Myotonia congenita is a rare genetic disorder in which the skeletal muscles have difficulty in relaxing. The most commonly affected muscles are those in the legs, although any skeletal muscle may be involved.
The two types of myotonia congenita share clinical similarities but also have few differences.
Symptoms of this form become apparent in infancy or toddler ages. The features are not progressive. The degree of severity can differ from one family member to another while males have a more serious clinical picture. Also, it is thought that cool temperatures may exacerbate symptoms in these patients.
This disease predominantly affects the muscles of the legs, hands and/ or eyelids . These individuals experience challenges with releasing of a hand grip, walking, running, and transitioning from lying or sitting positions to standing.
Further characteristics of the disease include spasms of muscles such as those of the face, trunk, and other parts of the body. When the extraocular muscles are involved, the patients develop acute episodes of diplopia or strabismus. Also, myotonia of the throat muscles causes trouble with swallowing, chewing and speaking following a period of rest.
The symptoms associated with the Becker variant become visible between the ages of 4 to 12 years old. However, there have been later onset cases reported. This disease is progressive and more severe, which is in contrast to the Thomsen type. Another difference from the other form is that cold exposure does not worsen symptoms.
These patients initially experience myotonia in the leg muscles, but the disease eventually progresses to involve the arms, trunk, and face .
On the physical examination, there is profound hypertrophy, and therefore, the individuals typically present with the "body-builder" physical appearance. They tend to exhibit muscle hypertrophy. Also, muscle weakness may be seen in such patients.
The evaluation of the patient includes a thorough personal and family history along with a comprehensive physical exam. The workup further consists of specialized studies.
The clinician should offer genetic counseling to the family members and the patients themselves when of appropriate age. Genetic testing may be an option as well. However, since certain mutations are implicated in both dominant and recessive traits, special consideration should be applied while interpreting the molecular analysis .
The physical exam reveals characteristic findings such as difficulty with relaxing a hand grip following the closing of the hand. Also, another positive clinical test is percussion myotonia, which is the persistent contraction that results after tapping the muscle.
This test will confirm the diagnosis. An EMG records the electrical activity of the skeletal muscle during rest and contraction . Specifically, the results are characterized by repetitive action potentials which develop after myotonic discharges or forceful contractions.
Some patients will be assessed with muscle biopsy, in which a sample of skeletal muscle tissue is obtained and submitted for analysis. The results reveal hypertrophic changes.
Additional studies may be performed to exclude similar diseases and to help confirm the diagnosis of myotonia congenita.
Finally, there is a rare study in which a sample of the muscle tissue is exposed to halothane. This in vitro testing will show a contracture response that mimics a picture similar to that of malignant hyperthermia.
The management of this disorder may involve pediatricians, orthopedists, physical and occupation therapists, psychologists, and other professionals to assist in social and vocational services. Furthermore, early intervention can allow the patients to overcome the symptoms.
The treatment for both types is usually symptomatic and supportive especially in those with stiffness and cramping. Studies have shown that stabilizers of the skeletal muscle membranes are the superior drugs. The myotonic features are treated with antiseizure medications such as carbamazepine, phenytoin , and acetazolamide .
Additionally, muscle relaxants may be helpful; these include dantrolene sodium and quinine sulfate. A sodium channel inhibitor called mexiletine may be used. Finally, the antihistamine, trimeprazine, can be beneficial. Since all of these medications are associated with side effects, the clinician should weigh the risks and benefits.
Genetic counseling is important for patients and their family members. They should be provided with information on modes of transmission, the probability of having affected and carrier offspring, and other details as well.
In Thomsen disease, the inheritance pattern is autosomal dominant , while the Becker form is transmitted through an autosomal recessive trait . The identified mutations are in the CLCN1 gene, which plays a pivotal role in the function of chloride ion channels.
Both variants are referred to as "ion channel diseases" since they emerge from the abnormal conductance of ions across the cell membranes of the skeletal muscles .
The estimated prevalence of myotonia congenita ranges from 1 to 10 per population of 100,000. Also, the onset can occur in infancy, early childhood, or even adulthood. If this disease manifests in early infancy, typically the family members recognize the abnormalities in the patient.
Both Thomsen and Becker myotonia congenita are associated with mutations in the CLCN1 gene. The mutations are located on the long arm of chromosome 7 (7q35) . CLCN1 is a gene that plays a role in the function of chloride ion channels in the cell membranes of skeletal muscles.
The chloride channels are responsible for the regulation of the electric excitability of the cell membranes. Hence, a defect in the CLCN1 gene will diminish the numbers of the channels and/or affect their function. These changes lead to decreased chloride flow and the resultant electrical instability accounts for the findings observed in myotonia congenita.
The sodium channel may also be involved in the pathogenesis of this disease. It is thought to participate in the hyperexcitability of the muscles seen with myotonia congenita.
Since myotonia congenita is a genetic disease, it cannot be prevented. However, patients and family members can seek genetic counseling and receive education to guide them in family planning. Moreover, they may be offered prenatal genetic testing as well.
Myotonia congenita is an inherited disorder in which involved skeletal muscles cannot relax after hyperexcitability or an enhanced response to a stimulus. The mutations responsible for this condition are found in the CLCN1 gene, which has two modes of transmission- autosomal dominant or autosomal recessive. Additionally, there are two forms of the condition: Thomsen disease , which is autosomal dominant, and Becker disease , which is autosomal recessive.
This rare disorder presents in early infancy, early childhood, or slightly later depending on the type. The symptoms are myotonia and rigidity, along with other associated features. Additionally, patients may experience difficulty with moving affected muscles following a period of rest. With regards to appearance, individuals may exhibit enlargement of the muscles and resemble a "herculean" appearance.
The diagnosis is achieved through assessment of the patient's personal and family history, key observations on physical examination, and specific findings on the electromyogram (EMG). Further studies may be warranted to exclude similar diseases.
The therapeutic approach is usually symptomatic and supportive as there are medications that can help alleviate the myotonia and stiffness. Other important components in the treatment involve physical exercises.
Overall, the prognosis of myotonia congenita is generally good.
Myotonia congenita is an inherited disorder that affects the relaxation of skeletal muscle cells. This occurs because of a mutation, or a change in the gene that controls the function of specific proteins that allow ions to travel across the cells. Symptoms may appear during the first few months, early childhood, or early adulthood.
There are two forms of this condition, and each one is inherited differently. One type is known as Thomsen myotonia congenital which is autosomal dominant. The other type is the Becker myotonia congenita, which is inherited in an autosomal recessive pattern.
The main symptom is that the skeletal muscles cannot relax quickly after the episode of contraction. This is observed when the patients cannot pull their hand right away after a handshake. It is a slow process to relax the muscle. Since the muscles have difficulty relaxing, this results in the stiffness known as myotonia.
Patients may also have difficulty with activities like walking, running, or changing positions. They may experience trouble with chewing, swallowing and talking. Some patients may suffer from shortness of breath or chest tightness at the beginning of the physical exercise.
The steps in diagnosis include taking detailed personal and family history, physical exam, and an electromyogram. For example, the clinician tests the patient by assessing whether the patient can release the grip quickly after closing his or her hand. Also, the doctor may tap a muscle and check for a prolonged contraction.
The electromyogram tests the electrical impulses from the affected muscles. The abnormal repetitive impulses can confirm this disease.
Note that these patients may have a "body builder" type of body with enlarged muscles.
Treatment of myotonia congenita involves a multidisciplinary approach that consists of pediatricians, orthopedics, physical therapists, and other healthcare professionals. The earlier the intervention, the better for the children.
Specific drug therapies aim to relieve the symptoms such as stiffness and cramping. For example, muscle relaxants and anticonvulsants can be useful. Phenytoin, quinine, and mexiletine are the examples of drugs commonly prescribed for this condition .
Special physical exercises are recommended to help relieve stiffness in these patients.