Limb-Girdle Muscular Dystrophy Health Article

Definition

Limb-girdle muscular dystrophy (LGMD) encompasses a diverse group of hereditary degenerative muscle disorders characterized by weakness and deterioration of the proximal skeletal muscles.

Description

The term limb-girdle muscular dystrophy is used to describe a group of muscular dystrophies that cause a muscle deterioration that primarily affects the voluntary muscles around the limb girdle. The muscles of the limb girdle include those around the shoulders and hips. As the disease develops, the distal muscles of the limbs can become affected. Most individuals' muscles of the heart are not affected, but exceptions can occur. There are at least 15 different LGMDs, each having a different range of symptoms. Each of the muscular dystrophies results in an absent, deficient, or abnormal protein that is required for normal structure and function of the muscles. It can be difficult to differentiate LGMD from other muscular dystrophies and muscle disorders that can also result in a weakness in the limb girdle.

Genetic profile

Each type of limb-girdle muscular dystrophy is caused by changes in a different type of gene that produces a protein normally involved in the functioning of the skeletal muscles. Each gene is found at a specific location on a chromosome. Every person inherits two copies of gene, one from their mother and one from their father. Each type of gene produces a specific type of protein. A change (mutation) in a gene can cause it to produce an abnormal protein, an increased or decreased amount of normal protein, or to stop producing protein altogether. Abnormal or decreased amounts of skeletal muscle proteins can affect the development or functioning of the muscle cells, causing the symptoms of LGMD. Most forms of LGMD are autosomal recessive, although some rare forms are autosomal dominant.

An autosomal recessive form of LGMD is caused by a change in both genes of a pair. One of the changed genes is inherited from the egg cell of the mother and one is inherited from the sperm cell of the father. Parents who have a child with an autosomal recessive form of LGMD are called carriers, since they each possess one changed LGMD gene and one unchanged LGMD gene. Carriers do not have any symptoms as they have only one unchanged gene, which produces enough normal protein to prevent the symptoms of LGMD. Each child born to parents who are both carriers for the same type of LGMD has a 25% chance of having LGMD, a 50% chance of being a carrier, and a 25% chance of being neither a carrier nor affected with LGMD. Parents who are each a carrier for a different type of LGMD are not at increased risk for having children affected with LGMD.

The autosomal dominant forms of LGMD are caused by a change in only one gene of a pair. This changed gene is inherited from either the mother or the father. If the changed gene is inherited, each child born to a carrier of LGMD has a 50% chance of inheriting the condition. Sometimes the change occurs spontaneously when the egg and sperm come together to form the first cell of the baby. In this case, other relatives, such as siblings, are probably not at increased risk for inheriting LGMD. People with an autosomal dominant form of LGMD have a 50% chance of passing the condition on to their children. Some people who possess an autosomal dominant LGMD gene change do not have any symptoms.

Demographics

The incidence of LGMD is difficult to estimate as it can have a wide variety of symptoms. The rate of incidence of LGMD is one in 14,500–123,000 people, and is found equally in men and women. LGMD is also difficult to differentiate from other muscular disorders. Some forms of LGMD are found more commonly in people of a certain ethnic background.