Makindo Medical Notes

About

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

Aetiology

• Type 1: Expanded trinucleotide (CTG) repeat in an untranslated region of the gene coding for dystrophia myotonica protein kinase (DMPK). Lengths greater than 34 repeats are abnormal. Prenatal testing during at-risk pregnancies can be performed.
• Type 2: proximal myotonic myopathy, have myotonia, cataracts, primarily proximal weakness (neck and finger flexors, followed by hip flexors and less severe. Dominantly inherited and results from an expanded CCTG repeat in a noncoding (intronic) region of the gene for zinc finger protein-9 (ZNF9). Onset is commonly in young adults.
• 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.

Photograph of a young man with myotonic dystrophy, showing frontal baldness, bilateral ptosis, and wasting of the temporalis, facial, and sternocleidomastoid muscles

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: Phenytoin 100 mg TDS is preferred and safest for managing myotonia. There is no treatment for the weakness that occurs
• Cardiac Monitoring: Annual ECGs and cardiology evaluations are recommended, with pacemaker implantation if necessary. A severe electrocardiographic abnormality and the presence of atrial tachyarrhythmia may predict risk of sudden death
• 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.