This is a rare genetic condition affecting the skeletal muscles used for movement and is characterized by difficulty relaxing certain muscles after contracting them (myotonia) and muscle stiffness. This occurs because of the abnormally repeated electrical signals in the muscles that cause their stiffness. There are two main forms of this disorder; Thomsen disease and Becker disease which are distinguished by the severity of the symptoms and their pattern of inheritance. The Becker disease can begin later in childhood than Thomsen disease causing severe muscle stiffness.
Causes
The genetic mutation identified in the gene CLCN1 causes the myotonia congenita which produces a protein essential for the normal function of the skeletal muscle cells. The muscle movements are controlled by the charged atoms (ions) that flow in and out of the muscle cells. The protein produced by the gene controls the flow of negatively charged chlorine atoms into these cells which prevent the muscles from contracting abnormally. The genetic mutation affects the chloride channels and hence the altered channels are unable to regulate the normal flow of ion. The disruption in chloride ion flow thus can trigger the prolonged muscle contractions which is one of the features of this condition. Thomsen disease is inherited in an autosomal dominant manner meaning a single copy of the defective gene in each cell can cause the disorder. The Becker disease is inherited in an autosomal recessive pattern meaning both the copies of the gene in each cell are defective.
Symptoms
This condition is characterized by the abnormal muscle stiffness, painless spasm of the muscle after forceful voluntary contraction and difficulty relaxing muscle (myotonia). Most of the affected individuals also develop an abnormal enlargement of the voluntary muscles that has the athletic appearance. Thomsen disease may become evident from infancy to around three years of age and are non-progressive. The common sites of the muscle abnormality include the muscles of the legs, hands and the eyelids. When the muscles of the throat become affected, it can present problems with chewing, swallowing and talking. The Becker disease can become apparent between the ages of four and 12 years although in rare cases it can develop as late as 18 years of age. The initial muscles affected are those of the legs which can progress to affect the muscles of the arms, trunk and the face.
Diagnosis
Myotonia congenita can be diagnosed based on the clinical evaluation, detailed history of the individual, the symptoms evident and with various specialized tests. An electromyography can record the electrical activity of the skeletal muscles at rest and with contraction. With the muscle biopsy, the affected tissue sample will be removed and examined under the microscope to identify the characteristic findings. The genetic testing can also detect the mutated gene responsible for the development of the disease.
Treatment
There is no cure for the disease and the treatment is focused on the symptoms evident. The treatment includes medications to help with the muscle stiffness and for the additional symptoms. The special exercise can also help alleviate the symptoms and to improve with the movement of the affected muscles. Additional services include special social support, physical therapy and vocational services. In some cases, genetic counseling may be beneficial for some of the affected cases and their family members.
References
https://rarediseases.org/rare-diseases/myotonia-congenita/
https://ghr.nlm.nih.gov/condition/myotonia-congenita#inheritance
https://medlineplus.gov/ency/article/001424.htm
http://www.myotoniacongenita.org/
