Myopathies Health Article

Definition

Myopathies are diseases of skeletal muscle that are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weak or shrunken (atrophied).

Description

There are many different types of myopathies. Some are inherited, some inflammatory, and some caused by endocrine or metabolic problems. Myopathies usually are not fatal. Typically they cause muscle weakness and movement problems. The shoulders and thigh muscles are usually, but not always, affected earlier than the muscles of the hands and feet. Most myopathies are degenerative, meaning they become more pronounced over time. Some weaknesses are transitory. Only rarely do individuals become dependent on a wheelchair. However, muscular dystrophy (technically a myopathy) is far more severe. Some types of muscular dystrophy are fatal in early adulthood.

Causes and symptoms

There is great variety among myopathies, but what they all share are effects on the skeletal muscles. The main causes of myopathies are genetic, inflammatory (caused by infection), endocrine (hormonal), and metabolic (errors in how cells function). Often the cause of the myopathy is not known (idiopathic disease).

Genetic myopathies

Among their many functions, genes are responsible for overseeing the production of proteins important in maintaining healthy cells. Muscle cells produce thousands of proteins. With each of the inherited myopathies, a genetic defect is linked to a lack of, or defect in, one of the proteins needed for normal muscle cell function.

There are several different kinds of myopathy caused by defective genes:

• central core disease
• centronuclear (myotubular) myopathy
• myotonia congenita
• nemaline myopathy
• paramyotonia congenita
• periodic paralysis (hypokalemic and hyperkalemic forms)
• mitochondrial myopathies

Most, but not all, of these genetic myopathies are inherited through an autosomal dominant pattern of inheritance. In this pattern of inheritance, one copy of each gene comes from each parent. Only one of these two copies needs to have the mutation (change) or defect in order for the child to have the disease. The parent with the defective gene has the disease, and each of this parent's children has a 50 percent chance of inheriting the disease. This percentage is not changed by results of other pregnancies. With this pattern of inheritance, male and female children are equally at risk of developing the disease.

However, for a child to have one type of myotonia congenita and some forms of nemaline myopathy, two defective genes must be inherited—one from each parents. This is called an autosomal recessive pattern of inheritance. Neither parent may have symptoms of the disease, but each carries a recessive defective gene for it. Each child of such parents has a 25 percent chance of inheriting both genes and showing signs of the disease, and a 50 percent chance of inheriting one defective gene from only one parent. If the child has inherited just one defective gene, he or she will be a carrier of the disease and can pass the gene on to his or her offspring, while showing no signs of the disease himself.

A few forms of centronuclear myopathy develop primarily in males. Females who inherit the defective gene are usually carriers without symptoms, like their mothers, but they can pass on the disease to their sons. Mitochondrial myopathies are inherited only through the mother, since sperm do not contain mitochondria.

The major symptoms associated with the genetic myopathies are:

• Central core disease: mild weakness of voluntary muscles, especially in the hips and legs; hip displacement; delays in reaching developmental motor milestones; problems with running, jumping, and climbing stairs develop in childhood.
• Centronuclear myopathy: weakness of voluntary muscles, including those on the face, arms, legs, and trunk; drooping upper eyelids; facial weakness; foot drop; affected muscles almost always lack reflexes.
• Myotonia congenita: voluntary muscles of the arms, legs, and face stiff or slow to relax after contracting (myotonia); stiffness triggered by fatigue, stress, cold, or long rest periods, such as a night's sleep; stiffness can be relieved by repeated movement of the affected muscles.
• Nemaline myopathy: moderate weakness of voluntary muscles in the arms, legs, and trunk; mild weakness of facial muscles; delays in reaching developmental motor milestones; decreased or absent reflexes in affected muscles; long, narrow face; high-arched palate; jaw projects beyond upper part of the face.
• Paramyotonia congenita: stiffness of voluntary muscles in the face, hands, and forearms; attacks spontaneous or triggered by cold temperatures; stiffness made worse by repeated movement; episodes of stiffness last longer than those seen in myotonia congenita.
• Periodic paralysis: attacks of temporary muscle weakness (muscles work normally between attacks); in the hypokalemic (low potassium) form, attacks triggered by vigorous exercise, heavy meals high in carbohydrates, insulin, stress, alcohol, infection, pregnancy; in the hyperkalemic (high potassium) form, attacks triggered by vigorous exercise, stress, pregnancy, missing a meal, steroid drugs, high potassium intake.
• Mitochondrial myopathies: symptoms vary quite widely with the form of the disease and may include progressive weakness of the eye muscles (ocular myopathy), weakness of the arms and legs, or multisystem problems primarily involving the brain and muscles.