MYOTONIA CONGENITA (CONGENITAL MYOTONIA)

The non-dystrophic myotonias are skeletal muscle ion channel disorders traditionally considered to be distinct from myotonic dystrophy because of the absence of progressive weakness and systemic features. This group of disorders involves: myotonia congenita and paramyotonia congenita with overall frequency of 1:100 000.

Myotonia ( myo – from Greek: muscle; and tonus from Latin: tension) describes an abnormal clinical and neurophysiological phenomenon characterized by delay in muscle relaxation as well as prolonged muscle stiffness (contraction). Affected individuals usually describe muscular stiffness upon initiating movement. Muscle stiffness typically remits with several repetitions of the same movement, giving rise to the so-called warm-up phenomenon.

Myotonia congenita (congenital myotonia) is most common inherited myopathy of ion canals. Based upon type of inheritance congenital myotonias can be divides into: autosomal-dominant Thomsen disease (OMIM 160800) and autosomal-recessive Becker disease (OMIM 255700). Both disorders are caused by mutations in CLCN1 gene responsible for chloride channel function (ClC-1) found in skeletal muscles. Clinical features of these two diseases are very similar. Difference is between age of onset, severity of clinical symptoms and type of inheritance.