The medical history of the patient (endurance sports) and physical examination (bradycardia, and maybe a third or fourth heart sound), can give important hints. ECG – typical findings in resting position are, for example, sinus bradycardia, atrioventricular block (primary and secondary) and incomplete (IRBBB) or complete right bundle branch block (RBBB) – all those findings normalize during exercise. Echocardiography – differentiation between physiological and pathological increases of the hearts size is possible, especially by estimating the mass of the wall (not over 130 g/m2) and its end diastolic diameter (not much less 60 mm) of the left ventricle. X-ray examination of the chest may show increased heart size (mimicking other possible causes of enlargement). Cardiac MRI - In athletes heart, there is balanced atrioventricular remodeling, reduced thickening of the heart after detraining, no late gadolinium enhancement, low to normal T1 signal, and normal extracellular volume.
Physical therapy and other rehabilitative measures may also be used to help muscle function. Genetic counseling is available. Epidemiology
In northern Scandinavia, the prevalence of myotonia congenita has been estimated at 1:10,000.Myotonia congenita is estimated to affect 1 in 100,000 people worldwide. Research
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.During the 1970s several murine models of myotonia appeared. One in particular has been used widely, the adr mouse or "arrested development of righting response". This model is often used in scientific work with muscular dystrophy, and displays myotonia due to lack of functional chloride channels. See also
Fainting goat
Tonic immobility
New Forest pony

References
External links
GeneReview/NCBI/NIH/UW entry on myotonia congenita
NINDS: Myotonia congenita
National Library of Medicine: Myotonia congenita