II. Epidemiology

  1. Prevalance: 1 in 8,000 to 10,000
    1. Prevalance as common as 1 in 2100 in some references
    2. Most common form of Muscular Dystrophy in adults
  2. Age of onset
    1. DM1 is typically diagnosed in infancy
    2. DM2 is frequently identified at age 20 to 30 years (but may be diagnosed in later adulthood)
  3. Race
    1. Rare in non-white groups

III. Pathophysiology

  1. Genetic mutation is Autosomal Dominant inheritance
  2. Subsequent generations experience worse symptoms

IV. Types: Genetic Mutations

  1. Myotonic Dystrophy 1 (DM1, Steinert Disease)
    1. DMPK Gene Mutation
    2. Typically identified near birth (congenital DM1) and is more severe than DM2
    3. Congenital DM1 is associated with the most significant manifestations
  2. Myotonic Dystrophy 2 (DM2)
    1. CNBP Gene Mutation
    2. More mild than DM1, and typically identified in adulthood
    3. Proximal Muscles are most affected (e.g. neck flexors, hip flexors and extensors)
    4. Muscle pain (myalgias) is more significant in DM2

V. Findings: Muscular

  1. Muscle Weakness
    1. Proximal Muscles are most affected (e.g. neck flexors, hip flexors and extensors) in DM2
    2. Facial Muscle Weakness including Ptosis, Dysarthria, and weak Muscles of Mastication
  2. Myotonia (esp. DM1)
    1. Delayed Muscle relaxation after contraction
  3. Myalgias
    1. Muscle pain is more common in DM2
  4. Muscle Atrophy

VI. Findings: Extramuscular

  1. Cataracts (early onset, before age 50 years old)
  2. Insulin Resistance
  3. Hypersomnia (esp. DM1)
  4. Obstructive Sleep Apnea
  5. Cognitive Deficits (esp. DM1)
    1. Executive Function deficits (e.g. word finding, organization)
  6. Cardiovascular effects
    1. Conduction abnormalities and Blocks
    2. Arrhythmias
    3. Dilated Cardiomyopathy
  7. Men
    1. Premature Balding
    2. Hypogonadism and Infertility

VII. Findings: Congenital DM1

  1. Infancy (diagnosis of congenital DM1)
    1. Hypotonia
    2. Feeding difficulty
    3. Respiratory Failure
    4. Club Foot
  2. Associated Findings at later ages
    1. See Muscular and Extramuscular findings above
    2. Gastrointestinal symptoms (e.g. Diarrhea, Constipation, Encopresis)
    3. Attention Deficit Disorder
    4. Autism Spectrum Disorder
    5. Intellectual Disability

VIII. Diagnosis

  1. Genetic Testing
  2. Lab abnormalities
    1. Serum Creatinine Kinase may be mildly increased
    2. Liver Function Tests elevated in up to 50% of patients
  3. Electrodiagnostic Tests
    1. Motor Nerve Conduction Studies (NCS)
      1. Decreased amplitude, but normal lactency and conduction velocity
    2. Electromyography (EMG)
      1. Rapid spontaneous discharges, waxing and waning in frequency and amplitude (esp. distal Muscles)
  4. Muscle Biopsy
    1. Myopathy

IX. Management

  1. Monitoring
    1. Baseline Electrocardiogram (EKG) and then yearly
    2. Pulmonary Function Tests at baseline and repeat at intervals
      1. Evaluate Pulmonary Function Assessment of Respiratory Muscle Strength
    3. Obstructive Sleep Apnea Screening and consider Sleep Study
    4. Endocrinopathy screening
      1. Diabetes Mellitus Screening
      2. Hypothyroidism screening
  2. Myotonia Management with Sodium Channel Blockers (avoid in AV Block)
    1. Mexiletene
    2. Tricyclic Antidepressants
    3. Benzodiazepines
  3. Referrals as needed
    1. Physical Therapy
    2. Occupational Therapy
    3. Orthotics and Prosthetics
    4. Speech and Language Pathology (Dysarthria)

X. References

  1. Vydra (2022) Myotonic Dystrophy, Stat Pearls, Treasure Island, FL, accessed 7/11/2023
  2. Thornton (2014) Neurol Clin 32(3):705-19 +PMID: 25037086 [PubMed]

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