Becker’s myotonia is a pathology with an autosomal recessive type of inheritance caused by hyperexcitability of muscle fibers of skeletal muscles and manifested by a violation of muscle relaxation after their contraction. The clinical picture of the disease consists of a myotonic symptom complex, manifested first in the lower and then in the upper extremities. A thorough history study, neurological examination data, EMG and muscle biopsy, genetic counseling helps to establish the diagnosis. Treatment is based on the use of medications that reduce myotonic spasms, diuretics and calcium preparations. Patients are shown physiotherapy and exercise therapy.
General information
The first medical description of congenital myotonia dates back to 1876. It was made by the Dane Julius Thomsen. For a long time, Thomsen’s myotonia was considered the only variant of this disease. However, in 1971, the German Emil Peter Becker described a more severe form of congenital myotonia, which has a different order of inheritance. In modern neurology, it is known as Becker’s myotonia.
Both diseases are allelic variants of mutations of the CLCN1 gene localized on the long arm of the 7th chromosome. The joint frequency of their occurrence in Europe is approximately 1 case per 100 thousand families, and in Scandinavia — 1 case per 10 thousand families.
Etiology and pathogenesis
The reason for the development of Becker’s disease lies in an inherited defect of the CLCN1 gene encoding the protein of the chlorine channels of skeletal muscle fibers. With functional inferiority of the protein or its absence, the ability of chlorine channels to pass chlorine ions into the muscle fiber decreases. As a result, the ionic equilibrium is disturbed and electrical instability of the fiber membrane occurs, which leads to hyperexcitability of the latter. As a result, in response to normal nerve impulses, there is an increased excitation of muscle fibers, clinically manifested by delayed relaxation of muscles after their contraction.
The disease is inherited autosomal recessive, i.e. its development in a child occurs only in cases when both parents are carriers of the pathological gene.
Symptoms
Unlike Thomsen’s disease, the manifestation of the first symptoms of which neonatologists observe immediately after birth or during the newborn period, Becker’s myotonia manifests in older children. For girls, the age of manifestation of the disease varies from 4 to 12 years, for boys it is most often 18 years.
The basis of the clinical picture of Becker’s myotonia is the myotonic phenomenon. It is characterized by myotonic spasm occurring after active movement, followed by delayed relaxation of the muscles involved in the movement; the occurrence of generalized myotonic spasm when trying to perform a rapid motor act; a decrease in spasticity during repeated movements. Typically, a decrease in the manifestation of myotonic syndrome when the patient is warm and its increase in the cold.
The most typical and accessible signs of the myotonic phenomenon include: the symptom of a fist — the inability to quickly unclench the fingers after their compression into a fist, flexion and reduction of the thumb of the hand when struck with a neurological hammer on the elevation of its tenor, the appearance of a characteristic local contraction in the form of a roller when tapping a hammer on a muscle.
In Becker’s disease, the myotonic phenomenon is more pronounced than in Thomsen’s disease; in a number of patients, it is accompanied by muscle pain. In addition to damage to the distal muscle groups, myotonia of the muscles of the proximal extremities may be noted. The appearance of the first signs of the disease in the leg muscles is characteristic. Then, after a few years, the myotonic phenomenon appears in the muscles of the arms. In the later stages of Becker’s disease, facial muscles are involved in the pathological process. There may be a decrease in muscle strength. Muscle hypertrophy typical of Thomsen’s myotonia is quite rare.
Diagnostics
The detection of manifestations of the myotonic phenomenon during the neurological examination and the absence of other pathological abnormalities in the status makes it possible to diagnose myotonia without difficulty. It is much more difficult for a neurologist to determine its type. anamnesis of the disease plays an important role here (its debut, the order of development of symptoms, features of the course), family history (the presence of cases of Becker’s myotonia in relatives), features of the clinic (for example, the absence of muscular hypotrophy typical of dystrophic myotonia).
Diagnostic search usually includes: biochemical blood analysis, electromyography (EMG) or electroneurography (ENG), muscle biopsy, consultation of a geneticist and genetic analysis. In terms of differential diagnosis, it is necessary to exclude Thomsen’s myotonia, dystrophic myotonia, myopathies, regid man syndrome, pseudomyotonia in hypothyroidism.
There are no specific biochemical markers for Becker’s myotonia. In a number of patients, a significant increase in creatine phosphokinase activity is observed in the blood serum. Electromyography data make it possible to detect the presence of pathognomonic discharges for myotonia while maintaining the parameters of DE (motor units) potentials. Morphological examination of muscle biopsies reveals hypertrophy of fibers and centralization of their nuclei, but is not specific for Becker’s myotonia. Confirmation of the disease is the data of molecular genetic analysis, indicating the presence of a mutation in the CLCN1 gene.
Treatment
Like most gene pathologies, the disease does not have a radical specific treatment. Therapy is mainly aimed at reducing the phenomena of myotonic spasm. For this purpose, the use of diphenine, novocainamide, carbamazepine, phenytoin, etc. is recommended. To reduce deviations in the ion balance, patients are prescribed diuretics (acetazolamide) and calcium preparations, a diet is prescribed that restricts the intake of potassium salts into the body. Of the methods of physiotherapy, electrophoresis with calcium is used. An important element of treatment is the development of a special complex of physical therapy by the patient.
Patients with Becker’s myotonia should avoid sudden and rapid movements, hypothermia, and staying in the cold. This will allow them to significantly reduce the frequency of myotonic seizures and ease the course of the disease.
Prognosis and prevention
The prognosis of recovery is unfavorable. With systematic and complex treatment, excluding factors that provoke myotonic spasms, it is possible to achieve a slower progression of the disease. Since only rapid movements cause difficulties in patients, most patients adapt to their disease, successfully undergo social and labor adaptation, although they have limited ability to work. As a rule, adult patients receive group III disability.
For preventive purposes, families in which cases of Becker’s myotonia have been noted are recommended genetic counseling during pregnancy planning, pregnancy management with prenatal DNA diagnostics.