What is myopathy?
Myopathies are diseases of skeletal muscle which are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weak or wasted. Myopathies are usually degenerative, but they are sometimes caused by drug side effects, chemical poisoning, or a chronic disorder of the immune system. 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 problem with, one of the proteins needed for normal muscle cell function.
There are many different types of myopathies, some of which are inherited, some inflammatory, and some caused by endocrine problems. Myopathies are rare and not usually fatal. Typically, effects are mild, largely causing muscle weakness and movement problems, and many are transitory. Only rarely will patients become dependent on a wheelchair. However, muscular dystrophy (which is technically a form of myopathy) is far more severe. Some types of this disease are fatal in early adulthood. Genetic myopathies include central core disease, centronuclear (myotubular) myopathy, myotonia congenita, nemaline myopathy, paramyotonia congenita, periodic paralysis and mitochondrial myopathies. These forms vary by symptoms, severity and genetic mutation. Both dominant and recessive modes of inheritance are also present. Certain forms of centronuclear myopathy, also known as myotubular myopathy, have been found to be X-linked and primarily affects males.
Congenital myopathies are a heterogeneous group of disorders that cause hypotonia in infancy or weakness and delayed motor milestones later in childhood. An autosomal dominant form of nemaline myopathy is linked to chromosome 1 (-tropomyosin gene), and a recessive form to chromosome 2. Other forms are caused by mutations in the gene for the ryanodine receptor (the calcium release channel of the sarcoplasmic reticulum) on chromosome 19q. Skeletal abnormalities and dysmorphic features are common. Diagnosis is made by histochemical and electron microscopic examination of a muscle sample to identify specific morphologic changes. Congenital muscular dystrophy (CMD) is evident at birth and causes general muscle weakness and joint deformities. Central core disease is a slowly progressing skeletal muscle disorder that, unlike most of the muscular dystrophies, is not life-threatening. Myotonia congenital, also known as Thomsen's disease, is a nonprogressive muscle disorder that develops from infancy to childhood. Paramyotonia congenital, also known as Eulenberg's disease, is evident at birth, and like myotonia congenita, is characterized by stiff muscles that take a long time to relax after contraction, it is not progressive, and it does not cause muscle weakening. Myotubular myopathy, also known as centronuclear myopathy, is a slowly progressive disease that causes drooping of the eyelids, foot drop, facial weakness, and other muscle weakness. Nemaline myopathy, also known as Rod body disease, develops from birth to adulthood and is nonprogressive and usually not fatal.
Mitochondrial myopathies range from mild, slowly progressive weakness of the extraocular muscles to severe, fatal infantile myopathies and multisystem encephalomyopathies. Some syndromes have been defined, with some overlap between them. Established syndromes affecting muscle include progressive external ophthalmoplegia, the Kearns-Sayre syndrome (with ophthalmoplegia, pigmentary retinopathy, cardiac conduction defects, cerebellar ataxia, and sensorineural deafness), the MELAS syndrome (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes), the MERFF syndrome (myoclonic epilepsy and ragged red fibers), limb-girdle distribution weakness, and infantile myopathy (benign or severe and fatal).
In some cases, myopathies can be caused by a malfunctioning gland (or glands), which produces either too much or too little of the chemical messengers called hormones. Hormones are carried by the blood and one of their many functions is to regulate muscle activity. Problems in producing hormones can lead to muscle weakness. Hyperthyroid myopathy and hypothyroid myopathy affect different muscles in different ways. Hyperthyroid myopathy occurs when the thyroid gland produces too much thyroxine, leading to muscle weakness, some muscle wasting in hips and shoulders, and, sometimes, problems with eye muscles. The hypothyroid type occurs when too little hormone is produced, leading to stiffness, cramps, and weakness of arm and leg muscles.
Some myopathies are inflammatory, leading to inflamed, weakened muscles. Inflammation is a protective response of injured tissues characterized by redness, increased heat, swelling, and/or pain in the affected area. Examples of this type include polymyositis, dermatomyositis, and myositis ossificans. Dermatomyositis is a disease of the connective tissue that also involves weak, tender, inflamed muscles. In fact, muscle tissue loss may be so severe that the person may be unable to walk. Skin inflammation is also present. The cause is unknown, but viral infection and antibiotics are associated with the condition. In some cases, dermatomyositis is associated with rheumatologic disease or cancer. Polymyositis involves inflammation of many muscles usually accompanied by deformity, swelling, sleeplessness, pain, sweating, and tension. It, too, may be associated with cancer. Myositis ossificans is a rare inherited disease in which muscle tissue is replaced by bone, beginning in childhood.
Metabolic myopathies are characterized by the absence of a substance that is essential for normal muscle function and are associated with genetic defects. In many of these disorders, the symptoms increase after exercise, and a person may experience severe muscle pain during exercise. This is usually due to a lack of oxygen and the absence of the chemicals necessary for maintaining the energy level of the muscle. There are numerous metabolic myopathies. McArdle's disease results from a genetic defect that causes phosphorylase deficiency. It usually develops in adolescence and is characterized by cramps after exercise, and sometimes muscle weakening. Most people can avoid progression of the disease by avoiding strenuous exercise, although about one-third of all people with McArdle's disease eventually have permanent muscle weakness. Phosphofructokinase deficiency, also known as Tarui's disease, is also caused by a genetic defect. Symptoms include cramping after exercise and sometimes muscle weakness. Carnitine palmityltransferase deficiency causes muscle tissue breakdown and pain. An inherited autosomal recessive genetic mutation is implicated. This condition appears to be more common in men than women.
While considered to be a separate group of diseases, the muscular dystrophies also technically involve muscle wasting and can be described as myopathies. These relatively rare diseases appear during childhood and adolescence, and are caused by muscle destruction or degeneration. They are a group of genetic disorders caused by problems in the production of key proteins. Duchenne muscular dystrophy (DMD) is caused by a defective recessive gene on the X chromosome and occurs only in boys. On average, one in three cases results from a new genetic mutation in the mother or grandmother and has not been passed down from generation to generation for very long.Becker muscular dystrophy (BMD) resembles DMD—the muscles are affected in much the same way, and it can cause heart complications—but it develops during adolescence or adulthood, not early childhood. Emery-Dreifuss muscular dystrophy (EDMD) is a slowly progressing disease that affects children and young teenagers. Limb-girdle muscular dystrophy (LGMD) is a slowly progressing weakening of the shoulder and pelvis muscles that onsets anywhere from childhood to middle age. Myotonic dystrophy is also known as Steinart's disease. It is a slowly progressing disease that onsets anytime from childhood through middle age. Oculopharyngeal muscular dystrophy (OPMD) is a slowly progressing disease that first affects the eyelid and throat muscles and causes swallowing difficulties. Distal muscular dystrophy is a slow progressive weakening of the hands, forearms, and lower legs.