Myotonic dystrophy - Dystropica myotonica

Cardiac conduction defects are seen with Myotonic dystrophy unlike the other muscular dystrophies which tend to be associated with cardiomyopathies

About

• Myotonic dystrophy is inherited as an autosomal dominant condition with variable penetrance.

Aetiology

• Expansion of a cytosine- thymine-guanine (CTG) repeat
• The repeat lies at the 3' end of the dystrophia myotonica-protein kinase (DMPK) gene on 19q13.3.
• 50-80 CTG trinucleotide repeats in the DMPK gene associated with early-onset cataracts.
• Most severe Congenital MD 500-2000 repeats and maternally inherited
• Anticipation (worsening phenotype) onset in younger patients per generation

Clinical

• Frontal balding, ptosis and weakness of neck flexors, cataracts are common
• Muscular atrophy and weakness which progressive rarely significantly affects mobility
• Cardiac conduction defects may require pacing [Other muscular dystrophies tend to cause cardiomyopathies]
• Impaired glucose tolerance and Insulin resistance
• Weakness of respiratory muscles - sleep apnoea
• Mental impairment is seen in over 50%
• Immunologic deficiencies are common
• Dysphagia and constipation and pseudo obstruction

Investigations

• DNA analysis of blood to demonstrate the abnormal expansion of the CTG repeat
• The CK may be elevated x 2-10
• The plasma IgG level is often low
• Muscle biopsy: shows atrophy and degeneration
• EMG: myopathic motor units and myotonic potentials
• ECG: increased PR interval, atrial flutter, other arrhythmias

Management

• Phenytoin is used for myotonia and is the safest drug
• Annual ECG and cardiology review for pacing if needed.
• Avoid sedatives that may depress respiration
• Women with myotonic dystrophy may have ineffective or abnormal uterine contractions during labour and delivery.
• Anaesthesia: Prolonged contraction in response to depolarizing relaxants and so Suxamethonium absolutely contraindicated. Myotonia provoked by mechanical stimulus and diathermy. Non-depolarizing relaxants are effective at normal doses. The reversal appears to be safe despite the theoretical risk of myotonia Intravenous regional anaesthesia should be effective