Muscular Dystrophies

Muscular dystrophy (MD) is a broad term that describes a genetic (inherited) disorder of the muscles. Muscular dystrophy causes the muscles in the body to become very weak. The muscles break down and are replaced with fatty deposits over time.

Other health problems commonly associated with muscular dystrophy include:

• Heart problems
• Scoliosis – a lateral, or sideways, curvature and rotation of the back bones (vertebrae), giving the appearance that the person is leaning to one side
• Obesity

The most common forms of muscular dystrophy are Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy. The two forms are very similar, but Becker muscular dystrophy is less severe than DMD. Girls rarely are affected by either of these two forms of muscular dystrophy.

Causes of muscular dystrophy

Duchenne muscular dystrophy is a genetic disease, which means it is inherited. Our genes determine our traits, such as eye color and blood type. Genes are contained in the cells of our bodies on stick-like structures called chromosomes. There are normally 46 chromosomes in each cell of our body, or 23 pairs. The first 22 pairs are shared in common between males and females, while the last pair determine gender and are called the sex chromosome pair: females have two X chromosomes, while males have one X and one Y chromosome. 

Duchenne muscular dystrophy is caused by an X-linked recessive gene. "X-linked" means that the gene causing the trait or the disorder is located on the X chromosome. Genes on the X chromosome can be recessive or dominant, and their expression in females and males is not the same because the genes on the Y chromosome do not exactly pair up with the genes on the X. X-linked recessive genes are expressed in females only if there are two copies of the gene (one on each X chromosome). However, for males there only needs to be one copy of an X-linked recessive gene in order for the trait or disorder to be expressed. For example, a woman can carry a recessive gene on one of the X chromosomes unknowingly and pass it on to a son, who will express the trait or disease. 

There are nine types of muscular dystrophy, with each type involving an eventual loss of strength, increasing disability and possible deformity. Each type differs in the muscles affected, the age of onset and its rate of progression. Some types are named for the affected muscles.

Type: Becker

Age at onset: Adolescence to early adulthood

Symptoms: Symptoms are almost identical to Duchenne, but less severe; progresses more slowly than Duchenne; survival into middle age

Type: Congenital

Age at onset: Birth

Symptoms: General muscle weakness and possible joint deformities; disease progresses slowly; shortened life span

Type: Duchenne

Age at onset: 2 to 6 years old

Symptoms: Symptoms include general muscle weakness and wasting; affects pelvis, upper arms and upper legs; eventually involves all voluntary muscles; survival beyond 20s is rare

Type: Distal

Age at onset: 40 to 60 years old

Symptoms: Symptoms include weakness and wasting of muscles of the hands, forearms and lower legs; progression is slow; rarely leads to total incapacity

Type: Emery-Dreifuss

Age at onset: Childhood to early teens

Symptoms: Symptoms include weakness and wasting of shoulder, upper arm and shin muscles; joint deformities are common; progression is slow; sudden death may occur from cardiac problems

Type: Facioscapulohumeral

Age at onset: Childhood to early adulthood

Symptoms: Symptoms include facial muscle weakness and weakness with some wasting of shoulders and upper arms; progression is slow with periods of rapid deterioration; life span may be many decades after onset

Type: Limb-Girdle

Age at onset: Late childhood to middle-age adulthood

Symptoms: Symptoms include weakness and wasting, affecting shoulder girdle and pelvic girdle first; progression is

Type: Myotonic

Age at onset: 20 to 40 years old

Symptoms: Symptoms include weakness of all muscle groups accompanied by delayed relaxation of muscles after contraction; affects face, feet, hands and neck first; progression is slow, sometimes spanning 50 to 60 years

Type: Oculopharyngeal

Age at onset: 40 to 70 years old

Symptoms: Symptoms affect muscles of eyelids and throat, causing weakening of throat muscles, which in time causes inability to swallow and emaciation from lack of food; progression is slow