Muscular Dystrophy Health Article

Definition

Muscular dystrophies (MD) are inherited disorders characterized by progressive weakness and degeneration of the skeletal or voluntary muscles which control movement, without a central or peripheral nerve abnormality. The muscles of the heart and other involuntary muscles are also affected in some forms of MD, and a few forms involve other organs as well. The major forms of muscular dystrophy include myotonic, Duchenne, Becker, limb-girdle, facioscapulohumeral, congenital, oculopharyngeal, distal, Emery-Dreifuss and Fukuyama muscular dystrophy.

Description

The commonest form of these inherited disorders is the Duchenne muscular dystrophy (DMD). The disorder was originally described in the mid-nineteenth century by the English physician Edward Meryon. At a meeting of the Royal Medical and Chirurgical Society in 1851, and later published in the transactions of the society, he described in detail the clinical presentation of Duchenne muscular dystrophy, beginning in early childhood with progressive muscle wasting and weakness and leading to death in late adolescence. Furthermore, his detailed histological studies led him to conclude that the muscle membrane or sarcolemma was broken down and destroyed.

Duchenne muscular dystrophy will usually produce symptoms between the ages of three and seven in young boys. It begins with a weakness in the pelvic area first and then progresses to the shoulder muscles. As the disorder escalates, the muscles enlarge although the muscle tissue is weak. The heart muscle will also enlarge, creating problems with the heartbeat that can be detected on an electrocardiogram. In most cases, the affected child has a waddling walk, often falls, has difficulty rising from a sitting position, has a difficult time climbing stairs, is unable to fully extend the arms and legs, and may develop scoliosis (an abnormally curved spine). In most cases, children with DMD are confined to a wheel chair between the ages of ten and twelve.

Most people with Becker muscular dystrophy (BMD) first experience difficulties between the ages of five and fifteen years, although onset in the third or fourth decade or even later can occur. By definition, patients with BMD are able to walk beyond age fifteen, while patients with DMD are typically in a wheelchair by the age of twelve. Patients with BMD have a reduced life expectancy, but most survive into the fourth or fifth decade. Mental retardation may occur in BMD, but it is not as common as in DMD. Cardiac (heart muscle) involvement occurs in BMD and may result in heart failure.

Myotonic muscular dystrophy (MMD) affects the muscles in the hands and feet. Limb-girdle muscular dystrophy (LGMD) begins late in childhood affecting mainly the muscles of the shoulders and hips. Facioscapulohumeral muscular dystrophy (FSHD) affects only the muscles of the upper arms, face and shoulder girdle. Landouzy-Dejerine muscular dystrophy (LDMD), which is transmitted by an autosomal dominant gene, affects the face, shoulder and lower leg muscles.

Other disorders related to muscular dystrophy include Steinert's disease, Thomsen's disease, and Pompe's disease. Steinert's disease affects both males and females, causing the muscles to be unable to relax after contracting, while Thomsen's disease causes a stiffness of the legs, hands and eyelids. Pompe's disease, which is a glycogen storage disease, affects the liver, heart, nerves and muscles.

United States

The incidence of muscular dystrophy varies, depending on the specific type. Duchenne muscular dystrophy is the most common condition. It is inherited on the X chromosome, primarily affects boys, and is the most severe type of the disease. Although women with the defective gene are carriers, they usually show no symptoms. DMD has an inheritance pattern of 1 case per 3,500 live male births, and one-third of cases are due to spontaneous new mutations.

Becker muscular dystrophy is the second most common form, with an incidence of 1 case per 30,000 live male births. Like DMD, BMD is linked to the X chromosome. Other types of MD are rare. Limb-girdle muscular dystrophy includes several different illnesses, which can be inherited by both males and females, as can facioscapulohumeral muscular dystrophy.

International

The incidence of muscular dystrophies internationally is similar to that of the United States, however some types are especially frequent in certain populations and are rare elsewhere. For example, autosomal dominant distal muscular dystrophy occurs more often in Scandinavia than elsewhere, Fukuyama muscular dystrophy in Japan, oculopharyngeal muscular dystrophy in French Canada, and several autosomal recessive LGMD in communities in Brazil, North America, and the Middle East.