Becker’s disease is an autosomal recessive type of myotonia con-genita, non-dystrophic myotonia, first described in the 1970s by Peter Emil Becker . The worldwide prevalence of myotonia congenita is about 1:100,000 while in some countries (e.g. Norway) the incidence may be 10 times higher [2,3]. It is linked to mutations in CLCN1 (the same as the autosomal dominant in Thomsen’s disease), the gene encoding the skeletal muscle chloride channel. The mutation in Becker’s disease leads to a reduced flow of chloride ions during repolarisation leading to sustained muscle contraction . The reduced chloride conductance of the mutated chloride channels in Becker’s myotonia causes hyper-excitability of the muscle fibre membrane leading to bursts of aberrant action potentials.
The clinical picture is characterised by slowed relaxation following forceful voluntary contrac-tions (myotonic stiffness). Myotonia tends to improve with exercise, the so-called ‘warm-up’ phenomenon. It usually presents during the first or second decade of life with slow pro-gression in later decades.
Symptoms are more severe than in Thomsen’s disease and usually involve the lower limbs first. Muscle hypertrophy is a common symptom. Sometimes it is accompanied by gradually progressive weakness and by peculiar transient episodes of proximal weakness, involving the hands and arm muscles in particular, and is connected to specific types of mutations .
More than 150 different mutations of the CLCN1 gene have been reported, some of them are associated with Becker’s disease (recessive form, more severe) and others to Thomsen’s disease (dominant form, milder). Laboratory diagnostics of myotonia congenita is based on sequencing the CLCN1 gene. Identification of mutations in the CLCN1 gene in the patient and parents differentiate between the two clinical forms of the disease. Since the disease shares symptoms with paramyotonia congenita and other diseases with myotonia, the pool of genes involved in the differential diagnosis is large enough to sequence all of them at the same time, currently by the new techniques of sequencing (NGS).
In addition to searching for a diagnosis based on NGS sequencing, some of the genes related to malignant hyperthermia (mainly RYR1 and CACNA1S genes) should be analysed if patients with myotonia congenita or any other of myopathy who are facing surgery.