Autosomal dominant disease characterised by trinucleotide repeat of CTG. Also known as Dystrophica Myotoncia the disease is causes an increase in muscle tone (Myotonia) with associated degeneration and shrinkage of muscle fibres (Dystrophy).

• DM1 is the most common
  • Caused by mutation of the DMPK gene of chromosome 19 (Dystrophia Myotonica protein kinase gene)
  • The mutation  is a  trinucleotide expansion of CTG on the DMPK gene
• DM2 less common
  • Caused mutation of the CNBP gene in chromosome 3
    • The mutation is a tetranucleotide expansion of CCTG on the CNBP gene.
  • DM2 is associated with proximal muscle weakness

Myotonia is defines as continued contraction of the muscle after voluntary contraction ceases.

Clinical features

• Delayed relaxation after contraction - hand shake slow to release
• Myotonic facies (expressionless, mouth open with a lean face)
  • Facial muscle weakness
  • Sternocledoid muscle weakness
• Difficulty opening eyes after firm closure
• Ptosis (often bilateral)
• Cataracts
• Balding
• Distal muscle wasting and weakness - hands
• Reduce reflexes
• Foot drop

Complications

• Diabetes - polyuria, polydipsia
• Cardiomyopathy - features of heart failure
• Conduction abnormality - pacemaker
  • Supraventriular tachycardia (most common)
  • 1st degree heart block (40%)
• Hypogonadism - gynacomastia and testicular atrophy
• Goitre

• Genetic mutations -> abnormal mRNA
  • abnormal mRNA like these do not leave the nucleus
• The abnormal mRNA can cause these hairpin structures which disrupt existing proteins in the nucleus
• The abnormal mRNA causes existing proteins to lose function or gain function:
  • Loss of function of the MBNL proteins
  • Gain of function of CUGBP proteins
  • These proteins are normally involved in splicing, mRNA transport, stability and decay
  • These protein dysfunction are thought to be the pathological hallmark of Myotonic Dystrophy resulting in:
    • Disruption of alternative splicing 
    • Disruption mRNA transport
    • Disruption of mRNA decay
    • Abnormal protein production and function causing complications of Myotonic dystrophy.

Other investigations are mainly to look for complications of Myotonic Dystrophy

• Ophthalmology review
• Electrocardiogram
• 24 holter monitor
• Echocardiogram
• Sleep study
• Pulmonary function test
• HbA1c
• TFT

Diagnosis

• Genetic testing 
• Nerve conduction study - myotonia 

• Medications that reduce sustained myotonia
  • sodium channel blockers such as mexiletine
  • tricyclic antidepressants
  • benzodiazepines
  • calcium antagonists

Treatment again targets complications of Myotonic Dystrophy

• Pacemaker, ICD
• CPAP machine
• Incentive spirometry
• Insulin
• Thyroxine
• Speech pathology for dysphagia
• Physical and occupational therapy is recommended for strengthening weakened muscles, evaluation for orthotics, and durable medical equipment needs.

Endocrine complications

• Type II diabetes melitius
• Hypogonadism
• Nodular thyroid enlargement

Cardiovascular complications

• Resting ECG changes - everything prolonged (PR, QRS, QT)
• Arrhythmia
  • Supraventricular tachycardia (most common)
  • VT
• Conduction blocks
  • 1st (common)
  • 2nd
  • complete
• Mitral valve prolapse
• Cardiomyopathy

Gastrointestinal complications

• Dysphagia
• Reflux
• Delayed gastric emptying
• Malabsorption
• Bacterial overgrowth
• Megacolon

Respiratory complications

• Hypoventilation - poor sleep
• Respiratory failure follow anaesthesia
• Recurrent pneumonia