Learning objectives

• Describe the causes and symptoms of myotonic dystrophy
• Diagnose and treat myotonic dystrophy
• Manage myotonic dystrophy patients presenting for surgery

Background

• Myotonic dystrophy (DM) is an autosomal dominant disorder characterized by muscle dystrophy starting in early adulthood 
• Myotonic dystrophy type I (DM1, Steinert disease) and type II (DM2, proximal myotonic myopathy, milder form of type I)
• Multisystem disorder affecting somatic and smooth muscle, as well as ophthalmological, cardiovascular, endocrine, and central nervous systems

Etiology

• Genetic disorder caused by an expansion of DNA tandem repeats, resulting in an RNA gain of function mutation
• DM1 is caused by an expansion of a CTG repeat in the 3’-untranslated region of the DM1 protein kinase gene
• DM2 is caused by the expansion of a CCTG repeat in the intron of the CCHC-type zinc finger nucleic aced-binding protein gene
• DM is the most common muscular dystrophy in the European population
• DM1 is more common than DM2

Signs & symptoms

• Can range from potentially lethal in infancy to mild in late adulthood
• DM1 is classified into three types:
  • Congenital myotonic dystrophy 
    • Fetal-onset involvement of muscle and central nervous system
    • Reductions in fetal movement and polyhydramnios
    • Equinovarus and ventriculomegaly on fetal ultrasound
    • Neonatal mortality rate ~18%
    • Childhood/adulthood: Characteristic tented appearance of the upper lip that results from facial diplegia, marked dysarthria, expressive aphasia, hypotonia rather dan myotonia
    • Frequent respiratory involvement
  • Mild myotonic dystrophy 
    • Mild muscle weakness, myotonia, and cataracts
    • Onset between 20-70 years of age (typically after 40)
    • Usually normal lifespan
  • Classic myotonic dystrophy
    • Onset during the second, third, or fourth decade of life
    • Myotonia is the primary initial symptom
    • Characterized by “warm-up phenomenon”: Symptoms appear more pronounced after rest and improve with muscle activity
    • Distal muscle weakness is the main symptom, leading to impairment of fine motor tasks with the hands and impaired gait
    • “Myopathic face”: due to weakness and wasting of facial, levator, palpebrae, and masticatory muscles
    • Cardiac conduction abnormalities are common
    • Reduced lifespan
• DM2:
  • Manifests in adulthood (median age 48 years) with a variable presentation
  • Early-onset cataract, varying grip myotonia, proximal muscle weakness or stiffness, hearing loss, myofascial pain
  • Weakness and/or myalgias are the most common initial symptoms
  • Mostly axial and proximal muscle weakness affecting the neck flexors, long finger flexors, hip flexors, and hip extensors
  • Abdominal, musculoskeletal, and exercise-related pain
  • Sometimes misdiagnosed as fibromyalgia

Diagnosis

• Genetic testing
• Elevations in alkaline phosphatase, gamma-glutamyl transferase, serum aspartate aminotransferase, and serum alanine aminotransferase in 30-50% of patients
• Electrodiagnostic testing: 
  • Motor nerve conduction studies: Decreased amplitude with normal latency and normal conduction velocity
  • Sensory nerve conduction studies: Typically normal
  • Electromyography:
    • Sustained runs of positive sharp waves
    • Trains of negative spikes
    • Fluctuating amplitude and frequencies
• Muscle biopsy: Type I fiber atrophy, Type 2 fiber hypertrophy, irregular fiber size, rows of internal nuclei, fibrosis, myofibrils oriented perpendicular to muscle fiber 

Differential diagnosis

• Schwartz–Jampel Syndrome
• Duchenne muscular dystrophy
• Hyperkalemic Periodic Paralysis (HPP)
• Paramyotonia Congenita (PC)
• Myotonia Congenita
• Myotubular myopathy
• Acid maltase deficiency
• Debrancher deficiency
• Inflammatory myopathies
• Hypothyroid myopathy
• Chloroquine myopathy
• Statin myopathy
• Cyclosporine myopathy

Treatment

• No curative treatment, therapy is supportive and consists of monitoring and treating the issues associated with DM

Complications

Anesthetic management

Suggested reading

• Vydra DG, Rayi A. Myotonic Dystrophy. [Updated 2022 Jun 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557446/
• Pollard BJ, Kitchen, G. Handbook of Clinical Anaesthesia. Fourth Edition. CRC Press. 2018. 978-1-4987-6289-2.
• Marsh S, Pittard A. Neuromuscular disorders and anaesthesia. Part 2: specific neuromuscular disorders. Continuing Education in Anaesthesia Critical Care & Pain. 2011;11(4):119-23.