Myotonia congenita is a disorder that affects muscles used for movement (skeletal muscles). Beginning in childhood, people with this condition experience bouts of sustained muscle tensing (myotonia) that prevent muscles from relaxing normally. Although myotonia can affect any skeletal muscles, including muscles of the face and tongue, it occurs most often in the legs. Myotonia causes muscle stiffness that can interfere with movement. In some people the stiffness is very mild, while in other cases it may be severe enough to interfere with walking, running, and other activities of daily life. These muscle problems are particularly noticeable during movement following a period of rest. Many affected individuals find that repeated movements can temporarily alleviate their muscle stiffness, a phenomenon known as the warm-up effect.
The two major types of myotonia congenita are known as Thomsen disease and Becker disease. These conditions are distinguished by the severity of their symptoms and their patterns of inheritance. Becker disease usually appears later in childhood than Thomsen disease and causes more severe muscle stiffness, particularly in males. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not seen in people with Thomsen disease.
The symptoms associated with the condition typically appear in childhood and vary from person to person.
The prolonged muscle contractions, which occur most commonly in the leg muscles in recessive mutations, and more commonly in the hands, face, and eyelids in dominant mutations, are often enhanced by inactivity, and in some forms is relieved by repetitive movement known as "the warm up effect". The warm up effect often diminishes quickly with rest. Some individuals with myotonia congenita are prone to falling as a result of hasty movements or an inability to stabilize themselves after a loss of balance. During a fall, a person with myotonia congenita may experience partial or complete rigid paralysis that will quickly resolve once the event is over. However, a fall into cold water may render the person unable to move for the duration of submergence. As with myotonic goats, children are more prone to falling than adults, due to their impulsivity.
The two major types of myotonia congenita are distinguished by the severity of their symptoms and their patterns of inheritance.
Becker disease usually appears later in childhood than Thomsen disease and causes more severe myotonia, muscle stiffness and transient weakness. Even though myotonia in itself is not normally associated with pain, cramps or myalgia may develop. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time. This muscle weakness is not observed in people with Thomsen disease. However, in recent times, as more and more of the individual mutations that cause myotonia congenita are identified, these limited disease classifications are becoming less widely used.
Early symptoms in a child may include:
- Difficulty swallowing
- Stiff movements that improve when they are repeated-Children with myotonia congenita often appear to be muscular and well-developed. The child may not have symptoms of myotonia congenita until age 2 or 3.
- Frequent falling
- Difficulties opening eyelids after strenuous contraction or crying (von Graefe's sign)
Possible complications may include:
- Aspiration pneumonia (caused by swallowing difficulties)
- Frequent choking or gagging in infants (also caused by swallowing difficulties)
- Abdominal muscle weakness
- Chronic joint problems
- Injury due to falls
- EMG abnormality
Myotonia congenita is caused by a change (mutation) to a gene. It is passed down from either one or both parents to the children (inherited). Myotonia congenita is believed to be caused by a problem in the part of the muscle cells that are needed for muscles to relax. Abnormal repeated electrical discharges occur in the muscles, causing a stiffness called myotonia.
Both conditions Myotonia congenita are caused by mutations in the CLCN1gene. However, the conditions have different modes of inheritance. The Becker type is inherited in anautosomal recessive fashion, and the Thomsen type is inherited in an autosomal dominant manner.
Genetic counseling may be of interest to couples who want to have children and have a family history of myotonia congenita.
The doctor may ask if there is a family history of myotonia congenita. Tests include:
- Muscle biopsy
- Test of the electrical activity in muscles (EMG)
People can do well with this condition. Symptoms only occur when a movement is first started. After a few repetitions, the muscle relaxes and the movement becomes normal. Symptoms may improve later in life.
Some cases of myotonia congenita do not require treatment, or it is determined that the risks of the medication outweigh the benefits. If necessary, however, symptoms of the disorder may be relieved with quinine, phenytoin, carbamazepine, mexiletine and other anticonvulsant drugs. Physical therapy and other rehabilitative measures may also be used to help muscle function. Genetic counseling is available.
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