Myotonia Congenita

Myotonia congenita is an inherited disorder that affects skeletal muscles (muscles used for movement). Beginning in childhood, affected individuals experience frequent episodes of myotonia ( muscle tensing that fail to relax normally after contracting). This can be severe enough to interfere with normal everyday activities.

Features

• Myotonia of the skeletal muscles most often occurs after long periods of rest, for example, waking in the morning, or upon standing up and starting to walk after prolonged sitting. Myotonia can also be brought on by cold, fatigue or emotional stress.
• Myotonia can often be relieved by exercise or repeated movements, a phenomenon known as the ‘warm-up effect’.
• Myotonia is not accompanied by pain.
• Face may also be affected and opening the eyelids can sometimes be difficult.
• Muscle enlargement, which may be pronounced, often affects the calves, thighs, shoulders and forearms.

How is myotonia congenita inherited?

There are two types of myotonia congenita, both of which are inherited

• Becker myotonia congenita – this is a more common type that is inherited as an autosomal recessive trait.
• Thomsen myotonia congenita - the less common form is inherited as an autosomal dominant trait

Cause

The underlying genetic fault causes a structural change of the chloride channel (or pore) in the muscle cell wall.