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Myotonic dystrophy

  • Myotonic dystrophy is an inherited multisystem disorder characterised by progressive muscle weakness, difficulty relaxing clenched hands (myotonia), cataracts, and ECG changes.
  • The condition is extremely variable and there is a well-recognised severe congenital form of the disease.
  • Myotonic dystrophy is an autosomal dominant condition, the commonest form caused by a mutation in the dystrophia myotonica protein kinase gene (DMPK) on chromosome 19.
  • Myotonic dystrophy is the most common of the inherited neuromuscular disorders in adults, with an prevalance of approximately 1 in 8,000.

Clinical features

The clinical features of myotonic dystrophy span a continuum from mild to severe, and have been grouped into four overlapping phenotypes:

Mild

  • Characterised by cataract and mild myotonia (difficulty relaxing clenched hands).

Classic

  • Characterised by progressive muscle weakness and wasting, myotonia, cataract, and often cardiac conduction abnormalities.

Childhood onset

  • Characterised by progressive muscle weakness and stiffness beginning in childhood, but with no symptoms at birth. Some children have learning difficulties.

Congenital myotonic dystrophy

  • This is the most serious form of the condition. It presents at birth with hypotonia and severe generalised muscle weakness including breathing and swallowing difficulties. Learning difficulties can be part of the condition.

Diagnosis

  • The diagnosis of myotonic dystrophy is suspected in individuals with characteristic muscle weakness and/or myotonia.
  • Myotonic dystrophy is confirmed by DNA testing, which detects mutations in virtually 100% of individuals with the condition.

Genetic basis

  • Myotonic dystrophy is an autosomal dominant condition. A person with an autosomal condition has one usual copy of a gene of a particular pair, while the other copy contains an alteration. Each child of someone with myotonic dystrophy can inherit either the usual or the altered gene, and so has 50%, or 1 in 2, chance of inheriting the gene alteration that causes the condition.
  • The mutation in the DMPK gene is an amplified CTG triplet (trinucleotide) repeat and the severity of the condition varies with the number of these repeats. The usual form of the gene has between 5 and 30 repeat copies. Mildly affected individuals have from 50 to 80 repeats, and children with the severe congenital form of the condition have 2,000 or more copies of the repeat.
  • The CTG repeat number usually increases from one generation to the next (amplification), although very large amplifications occur only when the gene is transmitted through females. This explains why myotonic dystrophy increases in severity with successive generations (anticipation) and why babies with the severe congenital form of the condition are almost exclusively born to women with the condition.

Clinical management

  • The variability of the condition means that each affected individual needs to be assessed on an individual basis.
  • Once the diagnosis is confirmed, an affected individual should have a thorough initial assessment to include baseline cardiac, ophthalmic and neurological examinations and an assessment of cognitive ability. In adults a fasting blood glucose and thyroid function tests are also indicated.
  • For those with complex symptomatology, a multidisciplinary approach to management in a specialist clinic is advised.
  • There should be a clear plan for when medical help is needed, and specialist advice should always be sought in relation to surgery and anaesthesia, sedation, pregnancy, and during concurrent illnesses.
  • Cholesterol-lowering medications, such as statins, can cause muscle pain and weakness and should be avoided.

Genetic testing

Genetic testing can be used to:

  • confirm the diagnosis in someone with possible myotonic dystrophy (diagnostic testing);
  • offer presymptomatic (predictive) testing in adults; and
  • offer prenatal genetic diagnosis.

Genetic testing is widely available across the UK and usually provided through specialist clinics or regional genetic centres.

 

This information is intended for educational use and was current in August 2013. For clinical management, it is recommended that local guidelines and protocols are used.