But in either form of myotonia congenita, the terms 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). Differential diagnosis
Sodium channel myotonias (SCN4A)

Potassium-aggravated myotonia (acetazolamide responsive myotonia)
Paramyotonia congenita
Hyperkalemic periodic paralysisDystrophies

Myotonic dystrophy (myotonic muscular dystrophy: Type 1 and Type 2)Potassium channel disorders (KCNJ2)

Andersen-Tawil syndromeOther disorders

Thyroid disorders
Neuromyotonia (Isaacs Syndrome)
Schwartz–Jampel syndrome
Stiff person syndrome
Brody myopathy (Brody Disease, Brodys Disease, Brodys Myopathy)

Treatment
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.
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.