|Inheritance||Autosomal dominant (Thomsen) or recessive (Becker)|
|Genetics||CLC1N pathogenic variant|
Myotonia Congenita is a genetic disease characterised by skeletal muscle stiffness from childhood. It sits in a group of disorders known as nondystrophic myotonias to differentiate them from the myotonic dystrophies as the tissues are not dystrophic. It is also known as chloride channel myotonia because it is a chloride channelopathy. The nondystrophic myotonias can be painful and so the Musculoskeletal Physician should be aware of them.
There is a loss of function mutation in the CLCN1 gene. Myotonia Congenita can be autosomal dominant which is called Thomsen disease or autosomal recessive which is called Becker disease. The same pathogenic variant can be associated with both forms.
The prevalence is 1 in 100,000.
Patients have episodes of muscle stiffness (myotonia) or cramping that typically begins in childhood. However the age of onset is highly variable. AR myotonia congenita typically occurs later than the AD form, but in both forms the age of onset can be as late as ones 20s to 30s.
The AR form typically has more severe manifestations including progressive, minor distal weakness, and transient weakness with movement following a period of rest. The stiffness is often alleviated by brief exercise which is called the "warm-up" effect.
Pain is common but usually not as severe as in myotonic dystrophy type 2.
Unlike with Myotonic Dystrophy, systemic manifestations do not generally occur. However a minority may have cardiac conduction defects.
On examination there may be percussion myotonia. All skeletal muscle groups can be affected including the eyes, face, and tongue. The muscles may appear hypertrophic.
|Feature||AR Myotonia Congenita||AD Myotonia Congenita|
|Age of onset||Early childhood||Early infancy|
|Muscle stiffness||Generalized, legs>arms||Generalized, arms>legs|
|Transient muscle weakness||Common||Not present|
|Distal muscle weakness||Rarely||Not present|
|Proximal muscle weakness||Rarely||Not present|
CK is typically elevated <3-4 times the upper limit of normal. EMG may show myotonic bursts.
The diagnosis is made through molecular genetics by finding a heterozygous pathogenic variant or biallelic pathogenic variants of the CLCN1 gene. There are no clinical diagnostic criteria.
The following are all the myotonic disorders. The presence of muscle wasting and systemic manifestations suggests myotonic dystrophy.
- Myotonic dystrophy
- DM type 1 (DMPK)
- DM type 2 (CNBP)
- Non-dystrophic myotonias
- Chloride channelopathies (CLCN1)
- Thomsen myotonia congenita
- Becker myotonia congenita
- Sodium channelopathies (SCN4A)
- Sodium channel myotonia
- Paramyotonia congenita
- Hyperkalaemic periodic paralysis
- Chondrodystrophic myotonia
- Chloride channelopathies (CLCN1)
- Drug induced (statins, fibrates, cyclosporine, chloroquine, terbutaline, formoterol, propranolol)
- Isolated electrical myotonia: Pompe Disease, Polymyositis, Myotubular congenital myopathy, hypothyroidism, malignant hyperpyrexia, severe denervation.
Mexiletine has the best documented effect, which is a sodium channel blocker.
GeneReviews - Myotonia Congenita
- ↑ Statland, Jeffrey M.; Wang, Yunxia; Richesson, Rachel; Bundy, Brian; Herbelin, Laura; Gomes, Joe; Trivedi, Jaya; Venance, Shannon; Amato, Anthony; Hanna, Michael; Griggs, Robert (2011-07). "An interactive voice response diary for patients with non-dystrophic myotonia". Muscle & Nerve. 44 (1): 30–35. doi:10.1002/mus.22007. ISSN 1097-4598. PMC 3233757. PMID 21674518. Check date values in:
- ↑ https://www.ncbi.nlm.nih.gov/books/NBK1355/
- Reviews from the last 7 years: review articles, free review articles, systematic reviews, meta-analyses, NCBI Bookshelf
- Articles from all years: PubMed search, Google Scholar search.
- TRIP Database: clinical publications about evidence-based medicine.
- Other Wikis: Radiopaedia, Wikipedia Search, Wikipedia I Feel Lucky, Orthobullets,