Muscular Dystrophy

Muscular dystrophy is one of a number of muscular diseases (myopathies) characterized by progressive muscle weakness and loss of muscle tissue.  It is caused by an inherited genetic abnormality whereas other myopathies, such as polymyositis, can be caused by an abnormality of the immune system.  These disorders can develop at any time from birth through adulthood.

There are many types of muscular dystrophy (MD).  These diseases are classified according to the muscle groups involved and the age of onset.  Some of these muscular dystrophies result in premature death; others may allow the person to live for decades with varying degrees of disability.

MD includes many inherited disorders such as:

• Duchenne muscular dystrophy
• myotonic dystrophy
• Becker's muscular dystrophy
• facioscapulohumeral muscular dystrophy
• limb-girdle muscular dystrophy
• Emery-Dreifuss muscular dystrophy
• myotonia congenita
• oculopharyngeal dystrophy
• distal muscular dystrophy
• mitochondrial myopathies

These disorders are distinguished from each other by the type of inheritance (sex-linked, dominant gene, recessive gene), the age when symptoms appear, and the types of symptoms that develop.

Myotonic dystrophy
Also known as Steinert's disease, this is the most common adult form of muscular dystrophy.  Its name underscores an unusual symptom found only in this form of dystrophy – myotonia – which is similar to a spasm or stiffening of muscles after use.

Myotonic dystrophy results from a gene flaw on chromosome 19.  The gene containing the defect is for an enzyme named myotonin protein kinase.  Some researchers believe the defect may affect other nearby genes.

The disease causes muscle weakness and affects the central nervous system, heart, gastrointestinal tract, eyes (causing cataracts) and endocrine (hormone-producing) glands.  Although muscle weakness progresses slowly, this symptom can vary greatly, even among members of a single family.  Most often muscle weakness doesn't hamper daily living for many years after symptoms first occur.

Many people with myotonic dystrophy seem to require more sleep than the average person.  Mild mental retardation is present in some people with the disease, and a peculiar emotional and mental "indifference" has also been found to accompany myotonic dystrophy in some patients.

Duchenne muscular dystrophy
The most common childhood form of muscular dystrophy, early signs usually occur between the ages of 2 and 6 and include frequent falling, difficulty getting up from a sitting or lying position, and a waddling gait.  Another hallmark is the apparent enlargement of the calf and sometimes other muscles, which is really due to an accumulation of fat and connective tissue in the muscle.  A blood sample shows a very high level of creatine kinase (CK), an enzyme that leaks out of damaged muscle.  Posture changes as the child grows.

Progression varies somewhat from child to child.  The use of orthopedic devices and physical therapy can prolong the ability to walk.  Frequently, however, a wheelchair will be needed by age 12.  Mild mental retardation has been noted in some (by no means all) boys with Duchenne dystrophy.

Breathing becomes affected during the later stages of Duchenne, leading to respiratory infections.  These are often successfully treated with antibiotics and respiratory therapy.  Severe respiratory and heart problems mark the disease's final stages, usually in the child's teens or early 20s.

Incidence; Causes and Development; Contributing Risk Factors

Muscular dystrophy is a rare disease.  Even Duchenne muscular dystrophy — one of the more common types — affects only 1 out of every 50,000 people.  The other types of muscular dystrophy are even rarer.

It occurs in both sexes and in all ages and races, but the most commonly in young boys.

All muscular dystrophies are inherited, and each type is associated with a distinct genetic mutation.

Because these are inherited disorders, risk factors include a family history of muscular dystrophy.

Signs and Symptoms

MD is characterized by muscle weakness in various parts of the body, depending on its type.  Muscle weakness worsens over time; initial symptoms usually develop gradually.

Symptoms vary depending on the type of muscular dystrophy; all of the muscles may be affected, or only specific groups.

People with muscular dystrophy might have some of the following symptoms, which arise from the atrophy that takes place:

• Delayed development of muscle skills
• Problems walking (frequent falls)

• Joint contracture – shortening of muscles around a joint – are common.  This causes abnormal positioning of the joint, along with possible pain.  Shortening of the muscle fibers, fibrosis of the connective tissue and scarring slowly destroy muscle function.  Some types of MD involve the heart muscle, causing cardiomyopathy or arrhythmias.

  • Muscular deformities, such as clubfoot or clawhand
  • Skeletal deformities, often scoliosis (curved spine)
• Drooling
• Drooping eyelids ("ptosis")
• Mental retardation is also possible
• Hypotonia (poor muscle tone)

As the disease progresses, it may lead to:

• Heart Trouble

  • Arrhythmia
  • Heart Failure

• Lung Ailments

  • Lung Infections, such as Pneumonia
  • Choking
  • Collapsed Lung
  • Respiratory Failure

Diagnosis and Tests

The different types are distinguished by factors such as:

• age at which symptoms usually start
• pattern of muscle weakness
• speed at which the disease progresses
• involvement of other tissues besides muscle
• pattern of inheritance.

Treatment and Prevention

There are no known cures for the various muscular dystrophies.  Treatment is aimed at controlling symptoms to maximize the quality of life.  Physical therapy may help people with this disorder to maintain muscle strength and function.  Orthopedic appliances such as braces and wheelchairs can improve mobility and self-care abilities.  In some cases, surgery on the spine or lower extremities may help improve function and slow deterioration.

Prognosis; Complications

The outcome varies with the type of muscular dystrophy.  All types of MD progressively worsen, but the speed of decline and extent of disability they cause varies widely.  Some types of muscular dystrophy, such as Duchenne muscular dystrophy (DMD), are deadly, with the average life expectancy for DMD patients being 27 years.  Other types cause little disability and people with them have a normal lifespan.

Complications include: deformities, tightening of muscles around the joints (contractures), scoliosis, joint contractures, permanent and progressive disability, decreased mobility, decreased ability to care for self, mental impairment (varies), cardiomyopathy with heart failure, respiratory failure (lung failure).