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Myotonic syndromes

Last updated: July 14, 2021

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Myotonic syndromes are a heterogeneous group of inherited disorders with similar pathological mechanisms. Myotonic syndromes are subdivided into dystrophic myotonic syndromes and nondystrophic myotonic syndromes. Nondystrophic myotonic syndromes are channelopathies and include Thomsen disease, Becker disease, and Eulenberg disease. The channelopathies are autosomal dominant or autosomal recessive conditions caused by defective ion channels in the skeletal muscle sarcolemma. All three diseases manifest with myotonia, muscle stiffness, and weakness. Thomsen disease and Becker disease are furthermore characterized by muscular hypertrophy. 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.

Epidemiology [1][2]

Etiology

Clinical features [1]

Clinical features of myotonic dystrophies

Myotonic dystrophy type I (DM1, Curschmann-Steinert disease)

Myotonic dystrophy type II (DM2, proximal myotonic myopathy)

Clinical features
Typical location
  • The proximal muscles: neck flexors, hip flexors, elbow flexors, finger flexors
Symptom onset
  • Congenital, juvenile, or adult onset
  • Usually adult onset

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.

Diagnostics [1][3]

Treatment [1][2]

Prognosis

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

Epidemiology [4]

Etiology and clinical features [5] [4]

Myotonia congenita (Thomsen disease) Myotonia congenita (Becker disease) Paramyotonia congenita (Eulenburg disease)
Inheritance
Pathophysiology
Symptom onset
  • Childhood (first decade of life)
  • 5–35 years
  • Congenital
Typical location
  • Lower extremities more than upper extremities
  • Face, eyes, tongue, and hands are most commonly affected
Clinical features
  • Myotonia (e.g., delayed release from a handshake)
  • Transient muscle weakness
  • Muscle stiffness
  • Warming-up effect: Symptoms improve through repeated contractions.
  • Muscular hypertrophy (athletic physique)
  • Painful myotonia and episodic muscle weakness
  • Precipitating factors
    • Cold temperatures
    • Strenuous exercise (paradoxical myotonia)

Diagnostics

  • Percussion myotonia: percussion of a muscle results in myotonia with delayed muscle relaxation (e.g., tapping the thenar eminence with a reflex hammer causes the thumb to abduct)
  • EMG: repetitive discharges with oscillating frequency and amplitude (crescendo-decrescendo sound )
  • Genetic testing: confirms the diagnosis

Treatment [5]

Prognosis

  • Incurable
  • Varying degree of life-long disability but no effect on life expectancy
  • No symptom progression throughout the course of the disease
Differential diagnoses of myotonic syndromes [1]
Onset Pathophysiology Clinical features
Myotonic dystrophies Type 1 (DM1)
  • Congenital, juvenile, or adult-onset
Type 2 (DM2)
  • Adulthood
Nondystrophic myotonic syndromes

Thomsen disease

  • From childhood

Becker disease

  • 5–35 years

Eulenburg disease

  • Paradoxical myotonia (worsens with exercise)
  • Muscle weakness exacerbated by cold temperatures
Duchenne muscular dystrophy
  • 2–5 years of age
Becker muscular dystrophy
  • Usually > 15 years of age

The differential diagnoses listed here are not exhaustive.

  1. Thornton CA. Myotonic dystrophy. Neurol Clin. 2014; 32 (3): p.705-719. doi: 10.1016/j.ncl.2014.04.011 . | Open in Read by QxMD
  2. Matthews E, Fialho D, Tan SV, et al. The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment. Brain. 2009; 133 (1): p.9-22. doi: 10.1093/brain/awp294 . | Open in Read by QxMD
  3. Myotonic syndromes. https://www.ncbi.nlm.nih.gov/pubmed/12351998. Updated: October 1, 2002. Accessed: April 2, 2017.
  4. Trivedi JR, et al.. Nondystrophic Myotonia: Challenges and Future Directions. Experimental Neurology. 2013 .
  5. Matthews E., et al.. The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment. Brain. 2009 .