Muscular Dystrophy

Muscular dystrophy is a group of hereditary disorders characterized by progressive symmetrical wasting of skeletal muscles but no neural or sensory defects. Four main types of muscular dystrophy occur: Duchenne's (pseudohypertrophic) muscular dystrophy, which accounts for 50% of all cases; Becker's (benign pseudo­hypertrophic) muscular dystrophy; Landouzy-Dejerine (facioscapulohumeral) dystrophy; and Erb's (limb-girdle) dystrophy. Duchenne's and Becker's muscular dystrophies affect males almost exclusively. The other two types affect both sexes about equally.

Depending on the type, the disorder may affect vital organs and lead to severe disability, even death. Early in the disease, muscle fibers necrotize and regenerate in various states. Over time, regeneration slows and degeneration dominates. Fat and connective tissue replace muscle fibers, causing weakness.

The prognosis varies. Duchenne's muscular dystrophy typically begins during early childhood and causes death within 10 to 15 years. Patients with Becker's muscular dystrophy may live into their 40s. Landouzy-Dejerine and Erb's dystrophies usually don't shorten life expectancy.

Causes

Muscular dystrophy is caused by various genetic mechanisms. The basic defect can be mapped genetically to band Xp 21. Duchenne's and Becker's muscular dystrophies are X-linked recessive, and Landouzy­Dejerine dystrophy is autosomal dominant. Erb's dystrophy may be inherited in several ways but usually is autosomal recessive.

Exactly how these inherited defects cause progressive muscle weakness isn't known. They may create an abnormality in the intracellular metabolism of muscle cells. The abnormality may be related to an enzyme deficiency or dysfunction, or to an inability to synthesize, absorb, or metabolize an unknown substance vital to muscle function.

Signs and Symptoms

Principal symptoms include:

• Progressive muscle wasting, weakness, and loss of function
• In Duchenne's MD, delayed development of basic muscle skills and coordination in children. Common signs include poor balance with frequent falls, walking difficulty with waddling gait and calf pain, and limited range of movement
• Obesity
• Joint contractures
• Cataracts, frontal baldness, drooping eyelids, gonadal atrophy, and mental impairment (with myotonic dystrophy)
• Intellectual impairment

Diagnostic tests

Several tests are used to help confirm the diagnosis:

• Muscle biopsy shows fat and connective tissue deposits and confirms the diagnosis. It also shows degeneration and necrosis of muscle fibers and, in Duchenne's and Becker's muscular dystrophies, a deficiency of the muscle protein dystrophin.
• Immunologic and biological techniques available in specialized medical centers facilitate prenatal and postnatal diagnosis of Duchenne's and Becker's muscular dystrophies. These techniques also help identify a person as a carrier. In addition, newer techniques are replacing muscle biopsy and serum creatinine kinase (CK) test as diagnostic procedures.
• Electromyography typically demonstrates short weak bursts of electrical activity in affected muscles
• Urine creatinine, serum CK, lactate dehydrogenase, alanine aminotransferase, and aspartate aminotransferase levels are elevated. CK levels increase before muscle weakness becomes severe, providing an early indicator of Duchenne's and Becker's muscular dystrophies. These diagnostic tests are also useful for genetic screening because unaffected carriers also show elevated enzyme levels.
• Amniocentesis can't detect muscular dystrophy definitively, but because it reveals the sex of the fetus, it may be recommended for pregnant women known to carry the gene for Duchenne's or Becker's muscular dystrophy.
• Genetic testing can be used to detect the gene defect that leads to muscular dystrophy in some families.

Treatment

Currently, no treatment can stop the progressive muscle impairment. Orthopedic appliances, exercise, physical therapy, and surgery to correct contractures can help preserve the patient's mobility and independence.

Prevention

Genetic counseling is advised when there is a family history of muscular dystrophy. Women may have no symptoms but still carry the gene for the disorder. Duchenne muscular dystrophy can be detected with about 95% accuracy by genetic studies performed during pregnancy.