Myotonia congenita is a genetic, neuromuscular disorder characterized by the slow relaxation of the muscles. Symptoms may include muscle stiffness and hypertrophy (enlargement). The disorder is caused by a genetic mutation involving the chloride channel of the muscles. The muscle stiffness, which particularly occurs in the leg muscles, may be enhanced by cold and inactivity, and is often relieved by exercise.
Historically two forms of myotonia congenita have been classified. The less severe form has been termed Thomsen myotonia congenita and is inherited in a dominant fashion; the more severe form has been termed Becker myotonia congenita and is inherited in a recessive fashion. More recently, as the individual mutations that give rise to this condition have been identified, these classifications are less widely used. The onset of the myotonia congenita is typically before the age of 20; most cases of myotonia congenita are nonprogressive and relatively nonlimiting.
Goats of the "fainting goat" breed all suffer from this condition.
Mutations in the CLCN1 gene alter the usual structure or function of chloride channels. The altered channels cannot properly regulate ion flow, reducing the movement of chloride ions into skeletal muscle cells. This disruption in chloride ion flow triggers prolonged muscle contractions, which are the hallmark of myotonia.
The two forms of myotonia congenita have different patterns of inheritance. Thomsen disease is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. In most cases, an affected person has one parent with the condition.
Becker disease is inherited in an autosomal recessive pattern, which means two copies of the gene in each cell are altered. Most often, the parents of an individual with an autosomal recessive disorder each carry one copy of the altered gene, but do not show signs and symptoms of the disorder.
Because several CLCN1 mutations can cause either Becker disease or Thomsen disease, doctors usually rely on characteristic signs and symptoms to distinguish the two forms of myotonia congenita.
Here's what it is like to live with Myotonia Congenita. Let's say you have been sitting down for a while and you get up to walk up a set of stairs. The sudden use of your muscles causes you to almost come to a full stop by the time you hit the second or third stair. Then after a pause of a few seconds, slowly your muscles start to relax and you are able to loosen up and continue climbing up the rest of the stairs. Myotonia Congenita is annoying and sometimes embarrassing if you are walking up the stairs with somebody else that is wondering why you are slowing down or freezing up.
Depending upon the severity of the condition, Myotonia can create unusual social situations in which people will question if the person with Myotonia is joking, on drugs or suddenly injured because they will be moving perfectly fine and then will suddenly stiffen up. People with Myotonia may associate with others for considerable periods of time before anyone notices anything unusual. To avoid these uncomfortable situations, sometimes people with Myotonia may try to hide their disability, since the degree of muscle stiffness can be lessened by artificially tightening and loosening the muscles before a particular physical action is performed. Think of sitting in a chair and squeezing your muscles as hard as you can and then loosening them repeatedly. This can create a sort of "Chameleon" type of reality for people with Myotonia, in which they blend in well with others until the disability "gives them up."
The severity of the stiffness is controlled by two things: previous muscle activity and severity of the muscle contractions. As mentioned earlier, if you have not been using your muscles for a while, your muscles will "cool down," and then they will take longer to relax once you start moving again. If you are sitting in a chair for say 10 minutes and then stand up, you may not be able to take your first step after standing because your muscles are too stiff. Very slowly, you will be able to move one leg forward at a time until your muscles "warm up." Once the muscles are warmed up you could sit down in the chair and then immediately stand back up again. However, if you sat back down in the chair and remained there for 30 seconds, your muscles would cool down during that time. You could probably stand back up again and walk forward, but you may not be able to walk at the same speed you were walking before, until your muscles warmed back up again. Going from walking to running would also require a warming up period, since running requires more strenuous use of the muscles.
This takes us to the second factor of muscle stiffness--the severity of the muscle contraction. All of the skeletal muscles of the body are affected by the Myotonia. Depending on the muscle group, and the degree to which they contract, this will affect the length and severity of the stiffness. The larger the contraction the slower the relaxation time. If you are sitting in your chair and you slowly rise out of the chair your relaxation time will be quicker than if you try to burst out of the chair. This is because bursting out of the chair requires you to exert more force upon the chair and the floor than if you slowly rise up.
In sports, the best example of the effect of Myotonia Congenita is if you are in a sprint. If you have not loosened up properly, and you attempt to quickly take off from a standing start in a race, by the time you take 4-5 steps, your muscles almost completely freeze up. At this point you may fall over like large oak tree being cut down. If you don't fall over, then your muscles may just freeze and then slowly loosen up, allowing you to finish the race. Each time you start back up again your body will tighten up and you will have to stop or slow down until your muscles loosen up and then you can try to go forward again. Eventually, the muscles warm up to a degree where you may be able to run at a decent speed and maintain that speed. Of course, your competition is likely far ahead at that point. Someone with Myotonia Congenita may therefore avoid sports that require quick spurts of movement like sprinting and football, and stick to sports that require more of a constant motion such as cross country running or soccer where they can participate in the sport without much of a problem from their condition.
Stiffness of the muscles can also be triggered by sudden activity, a sufferer of Myotonia may find in a situation such as crossing the road, that in the face of an oncoming car they may be able to jump out of the way but the sudden actions would cause the muscles to go stiff, resulting in the sufferer being unable to balance and falling over 'stiff as an ironing board'.
Myotonia Congenita is sometimes thought to only affect muscles in the legs, but it also affects other muscles such as your arms, hands (you shake someone's hand and then find it difficult to quickly let go). Other cases that may show Myotonia would be making a fist and letting go again. Most people can make a fist and immediately let go again, but with Myotonia you have trouble letting the fist go. Eyes, diaphragm, and even muscles in the tongue which may briefly affect your speech for a few seconds until you loosen up and finish trying to say what you intended.