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Myotonic Dystrophy Type 1

Topic updated on 10/15/17 11:30am

Snapshot
  • A 35-year-old man presents to his primary care physician's office for difficulty chewing and walking and mild musclar pain. He notices himself dragging his feet while walking and pain in his legs. He experiences stiffness in his jaw and his hands. On physical exam, there is bilateral ptosis and atrophy of the temporalis muscle. There is weakness with turning of the neck and bilateral dorsiflexion. A serum creatine kinase is normal. Genetic testing shows CTG nucleotide expansion on the DMPK gene on chromosome 19.
Introduction
  • Clinical definition
    • a hereditary disorder that results in a number of clinical consequences such as
      • sustained muscle contraction (myotonia)
      • muscle weakness
      • cataracts
      • frontal balding
      • arrhythmias
  • Epidemiology
    • incidence
      • most common type of muscular dystrophy in adults
        • especially with European ancestry
    • risk factors
      • family history
  • Etiology
    • genetics
      • myotonic dystrophy (MD) type 1
        • autosomal dominant mutation in DMPK gene on chromosome 19
          • leads to a CTG trinucleotide expansion
  • Pathogenesis
    • unclear, but it is believed that the abnormal mRNA transcript of DMPK impairs RNA splicing, which leads to missplicing of other RNA transcripts
  • Prognosis
    • 30-50% of cases leads to disability in adulthood
    • most common causes of death are
      • respiratory diseases (e.g., respiratory failure)
      • cardiac diseases (e.g., arrhythmia)
Presentation
  • Symptoms
    • muscle stiffness
    • muscle weakness and pain
    • visual problems
    • excessive daytime sleepiness
  • Physical exam
    • muscle weakness that is typically found in the
      • face or jaw
        • commonly as ptosis
      • distal limb muscles
      • sternocleidomastoid muscle
    • muscle atrophy that is typically found in the
      • face
      • temporal
      • distal hands
      • legs
    • myotonia
    • cataracts
    • frontal balding
    • testicular atrophy
Studies
  • Labs
    • serum creatine kinase
      • normal-to-moderate increase
  • Genetic testing
    • gold standard
  • Slit-lamp examination
    • to evaluate for cataracts
      • classically posterior subcapsular cataracts
  • Electrocardiogram
    • critical in the evaluation of cardiac conduction defects
  • Making the diagnosis
    • based on clinical presentation and confirmed with genetic testing
Differential
  • Limb-girdle muscular dystrophy
  • Spinal muscular atrophy
Treatment
  • Management approach
    • there are no disease modifying treatments and thus management involves a multidisciplinary approach
      • for example
        • cardiac pacemakers in patients with life-threatening cardiac arrhythmias
        • surgery for cataract extraction
        • non-invasive positive pressure ventilation in patients with obstructive sleep apnea
Complications
  • Cardiovascular disease and arrhthmias
  • Hyperinsulinemia
  • Pregnancy complications such as
    • spontaneous abortion
    • preterm labor
  • Obstructive sleep apnea
  • Hypothyroidism
  • Increased cancer susceptibility
  • Cataracts



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