, especially while sleeping.
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Prognosis

·         The prognosis for people with muscular dystrophy varies according to the type and progression of the disorder.

·         Some cases may be mild and progress very slowly over a normal lifespan, while others produce severe muscle weakness, functional disability, and loss of the ability to walk.

·         Some children with muscular dystrophy die in infancy while others live into adulthood with only moderate disability.

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