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.
During a fall, a person with myotonia congenita may experience partial or complete rigid paralysis that will quickly resolve once the event is over.
[3] 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.
However, in recent times, as more of the individual mutations that cause myotonia congenita are identified, these limited disease classifications are becoming less widely used.
It has been shown that pregnancy[6] and the use of diuretics[7] aggravate myotonia, and both these conditions are linked to the loss of divalent cations such as magnesium and calcium.
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.
[13] 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.
[14] 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).
[citation needed] 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).
This is the gene encoding the protein CLCN1, that forms the ClC-1 chloride channel, critical for the normal function of skeletal muscle cells.
[16] In skeletal muscle fibers, a large transverse tubule system with a high surface-area to volume ratio exists.
[2] 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.
[citation needed] 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.
[citation needed] A so-called Finnish heritage disease, congenital myotonia is more common in Finland and among ethnic Finns.
[23][24] 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.
[25] This model is often used in scientific work with muscular dystrophy, and displays myotonia due to lack of functional chloride channels.