Врожденная миотония: симптомы, причины и особенности течения.

Myotonia congenita is a congenital neuromuscular channelopathy that affects skeletal muscles (muscles used for movement). It is a genetic disorder. The hallmark of the disease is the failure of initiated contraction to terminate, often referred to as delayed relaxation of the muscles (myotonia) and rigidity. Symptoms include delayed relaxation of the muscles after voluntary contraction (myotonia), and may also include stiffness, hypertrophy (enlargement), transient weakness in some forms of the disorder (from certain genetic mutations), severe masseter spasm, and cramping. The condition is sometimes referred to as fainting goat syndrome, as it is responsible for the eponymous 'fainting' seen in fainting goats when presented with a sudden stimulus. Of note, myotonia congenita has no association with malignant hyperthermia (MH).

Both Thomsen and Becker myotonia have high phenotype variability. Severity of symptoms can vary greatly between individuals and throughout the lives of the individuals themselves. This may be partly because there are over 130 currently known different mutations that can cause the disorder, each with their own specifics, and also because myotonia congenita is an ion channel disorder, and ion channels are sensitive to internal and external environmental factors. It has been shown that pregnancy and the use of diuretics aggravate myotonia, and both these conditions are linked to the loss of divalent cations such as magnesium and calcium. It has further been shown that in chemically induced myotonia in isolated rat muscle, myotonia could be dampened by increasing the magnesium and calcium content of the extracellular medium. This has also been shown for isolated human muscle. Adrenaline/epinephrine is well known to make myotonia worse in most individuals with the disorder, and a person with myotonia congenita may experience a sudden increase in difficulty with mobility in a particularly stressful situation during which adrenaline is released. Due to the invisible nature of the disorder, the fact that those with myotonia congenita often appear very fit and able bodied, the general lack of knowledge about the disorder by the general and medical community, and often by the individual themselves, and the potential for inconsistency with the symptoms, many people with myotonia congenita have experienced a degree of social persecution at one time or another because of the effects of their disorder.

This phenomenon was described along with the disease by Thomsen in 1876 but its etiology remains unclear. Patients report that myotonia congenita may present itself in the following ways (this is from first hand experience). If the person is sedentary and then decides to walk up a set of stairs, by the third or fourth step their leg muscles begin to stiffen significantly, requiring them to slow down almost to a complete stop. But as the muscles loosen up, a few steps later, they can once again begin to climb the steps at a normal pace. If this person plays any kind of a sport, a good warm up is mandatory. Otherwise if they need to quickly and intensively use their muscles, such as in a sprint race or a basketball game, their muscles will freeze up, causing them to slow down or almost come to a complete stop. But once the muscles are warmed up, they can once again function normally. This can happen in various muscles, even in muscles such as the tongue. For example, if a person has not spoken for awhile and then wants to speak, their tongue may be stiff at first causing the words to come out a little garbled, but after a few seconds of trying to speak, the tongue muscle will loosen up and then they can speak normally for the remainder of the time that they are conversing. Patients report that repeated contraction of muscle alleviates present myotonia with each contraction, such that myotonia is almost absent after a few contractions of the same muscle. The effect lasts about five minutes. There have been several proposed mechanisms for this phenomenon, but none have been demonstrated conclusively; one hypothesis is that the Na+/K+ ATPase is stimulated during the myotonic activity by increased intracellular Na+ in the cytosol of the muscle cell, increasing the activity of the Na+/K+ ATPase. However, in experiments with patients where the Na+/K+ ATPase had been blocked in the underarm by infusion of the Na+/K+ ATPase blocker Ouabain, no effect on warm up was observed. Another hypothesis states that the few remaining functional chloride channels in muscle may become more active with increased muscle activity. It has been proposed that inactivation of sodium channel protein type 4 subunit alpha, residing in skeletal muscle, could play an important role in the warm up phenomenon. In particular, slow inactivation of the channel is believed to have a spatial and temporal extent that is correlated to warm up and therefore may provide a plausible cause.

The disorder is caused by mutations in part of a gene (CLCN1) encoding the ClC 1 chloride channel, resulting in muscle fiber membranes having an unusually exaggerated response to stimulation (hyperexcitability). Three cases have been reported who were diagnosed with Thomsen's myotonia and proved on genetic testing not to have mutations in the chloride gene but rather in the alpha subunit of the voltage gated sodium channel (SCN4A). Like chloride channel mutations, patients with sodium channel mutations may have a very varied phenotype, making a diagnosis more difficult.

Two types of myotonia congenita exist, an autosomal dominant form and an autosomal recessive form. Autosomal dominant myotonia congenita (OMIM #160800) is also called Thomsen disease, after Danish/German physician Asmus Julius Thomas Thomsen (1815–1896), who himself had the disease and who wrote the first description of it in the medical literature (in 1876). Autosomal recessive myotonia congenita (OMIM #255700) is also called generalized myotonia, recessive generalized myotonia (RGM), Becker disease, and Becker myotonia, after the German professor Peter Emil Becker, who discovered its recessive nature. The term congenital in its sense of "clinically apparent from birth" applies only to Thomsen disease, as the clinical onset of Becker myotonia may be delayed up to the age of 4 to 6 years. But in either form of myotonia congenita, the term's strictest sense reflects that the disease is genetically present from birth, although the clinical onset may be delayed. With the advent of genetic testing, it has recently been found that some typically recessive mutations may occur in a dominant fashion in some individuals. The reason for this is not known. Because several CLCN1 mutations can cause either Becker disease or Thomsen disease, doctors usually rely on characteristic signs and symptoms to distinguish the two forms of myotonia congenita. However, myotonia caused by CLCN1 mutations can occasionally be clinically indistinguishable from myotonia caused by sodium channel mutations (SCN4A mutations) resulting in the similar disease paramyotonia congenita. A so called Finnish heritage disease, congenital myotonia is more common in Finland and among ethnic Finns. A molecular study of the CLCN1 gene in 24 families in northern Finland, including 46 affected individuals, showed that although the inheritance appeared to be dominant (Thomsen type), in fact it is recessive (Becker type).

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, ranolazine, procainamide, flecainide, 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.

In northern Scandinavia, the prevalence of myotonia congenita has been estimated at 1:10,000.

The name Thomsen's disease refers to the Danish physician Julius Thomsen (1815–1896) who described the condition in himself and his family. Myotonia can be achieved in preparations of intact isolated muscle by the administration of 9 Anthracenecarboxylic acid, a blocker of chloride channels. It is also possible to achieve myotonia in preparations of intact isolated muscle by greatly lowering or removing the extracellular content of chloride in the bathing medium. This model is often used in scientific work with muscular dystrophy, and displays myotonia due to lack of functional chloride channels.