Several uncommon forms of muscular dystrophy, all inherited, also cause progressive muscle weakness.
This 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 sudden death.
Doctors usually do blood tests to measure levels of the enzyme creatine kinase in the blood (see Diagnosis), electrical studies of muscle function (electromyography—see see Electromyography and Nerve Conduction Studies), muscle biopsy (examination of a sample of the weak muscle tissue under a microscope), and genetic testing.
Heart pacemakers may help prolong life.
Facioscapulohumeral (Landouzy-Dejerine) muscular dystrophy:
This 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. It is the third most common muscular dystrophy after Duchenne muscular dystrophy and myotonic dystrophy and affects 1 of 20,000 whites. Symptoms usually begin gradually 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). People also frequently have hearing loss and eye problems. The weakness is rarely severe, and many people are not disabled and have a normal life expectancy. However, other people are confined to a wheelchair in adulthood, and, in one form that develops in infancy, children have rapidly progressive muscle weakness and severe disability.
The diagnosis is based on characteristic symptoms, the person's age when symptoms began, family history, and the results of genetic testing.
There is no treatment for the weakness, but physical therapy may help maintain muscle function.
Limb-girdle muscular dystrophies:
These 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 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.
Doctors usually do a muscle biopsy and genetic testing.
Treatment is focused on maintaining muscle function and preventing the muscles from becoming permanently tightened (contracted).
These myopathies are muscle disorders inherited through faulty genes in mitochondria (the energy factories of cells, which carry their own genes) or in the nuclear genes that control mitochondrial function (see see Mitochondrial chromosomes). These rare disorders sometimes cause increasing weakness in one or a few muscle groups, such as the eye muscles (ophthalmoplegia), and often affect many other organs, such as the heart, intestines, or brain. One mitochondrial myopathy is called Kearns-Sayre syndrome.
Myotonia congenita (Thomsen disease):
This dystrophy 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) 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 disease):
This dystrophy is an autosomal dominant disorder affecting males and females. It is the most common muscular dystrophy among whites and affects about 1 in 8,000 people. Symptoms begin during adolescence or young adulthood.
The disorder causes myotonia (very stiff muscles caused by a delayed ability to relax the muscles after contracting them). Other main symptoms are weakness and wasting of arm and leg muscles (especially in the hands) and muscles in the face. Drooping eyelids are also common. The heart muscle also becomes weak (cardiomyopathy) and the heart rhythm may become abnormal. Symptoms begin during adolescence or young adulthood 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 intellectual disability. They usually die by age 50.
Mothers with myotonic dystrophy may have children with a severe form of myotonia that appears during infancy. Infants have severely reduced muscle tone (hypotonia, or "floppiness"), feeding and breathing problems, bone deformities, facial weakness, and delays in development of thought processes and physical movement. Up to 40% of infants do not survive, usually because of respiratory failure and perhaps cardiomyopathy. Up to 60% of survivors have intellectual disability.
Treatment with mexiletine or other drugs (for example, quinidine, phenytoin, carbamazepine, or procainamide) may relieve the stiffness, 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 (for footdrop) and other devices.
Last full review/revision July 2013 by Michael Rubin, MDCM