Myotonia Congenita

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Written by: Dr Jeremy Steinberg – created: 8 March 2023; last modified: 12 March 2023

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Myotonia Congenita
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.

Clinical Features

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.[1]

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.

Clinical Features of AR vs AD Myotonia Congenita[2]
Feature AR Myotonia Congenita AD Myotonia Congenita
Age of onset Early childhood Early infancy
Muscle stiffness Generalized, legs>arms Generalized, arms>legs
Muscle hypertrophy Generalized Generalized
Pain Very common Common
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.

Differential Diagnosis

The following are all the myotonic disorders. The presence of muscle wasting and systemic manifestations suggests myotonic dystrophy.

Myotonia Differential Diagnosis
  • Myotonic dystrophy
    • DM type 1 (DMPK)
    • DM type 2 (CNBP)
  • Non-dystrophic myotonias
    • Chloride channelopathies (CLCN1)
    • Sodium channelopathies (SCN4A)
      • Sodium channel myotonia
      • Paramyotonia congenita
      • Hyperkalaemic periodic paralysis
    • Chondrodystrophic myotonia
  • 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


  1. 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: |date= (help)

Literature Review