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Several uncommon forms of muscular dystrophy, all inherited, also cause progressive muscle weakness.
Emery-Dreifuss dystrophy is transmitted in various ways. Only males are affected, but females may be carriers of the gene that causes the disorder. Muscles become weak and waste away (atrophy) any time before age 20 years. The most affected muscles are those of the upper arms, lower legs, and heart. An affected heart commonly causes premature death. Heart pacemakers may help prolong life.
Facioscapulohumeral (Landouzy-Dejerine) muscular dystrophy is transmitted by an autosomal dominant gene. Therefore, a single abnormal gene is sufficient to cause the disorder, and the disorder can appear in either males or females. Symptoms usually begin between the ages of 7 and 20. The facial and shoulder muscles are always affected, so that a child has difficulty whistling, closing the eyes tightly, or raising the arms. Some people with the disease also develop a footdrop (the foot flops down). The weakness is rarely severe, and people who have Landouzy-Dejerine muscular dystrophy have a normal life expectancy.
Limb-girdle muscular dystrophies can be transmitted in various ways. They cause weakness in the muscles of either the pelvis (Leyden-Möbius muscular dystrophy) or the shoulder (Erb's muscular dystrophy). Males and females are affected equally. These inherited disorders often begin in early childhood but may not begin until adulthood. They rarely cause serious weakness.
Mitochondrial myopathies are muscle disorders inherited through faulty genes in mitochondria (the energy factories of cells, which carry their own genes). Because sperm do not contribute mitochondria during fertilization, all mitochondrial genes come from the mother (see Genetics: Mitochondrial Chromosomes). Therefore, although they are equally likely to occur in males and females, these disorders can never be inherited from the father. These rare disorders sometimes cause increasing weakness in one or a few muscle groups, such as the eye muscles (ophthalmoplegia), and may affect other organs, such as the heart or brain. One mitochondrial myopathy is called Kearns-Sayre syndrome.
Diagnosis and Treatment
Diagnosis usually requires removing a sample of the weak muscle tissue for biopsy and either examining it under a microscope or performing chemical tests on it. Specific treatments are not available, but gene therapy is under investigation.
Myotonic Myopathies
Myotonic myopathies are muscular dystrophies in which the muscles are not able to relax normally after contraction. Muscle weakness and spasms may also occur.
Myotonia congenita (Thomsen's disease) is a rare autosomal dominant disorder (only one affected parent is needed to pass the trait on to offspring) that affects males and females. Symptoms usually start in infancy. The hands, legs, and eyelids become very stiff because of an inability to relax the muscles. Muscle weakness, however, is usually minimal. The diagnosis is made from the child's characteristic appearance, inability to relax the grip of the hand rapidly after closing the hand, and prolonged contraction after the doctor taps a muscle. An electromyogram (a test in which electrical impulses from muscles are recorded—see Symptoms and Diagnosis of Brain, Spinal Cord, and Nerve Disorders: Electromyography and Nerve Conduction Studies) is needed to confirm the diagnosis. Myotonia congenita is treated with phenytoin, quinine, procainamide, or mexiletine to relieve muscle stiffness and cramping; however, each of these drugs has undesirable side effects. Regular exercise may be beneficial. People with myotonia congenita have a normal life expectancy.
Myotonic dystrophy (Steinert's disease) is an autosomal dominant disorder affecting males and females. It is the most common muscular dystrophy among whites. Symptoms begin during adolescence or young adulthood. The disorder causes weakness and stiff muscles, especially in the hands. Drooping eyelids are also common. Symptoms can appear at any age and can range from mild to severe. People with the most severe form of the disorder have extreme muscle weakness and many other symptoms, including cataracts, small testes (in men), premature balding in the front (in men), irregular heartbeats, diabetes, and mental retardation. They usually die by age 50. Treatment with mexiletine or other drugs (for example, quinine, phenytoin, or procainamide) has been used, but these drugs do not relieve the weakness, which is the most bothersome symptom to the person. Also, each of these drugs has undesirable side effects. The only treatment for muscle weakness is supportive measures, such as ankle braces and other devices.
Last full review/revision January 2008 by Michael Rubin, MDCM
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