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  Disease summary:

  
  
  
  
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    Myotonic dystrophy type 1 is a multisystemic disease with variable clinical manifestations.

    
    
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    Muscle weakness typically involves the face, long finger flexors, intrinsic hand muscles, and foot dorsiflexors.

    
    
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    Myotonia (impaired muscle relaxation) may be elicited with grip or percussion of the muscle. Myotonia is not present in infants or very young children with other features of myotonic dystrophy.

    
    
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    Early-onset cataract (age <50) is often present and may be the only disease feature in mildly affected individuals.

    
    
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    Severe cases of myotonic dystrophy type 1 may result in onset of symptoms at birth, known as congenital myotonic dystrophy. Features associated include hypotonia, respiratory failure, feeding difficulties, clubfoot, and cognitive delay.

    
    
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    Myotonic dystrophy type 1 affects the heart, smooth muscle, gastrointestinal (GI) system, endocrine system, central nervous system (CNS) in addition to the core features, as delineated in Table 131-1.

  
  
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  Hereditary basis:

  
  
  
  
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    Myotonic dystrophy type 1 is an autosomal dominant condition due to an unstable CTG repeat expansion in the 3’untranslated region of the DMPK gene on chromosome 19q13.3. It has complete penetrance but disease manifestations are subject to anticipation. As such, subsequent generations have earlier onset of more severe symptoms due to greater expansion of the trinucleotide repeat length.

  
  
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  Differential diagnosis:

  
  
  
  
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    Myotonic dystrophy type 2 may present similarly in adults, though the weakness is typically more proximal. Isolated myotonia without weakness can also occur in nondystrophic myotonias (myotonia congentia). Congenital myopathies, spinal muscular atrophy, and Pompe disease are all considerations in the evaluation of a congenital myotonic dystrophy patient.

Diagnostic Criteria

Myotonic dystrophy type 1 has the most varied presentation of any muscular dystrophy. However, certain features are highly associated with myotonic dystrophy type 1:

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  Early-onset cataracts (age <50)

  
  
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  Muscle weakness, especially long finger flexors

  
  
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  Myotonia

  
  
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  Affected family member

Clinical Characteristics

Myotonic dystrophy type 1 is characterized by weakness, myotonia, and early-onset cataracts. Many of the disease manifestations in Table 131-1 are commonly associated with the disease. It is typical for individuals within an affected kindred to have a variety of manifestations affecting different systems.

Congenital myotonic dystrophy typically occurs only from affected mothers with myotonic dystrophy type 1 and presents with hypotonia, generalized weakness, feeding difficulties, clubfoot deformity, respiratory failure requiring ventilation, or reduced fetal movement. Patients may require prolonged (>1 month) ventilation, in which case there is a 25% mortality rate in the first year. As these children grow older, they often have cognitive symptoms ranging from mild learning difficulties to severe mental retardation. Typical features of adult-onset myotonic dystrophy subsequently develop in the second and third decades.