Definition of Muscular Dystrophy

The collective term for a group of genetic disorders of the muscle resulting in progressive weakness. Most types of muscular dystrophy arise from a deficiency of the protein dystrophin, which is essential for skeletal (striated) muscle cell integrity and function. Without it the skeletal muscles deteriorate and movement becomes difficult or impossible.

About 50,000 Americans have muscular dystrophy

The three most common of the nine major types of muscular dystrophy are Duchenne’s muscular dystrophy, facioscapulohumeral muscular dystrophy, and myotonic muscular dystrophy.

Duchenne’s Muscular Dystrophy

Duchenne’s is an X-linked recessive mutation affecting the dystrophin gene. As such, it nearly exclusively affects boys. Symptoms begin to appear in early childhood with characteristic postures and gait.

Progression is steady, and most boys who have Duchenne’s lose the ability to walk by about age 12. The skeletal muscles of the upper chest become involved in adolescence, affecting breathing. Duchenne’s is usually fatal before age 20.

A milder presentation of similar symptoms and pattern of progression with a later age of onset (late childhood or early adolescence) is Becker’s muscular dystrophy. Though the course of the disease is ultimately fatal, most who have it live into their 30s.

Treatment is primarily supportive, with physical therapy to help preserve muscle strength and function. Corticosteroid medications may improve symptoms.

Facioscapulohumeral Muscular Dystrophy

An adult-onset type of muscular dystrophy, facioscapulohumeral muscular dystrophy affects men and women equally. Symptoms first appear as weakness in the muscles of the face and shoulder girdle (upper arms and shoulders). The shoulders often “wing” outward. Over the course of the disease, muscle weakness moves downward through the body though the lower arms are usually the last affected.

Symptoms are mild enough in about half of those who have this form of muscular dystrophy to permit fairly normal function and mobility throughout life. In others, symptoms may affect swallowing and mobility.

Myotonic Muscular Dystrophy

In myotonic muscular dystrophy the muscles lose the ability to relax after contraction, causing them to become stiff. Myotonic muscular dystrophy is the most common type of adult-onset muscular dystrophy and affects men and women equally.

The cause is a mutation in the gene that encodes for myotonica protein kinase. Other gene mutations may also contribute. Though progression is usually slow, myotonic muscular dystrophy affects other body systems as well. Cataracts and diabetes are common.

MAJOR TYPES OF MUSCULAR DYSTROPHY
Type of Muscular DystrophyKey CharacteristicsInheritance Pattern
Duchenne’smost common type
affects primarily muscles of the upper arms, upper legs, and pelvic girdle
first symptoms usually appear between ages 2 and 6
X-linked recessive
myotonicaffects primarily muscles of the face and neck, hands, and feet
gastrointestinal, cardiac, eye, neurologic, and endocrine involvement later in the disease
first symptoms appear in adulthood
autosomal dominant
Becker’saffects primarily muscles of the upper arms, upper legs, and pelvic girdle
very similar to Duchenne’s with milder symptoms
symptoms begin in late childhood or early adolescence
X-linked recessive
limb-girdleaffects primarily the muscles of the pelvic girdle and shoulder girdle
symptoms begin in late adolescence or early adulthood
autosomal recessive or autosomal dominant
facioscapulohumeralaffects primarily the muscles of the face, neck, and shoulders
symptoms begin in late adolescence or early adulthood
autosomal dominant
congenitalaffects all skeletal muscles
often affects the central nervous system, causing seizures
symptoms are present at birth
autosomal recessive
oculopharyngealaffects the muscles of the eyelids and throat
symptoms begin in middle to late adulthood
autosomal dominant
distalaffects the forearms, hands, lower legs, and feet
symptoms begin in adulthood
autosomal recessive or autosomal dominant
Emery-Dreifussaffects primarily the shoulders, upper arms, pelvis, and lower legs
symptoms typically appear first as contractures, then weakness
symptoms begin in late childhood or early adolescence
X-linked recessive

Symptoms and Diagnostic Path

In most forms of muscular dystrophy, the primary symptoms are muscle weakness and disturbances of posture and gait (walking style). The diagnostic path begins with detailed personal health history and family health history. Because muscular dystrophies are inherited disorders, the family health history is particularly important.

A comprehensive neurologic examination identifies the specific symptoms, which helps narrow the diagnosis. Blood tests may show excessive proteins that indicate muscle destruction. Each type of muscular dystrophy has fairly characteristic patterns of symptoms. Muscle biopsy shows damage to the muscle cells.

Treatment Options and Outlook

Treatment for all types of muscular dystrophy is primarily supportive. Physical therapy, braces, orthotics, and mobility aids extend the ability to walk and function independently. Corticosteroid medications slow the progression of symptoms in some types of muscular dystrophy, notably Duchenne’s.

Though all types of muscular dystrophy are lifelong, muscular dystrophy is not necessarily fatal. Many people with milder types of the disease live normal life expectancy with relative independence.

Risk Factors and Preventive Measures

Muscular dystrophy is always inherited, so the key risk factor is family history. People who have muscular dystrophy or family history of muscular dystrophy should consider genetic counseling to aid in family planning decisions.

There are no measures known to prevent muscular dystrophy, though research holds hope for gene therapy that can someday correct the mutations that cause the disease.

See also CATARACT; MYOPATHY.

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