Myotonia congenita
What is it?
Myotonia congenita is a congenital neuromuscular channelopathy that affects skeletal muscles. It is a genetic disorder. The hallmark of the disease is the failure of initiated contraction to terminate, often referred to as delayed relaxation of the muscles (myotonia) and rigidity. The condition is sometimes referred to as fainting goat syndrome, as it is responsible for the eponymous 'fainting' seen in fainting goats when presented with a sudden stimulus. Of note, myotonia congenita has no association with malignant hyperthermia (MH).
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Additional names
This group contains additional names:
- Thomsen disease
- Generalized myotonia
Signs & symptoms
Early symptoms may include:
* Difficulty swallowing
* Gagging
* Stiff movements that improve when they are repeated
* Frequent falling
* Difficulties opening eyelids after strenuous contraction or crying (von Graefe's sign)
Possible complications may include:
* Aspiration pneumonia (caused by swallowing difficulties)
* Frequent choking or gagging in infants (also caused by swallowing difficulties)
* Abdominal muscle weakness
* Chronic joint problems
* Injury due to falls
Diagnosis
Two types of myotonia congenita exist, an autosomal dominant form and an autosomal recessive form. Autosomal dominant myotonia congenita is also called Thomsen disease, Autosomal recessive myotonia congenita is also called generalized myotonia, recessive generalized myotonia (RGM), Becker disease, and Becker myotonia.
With the advent of genetic testing, it has recently been found that some typically recessive mutations may occur in a dominant fashion in some individuals. The reason for this is not known.
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. However, myotonia caused by CLCN1 mutations can occasionally be clinically indistinguishable from myotonia caused by sodium channel mutations (SCN4A mutations) resulting in the similar disease paramyotonia congenita.
Treatment
Some cases of myotonia congenita do not require treatment, or it is determined that the risks of the medication outweigh the benefits. If necessary, however, symptoms of the disorder may be relieved with quinine, ranolazine, procainamide, flecainide, phenytoin, carbamazepine, mexiletine and other anticonvulsant drugs. Physical therapy and other rehabilitative measures may also be used to help muscle function. Genetic counseling is available.
☝️ This is not a substitute for professional medical advice. Please consult with your physician before making any medical decision.
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