Muscular dystrophy (MD) refers to a group of genetic diseases described by progressive dysfunction and/or weakness of skeletal muscles. Muscles in charge of eye closure, facial expression, talking, swallowing, breathing, and moving upper or lower extremities might be involved to various extents in different kinds of muscular dystrophies. Some muscular dystrophies also affect the heart muscle causing cardiomyopathy or dysrhythmias.
Various structural and regulatory proteins are needed to maintain the integrity and proper function of the muscle. Pathologic mutations of the genes involved in synthesis and regulation of these proteins lead to disruption in the normal structure and function of the muscle tissue. Progressive muscle tissue damage causes weakness and sometimes loss of muscle bulk or replacement of normal muscle structure by fat or scar tissue. MDs are categorized based on the culprit gene(s), inheritance pattern, clinical presentation and muscle biopsy features:
- Dystrophinopathies, which include Duchenne and Becker muscular dystrophies
- Myotonic dystrophies
- Facioscapulohumeral muscular dystrophy (FSHD)
- Limb-girdle muscular dystrophies
- Oculopharyngeal muscular dystrophy
- Congenital muscular dystrophies
- Emery-Dreifuss muscular dystrophy
More than 30 genes have been identified to cause different types of muscular dystrophies. Many muscular dystrophies are now diagnosed through genetic testing.
MDs can affect people of all ages. Although some forms first become apparent in infancy or childhood, others may not appear until middle age or later. Duchenne muscular dystrophy is the most common form affecting children. Myotonic dystrophy type 1 is the most common form affecting adults.
Mutations can develop “de novo” and lack of family history of MD does not rule out the possibility of muscular dystrophies. The following are three primary types of inheritance by which the faulty gene that causes MD can be passed along to offspring:
- X-linked recessive: Genes that are X-linked recessive are carried by the female on one of the X chromosomes that determine the sex of the child. As such, only boys will develop conditions determined by these genes. Their mothers, known as carriers, will usually not show signs of the disease. A son of a carrier of MD has about a 50 percent chance of developing the disease. A daughter of a carrier has a 50 percent chance of becoming a carrier. If a boy is unaffected, he cannot pass on MD. However, daughters from a man with an X-linked dystrophy will all be carriers. Duchenne/Becker and Emery-Dreifuss are X-linked recessive MDs. Some female carriers of X-linked recessive MDs might develop similar, but usually milder, symptoms and they are called “manifesting carriers”.
- Autosomal recessive: For this type of inheritance, both parents must pass on the faulty gene, so both copies of the culprit gene are abnormal. Neither parent shows any symptoms, but each of their offspring, whether boy or girl, will have a 25 percent chance of developing the disease. Limb-girdle type 2 MDs are autosomal recessive.
- Autosomal dominant: In this case, having only one copy of the abnormal gene is enough to develop MD. This faulty gene can come from either parent, and it can affect either sex. Each child of an affected parent will have a 50 percent chance of developing MD. For this type of inheritance, the severity of MD can vary greatly. It can be so mild that it is not recognized, but it can also be severe. Myotonic dystrophies, facioscapulohumeral muscular dystrophy (FSHD), and oculopharyngeal muscular dystrophy (OPMD) are autosomal dominant.
Diagnosing muscular dystrophy
After carefully evaluating a patient's medical history, the doctor will perform a thorough physical exam. If MD is suspected, there are a variety of laboratory tests that can be used to confirm the diagnosis. These tests may include:
- Blood tests: When blood tests are performed to test for MD, the doctors are looking for an enzyme called creatine kinase (CK). Elevated CK level is noted when there is muscle damage and is a very helpful marker for MDs.
- Electromyogram (EMG): EMG is a test that measures the muscle's response to stimulation of its nerve supply (nerve conduction study) and examines the electrical activity of muscles (needle electrode examination). This test is very helpful in demonstrating that the weakness is due to a muscle disease rather than a nerve disease.
- Muscle biopsy: During a muscle biopsy, a small piece of muscle tissue is removed, sent to a pathology lab for specific staining, and then examined under a microscope. If MD is present, changes in the structure of muscle cells and the findings on different immunohistochemical stainings can help with the diagnosis of MDs.
- Genetic testing: Many muscular dystrophies can be definitively diagnosed by testing for the mutated genes. In many instances, this is the most specific test to diagnose MDs. If clinical features and family history point to specific types of MDs, neuromuscular specialists might directly go to genetic testing and skip muscle biopsy. This approach is common in diagnosing dystrophinopathies (like Duchenne), myotonic dystrophies, FSHD, OPMD, several forms of limb-girdle MDs, Emery-Dreifuss, and identifying carriers of X-linked MDs.
Treating muscular dystrophy
There is currently no cure for muscular dystrophy but there have been a lot of progress in understanding MDs and improving the life expectancy and quality of life of patients with MD. Steroids have now become standard of care in treating patients with Duchenne as they slow down the progression of disease. Long-term steroid use is associated with side effects and neuromuscular physicians consider those when deciding on when to start or stop steroids for patients with Duchenne MD. There are also gene therapy trials at different levels with the goal of slowing the progression of Duchenne MD. For the time being, treatment is aimed at preventing complications caused by:
- Muscle weakness
- Decreased mobility
- Heart defects
- Respiratory insufficiency
Physical and occupational therapies: Physical therapy, especially regular stretching, is important in helping to maintain range of motion for affected muscles and to prevent or delay development of contractures. Strengthening other muscles to compensate for weakness in affected muscles may be of benefit also, especially in earlier stages of milder MD. Regular exercise is important in maintaining good overall health, but strenuous exercise may damage muscles further. Customized wheelchairs, braces, handicap-friendly residential, or work environments all improve the patient’s independence and quality of life. Occupational therapy involves employing methods and tools to compensate for a patient's loss of strength and mobility. This may include modifications at home, dressing aids, wheelchair accessories, and communication aids.
Surgery: If a patient's contractures have become very pronounced, surgery may be used to relieve the tension by cutting the tendon of the affected muscle, then bracing it in a normal resting position while it heals. Surgery for scoliosis is often needed for patients with Duchenne or similar MDs.
Nutrition: Nutrition has not been shown to treat any conditioning MDs. Healthy diet is important in Duchenne patients who are on steroids as steroid-induced metabolic changes could compromise patient’s mobility and independence. Malnutrition in patients with swallowing difficulty or fatigue also needs to be addressed and alternative methods of feeding explored.
Cardiac care: Dysrhythmias or heart failure (cardiomyopathy) are a complication in dystrophinopathies (Duchenne and Becker), myotonic dystrophies, Emery-Dreifuss, and some limb-girdle MDs. Placement of cardiac pacemakers and/or different therapies for cardiomyopathy is sometimes needed in these diseases.
Respiratory care: When the muscles of the diaphragm and other respiratory muscles become very weak, a patient may require a ventilator device to continue breathing deeply enough. Air may also be administered through a tube or mouthpiece. It is therefore very important to maintain healthy lungs to reduce the risk of respiratory complications. Cough-assist devices might also be required to help clear respiratory secretions and prevent infections.
Like many other disorders, understanding and education about muscular dystrophy is the most important tool with which to manage and prevent complications. The following organizations can provide more information about muscular dystrophy:
- National Institute of Neurological Disorders and Stroke: Muscular dystrophy: Hope Through Research
- Mercuri E, Muntoni F. Muscular dystrophies. Lancet 2013; 381: 845–60
- Moxley RT III, Ashwal S, Pandya S, et al. Practice Parameter: Corticosteroid treatment of Duchenne dystrophy: Report of the Quality Standards Subcommittee of the American Academy of Neurology and the Practice Committee of the Child Neurology Society. Neurology 2005;64;13-20
- Harrison's Principles of Internal Medicine: Muscular Dystrophies and Other Muscle Diseases
- Rudolph's Pediatrics: Myopathies
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