Makindo Medical Notes

About

• Myotonic dystrophy, also known as Dystrophia Myotonica, is an autosomal dominant inherited condition with variable penetrance.

Aetiology

• Caused by an expansion of a cytosine-thymine-guanine (CTG) repeat in the DMPK gene located at 19q13.3.
• 50-80 CTG repeats are associated with early-onset cataracts, while congenital myotonic dystrophy with severe symptoms often has 500-2000 repeats.
• The condition worsens with each generation due to genetic anticipation, with earlier and more severe onset.

Clinical Features

• Common features include frontal balding, ptosis, neck flexor weakness, and cataracts.
• Muscular atrophy and progressive weakness, although typically not severely impacting mobility.
• Cardiac conduction defects that may necessitate pacemaker implantation (unlike other muscular dystrophies that primarily cause cardiomyopathies).
• Impaired glucose tolerance and insulin resistance.
• Respiratory muscle weakness, potentially leading to sleep apnea.
• Mental impairment in over 50% of cases, as well as common immunologic deficiencies.
• Gastrointestinal symptoms, including dysphagia, constipation, and pseudo-obstruction.

Investigations

• Genetic Testing: DNA analysis from blood to identify CTG repeat expansion.
• Creatine Kinase (CK): Levels may be elevated (2-10 times normal).
• Immunoglobulin G (IgG): Plasma levels may be low.
• Muscle Biopsy: Shows muscle atrophy and degeneration.
• Electromyography (EMG): Reveals myopathic motor units and myotonic potentials.
• ECG: Often shows increased PR interval, atrial flutter, and other arrhythmias.

Management

• Medication for Myotonia: Phenytoin is preferred and safest for managing myotonia.
• Cardiac Monitoring: Annual ECGs and cardiology evaluations are recommended, with pacemaker implantation if necessary.
• Respiratory Considerations: Avoid sedatives that may depress respiration.
• Labour and Delivery: Women may experience ineffective uterine contractions; consult for specialized obstetric care.

Anaesthetic Precautions

• Avoid suxamethonium due to prolonged contraction risks.
• Non-depolarizing relaxants are effective at standard doses, and reversal agents appear safe.
• Mechanical stimuli, diathermy, and depolarizing relaxants can provoke myotonia; consider intravenous regional anaesthesia when appropriate.