• Clinical science

Myotonic syndromes


Myotonic syndromes are a heterogenous group of inherited disorders with similar pathological mechanisms. Myotonic dystrophies, the most prevalent myotonic syndromes, are one of the most common forms of adult-onset muscular dystrophy. Both types, myotonic dystrophy type I (Curschmann-Steinert disease) and myotonic dystrophy type II (proximal myotonic myopathy), are autosomal dominant conditions with CTG trinucleotide repeat and CCTG tetranucleotide repeat expansions respectively. Type I is a severe (often life-threatening) form of disease, while type II is usually mild. Both present with skeletal muscle weakness and myotonia, muscle pain, heart conduction defects, cataracts, testicular atrophy, and frontal balding. Electromyography may confirm myotonia that is not identified during clinical examination; however, genetic tests usually confirm the diagnosis. As no curative therapy exists, treatment is symptomatic. Except for DM1, patients with myotonic syndromes have a normal lifespan.

Differential diagnoses

Myotonic dystrophies Duchenne muscular dystrophy Becker muscular dystrophy Non-dystrophic myotonic syndromes
Type 1 (DM1; Curschmann-Steinert disease) Type 2 (DM2; Proximal Myotonic Myopathy, PROMM)

Myotonia congenita Thomsen

(Thomsen's disease)

Myotonia congenita Becker

(Becker's disease)

Paramyotonia congenita Eulenburg

(Eulenburg's disease)

  • Congenital, juvenile, or adult-onset
  • Adulthood
  • 2–5 years of age
  • Usually > 15 years of age
  • From childhood
  • 5–35 years
  • Sodium channel
Clinical features
  • No atrophy, but hypertrophy
  • Initial difficulties in muscle relaxation
  • Primarily affected muscles: legs, eyes, tongue, jaw
  • Possibly upper extremities affected
  • Muscle weakness in the cold
  • Mild disease
  • Skeletal muscle weakness and myotonia is more common in proximal muscles: neck flexors, hip flexors, elbow flexors, finger flexors


The differential diagnoses listed here are not exhaustive.



Epidemiological data refers to the US, unless otherwise specified.


Clinical features

Myotonic dystrophy type I

(DM1, Curschmann-Steinert disease)

Myotonic dystrophy type II

(DM2, proximal myotonic myopathy)

  • Most common in the proximal muscles: neck flexors, hip flexors, elbow flexors, finger flexors
  • Mild clinical myotonia
  • Uncommon symptoms: respiratory muscle involvement, dysphagia, dysarthria, irritable bowel-like symptoms (e.g., abdominal pain, bloating), impaired sleep, daytime somnolence are uncommon

Myotonic dystrophy type I is caused by a CTG nucleotide repeat expansion and results in Cataracts, Toupee (premature hair loss in men), and Gonadal atrophy.





  • Incurable
  • Symptomatic treatment (e.g., analgesia, physiotherapy, walking aids, pacemaker, medical therapy for myotonia )
  • Regular monitoring (i.e., of respiratory compromise, dysphagia)
  • Genetic counseling and testing



  • The course of DM1 is chronic progressive. Cardiac complications reduce life expectancy.
  • DM2 has a milder disease course.