Muscular dystrophy

Muscular dystrophy (MD) is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion (walking or moving in some way). Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.

As of 2016, there is no specific treatment or cure for muscular dystrophy. However, physiotherapy, aerobic exercise, yoga, low intensity catabolic steroids such as prednisone, and deflazacort (for which the U.S. FDA has designated fast track status in January 2016 to conduct clinical trials) can help to maintain muscle tone. On the other hand, nutrient supplements such as tocopherol, Vitamin D3, calcium, whey protein, creatine, glutamine, meat, almond etc. may help to prevent contractures. However, recombinant DNA therapy approaches such as exon skipping to restore dystrophin production are currently being investigated.

Apart from the nine major types of muscular dystrophy listed below, several MD-like conditions, also known as "outliers" exist. Normal intellectual, behavioral, bowel and sexual function is noticed among these "outliers". MD-affected individuals with susceptible intellectual impairment are diagnosed through molecular characteristics but not through problems associated with disability. However, a third of patients who are severely affected with DMD may have cognitive impairment, behavioral, vision and speech problems.

Becker's muscular dystrophy

Becker muscular dystrophy (also known as Benign pseudohypertrophic muscular dystrophy) is an X-linked recessive inherited disorder characterized by slowly progressive muscle weakness of the legs and pelvis.

It is a type of dystrophinopathy, which includes a spectrum of muscle diseases in which there is insufficient dystrophin produced in the muscle cells, resulting in instability in the structure of muscle cell membrane. This is caused by mutations in the dystrophin gene, which encodes the protein dystrophin. Becker muscular dystrophy is related to Duchenne muscular dystrophy in that both result from a mutation in the dystrophin gene, but in Duchenne muscular dystrophy no functional dystrophin is produced making DMD much more severe than BMD.

Eponym

Becker Muscular Dystrophy is named after the German doctor Peter Emil Becker (November 23, 1908 Hamburg, Germany - 2000).

Genetics

The disorder is inherited with an X-linked recessive inheritance pattern. The gene is located on the X chromosome. Since women have two X chromosomes, if one X chromosome has the non-working gene, the second X chromosome will have a working copy of the gene to compensate. Because of this ability to compensate, women rarely develop symptoms. However, they may do so due to mosaicism. For example, carrier females of mutations are at increased risk for dilated cardiomyopathy. Since men have an X and a Y chromosome and because they don't have another X to compensate for the defective gene, they will develop symptoms if they inherit the non-working gene.

Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a rare recessive X-linked form of muscular dystrophy, affecting around 1 in 3,600 boys, which results in muscle degeneration and premature death. The disorder is caused by a mutation in the gene dystrophin, located on the human X chromosome, which codes for the protein dystrophin. Dystrophin is an important component within muscle tissue that provides structural stability to the dystroglycan complex (DGC) of the cell membrane. While both sexes can carry the mutation, females are rarely affected.

Symptoms usually appear in boys between the ages of 2 and 3 and may be visible in early infancy. Even though symptoms do not appear until early infancy, laboratory testing can identify children who carry the active mutation at birth. Progressive proximal muscle weakness of the legs and pelvis associated with loss of muscle mass is observed first. Eventually this weakness spreads to the arms, neck, and other areas. Early signs may include pseudohypertrophy (enlargement of calf and deltoid muscles), low endurance, and difficulties in standing without help or an inability to walk up stairs. As the condition progresses, muscle tissue experiences wasting and is eventually replaced by fat and fibrotic tissue (fibrosis). By age 10, braces may be required to aid in walking but most patients are wheelchair dependent by age 12. Later symptoms may include abnormal bone development that lead to skeletal deformities, including curvature of the spine. Due to progressive deterioration of muscle, loss of movement occurs, eventually leading to paralysis. Intellectual impairment may or may not be present but if present, does not progressively worsen as the child ages. The average life expectancy for individuals afflicted with DMD is around 25.

Muscle

Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. They are primarily responsible for maintaining and changing posture, locomotion, as well as movement of internal organs, such as the contraction of the heart and the movement of food through the digestive system via peristalsis.

Muscle tissues are derived from the mesodermal layer of embryonic germ cells in a process known as myogenesis. There are three types of muscle, skeletal or striated, cardiac, and smooth. Muscle action can be classified as being either voluntary or involuntary. Cardiac and smooth muscles contract without conscious thought and are termed involuntary, whereas the skeletal muscles contract upon command. Skeletal muscles in turn can be divided into fast and slow twitch fibers.

Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads.