Muscular degenerative diseases, or muscular dystrophies, are a group of hereditary diseases in which muscles are progressively destroyed. According to "Harrison's Principles of Internal Medicine," most are diagnosed early in life and lead to a limited life span. The genetic basis of some of these diseases has been identified. Genetic counseling can assist a family who has an affected child in assessing the risk that additional children will also suffer from the disease. Some treatments slow the progression of the disease for a few years, but muscular dystrophies have no cure.
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy appears in 30 out of every 100,000 male births. The gene responsible for the disease is located on the X chromosome, which means that females can be carriers but only males are affected. A definitive diagnosis is made by a muscle biopsy, which reveals an absence of dystrophin, a protein necessary for normal muscle function.
Symptoms first appear between 3 and 5 years of age. The child tends to fall and has difficulty playing physical games. Weakness affects the legs more than the arms, and proximal muscles, such as the thigh muscles, more than distal muscles such as those of the feet. Gower's sign appears--the boy gets up from the floor by putting his hands on the thighs and pushing. Later on, contractures develop in joints, causing the child to walk on his toes. Severe scoliosis develops, resulting in a deformed chest.
Children with Duchenne are usually confined to a wheelchair by age 12. Intelligence is often below normal. The heart muscle is also affected. Weak respiratory muscles along with the deformed chest predispose the patients to lung infections. Life expectancy is around 20 years.
Becker Muscular Dystrophy
Becker muscular dystrophy is similar to Duchenne in many ways, but its symptoms are less severe. Only boys are afflicted. Proximal muscles are affected first, primarily the thigh muscles. Later on the weakness spreads to the rest of the musculature. The calf muscles become hypertrophic, which means they appear larger than normal. Symptoms first appear between ages 5 and 15, though sometimes the disease can first manifest as late as the fourth decade of life. At age 15, patients are still walking, unlike children with Duchenne. Mental retardation is less common than in Duchenne. The cardiac muscle is often involved and can lead to heart failure--sometimes this is the only symptom. A definitive diagnosis is made by taking a muscle biopsy or performing a DNA analysis of white blood cells. In Becker muscular dystrophy, dystrophin is reduced but not absent altogether.
Myotonic Dystrophy
According to the Genetics Home Reference of the National Institues of Health, myotonic dystrophy is the most common form of dystrophy that starts in adulthood. Myotonic dystrophy involves not only the muscles but other systems. Cataracts, gonadal atrophy and daytime sleepiness are other common features. Frontal baldness and wasting of the facial muscles cause a typical myotonic appearance.
Neck muscles and distal muscles, such as the muscles of the hand, are affected early in the course of the disease. It becomes difficult to perform tasks that require normal hand function. Muscles are not only weak, they have a hard time relaxing. When a patient shakes hands, he can't let go for a while. Foot drop may appear, which means the patient has a difficult time lifting his foot when he walks.
Muscles of the pharynx, palate and tongue are affected, causing a nasal voice and difficulty swallowing. Respiratory muscles are weakened and can lead to respiratory failure. Heart involvement can cause arrhythmias and pacemakers are often used. Life expectancy is shortened. Death is usually caused by respiratory or cardiac failure.
References
- "Harrison's Principles of Internal Medicine, 17th edition"; Anthony Fauci et al.; 2008
- Genetics Home Reference: Myotonic Dystrophy
- "Adams and Victor's Principles of Neurology"; Maurice Victor and Allan Ropper; 2001
