Myotonia congenita: Thomsen and Becker types explained

People describe a sudden stiffness when they begin to move after resting: legs that will not respond when standing up from a chair or starting to run, hands that cannot release a grip, eyelids that stay closed after a hard blink. MedlinePlus describes bouts of sustained muscle tensing that stop muscles relaxing normally, most often affecting the legs.

The distinctive feature is that persistent movement makes things better. After a few steps the legs loosen. That is the warm-up effect. Cold weather, emotion, surprise and prolonged sitting can all make stiffness worse. Children are sometimes mistaken for clumsy or reluctant when they stumble on starting to run and then do fine.

In a healthy muscle fibre, chloride moving through CLCN1 channels stabilises the membrane so that after a nerve signal ends, the fibre stops firing. When the channel is altered, the fibre keeps firing and the muscle continues to contract. MedlinePlus explains that the gene provides instructions for a protein forming a chloride channel in skeletal muscle cells and that altered channels cannot properly manage chloride movement.

That muscle overactivity often leads to muscle bulk. Many people look unusually muscular, a pattern sometimes called herculean. Bulk is not the same as strength, but strength is usually well preserved.

The two historical names refer to inheritance pattern more than to different diseases. Because Becker myotonia shares a name with Becker muscular dystrophy, which is an unrelated X-linked condition, the two are easily confused in conversation and in records. Always check the full term when reading a letter.

A neurologist can often make the diagnosis from the history and from seeing stiffness that eases with repetition. A doctor may tap a muscle with a reflex hammer and see a prolonged dimple or contraction. Electromyography shows characteristic myotonic discharges, and a creatine kinase level is normal or only slightly raised. Genetic testing of CLCN1 provides confirmation and clarifies inheritance.

Other conditions with myotonia, such as myotonic dystrophy and sodium channel myotonias, are distinguished by their wider features and by genetics, which matters because the heart and eyes need no special surveillance in myotonia congenita. NHS genetic testing is free when arranged by a hospital specialist.

Many people need no medication. Practical steps include warming up before activity, building gradual exercise into the day, avoiding sudden bursts after rest and keeping warm in cold weather. Physiotherapists and occupational therapists can advise on pacing and on adjustments at school or work.

When stiffness interferes with function, specialists may consider a sodium channel blocking medicine such as mexiletine. It is prescription-only, needs a baseline heart tracing and ongoing monitoring, and its suitability is a matter for the neuromuscular team. Anaesthetists should be told about the diagnosis in advance, since some muscle relaxants used in surgery can provoke severe stiffness in people with myotonia.

With dominant disease, an affected parent has an even chance of passing it on. With recessive disease, both parents are usually carriers with no or very mild symptoms, and each child has a one in four chance of being affected. Because carriers can occasionally have mild signs, clinical genetics may advise on testing relatives. Symptoms sometimes worsen in pregnancy, so antenatal teams should be told in advance.