A genetic congenital neuromuscular disorder affecting the skeletal muscles. It is caused by mutations in the chloride channel gene (CLCN1 gene). It is characterized by muscle stiffness, hypertrophy, pain, and cramping.

Inherited myotonic disorders with early childhood onset MYOTONIA. Muscular hypertrophy is common and myotonia may impair ambulation and other movements. It is classified as Thomsen (autosomal dominant) or Becker (autosomal recessive) generalized myotonia mainly based on the inheritance pattern. Becker type is also clinically more severe. An autosomal dominant variant with milder symptoms and later onset is known as myotonia levior. Mutations in the voltage-dependent skeletal muscle chloride channel are associated with the disorders.

Myotonia congenita is characterized by muscle stiffness present from childhood; all striated muscle groups including the extrinsic eye muscles, facial muscles, and tongue may be involved. Stiffness is relieved by repeated contractions of the muscle (the "warm-up" phenomenon). Muscles are usually hypertrophic. Whereas autosomal recessive (AR) myotonia congenita is often associated with more severe manifestations (such as progressive minor distal weakness and attacks of transient weakness brought on by movement after rest), autosomal dominant (AD) myotonia congenita is not. The age of onset varies: in AD myotonia congenita onset is usually in infancy or early childhood; in AR myotonia congenita the average age of onset is slightly older. In both AR and AD myotonia congenita onset may be as late as the third or fourth decade of life.

Myotonia congenita is an inherited disorder causing muscle stiffness and hypertrophy beginning during childhood. There are 2 main types with different modes of inheritance and manifestations. Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness.

Myotonia congenita is a genetic disease characterized by the inability of the skeletal muscles to quickly relax after voluntary movements. Symptoms typically begin in childhood and vary from person to person. They may include muscle stiffness, muscle weakness, and attacks of weakness brought on by movement after rest. There are two forms...