Cardiac conduction defects are seen with Myotonic dystrophy unlike the other muscular dystrophies which tend to be associated with cardiomyopathies
- Myotonic dystrophy is inherited as an autosomal dominant condition with variable penetrance.
- 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
- 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
- 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
- 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