Myotonic Dystrophy Type 1

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

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