What is muscular dystrophy?
Muscular dystrophy (MD) refers to a group of genetic diseases described by progressive dysfunction and/or weakness of skeletal muscles.
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
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.
What causes 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.
What are symptoms of muscular dystrophy?
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.