Myotonic Dystrophy

Myotonic dystrophy is an autosomal dominant multisystem disorder.

• DM1: CTG repeat expansion in DMPK (prevalence 1 in 8,000), anticipation with maternal transmission to congenital form.

• DM2: CCTG expansion in CNBP, milder, proximal weakness.

• Spliceopathy from toxic RNA causes multisystem disease.

• Complications: cardiac conduction disease and arrhythmia (sudden death), cataracts, respiratory failure, diabetes, infertility, cognitive disorder, anaesthetic risk.

• Distal weakness (hand grip, foot drop), facial and neck flexor weakness (hatchet face, ptosis), myotonia (delayed relaxation after grip, percussion myotonia).

• Frontal balding, cataracts, testicular atrophy, insulin resistance, daytime sleepiness, dysphagia.

• Congenital DM1: hypotonia, respiratory failure, feeding difficulty, talipes, intellectual disability.

• Constipation, gallstones.

• Genetic testing: DMPK CTG repeat size (above 50 abnormal) or CNBP.

• EMG: myotonic discharges ("dive bomber").

• CK normal to mildly raised.

• Baseline ECG, Holter, echocardiogram; spirometry; HbA1c; slit lamp; sleep study; TSH, lipids, liver tests.

• ECG annually (PR over 200 ms, QRS over 100 ms signal risk); pacemaker or ICD for significant conduction disease or arrhythmia (electrophysiology study if borderline).

• Myotonia: mexiletine 150 to 200 mg three times daily if symptomatic (check ECG); avoid class I antiarrhythmics otherwise.

• Daytime sleepiness: modafinil 100 to 200 mg daily, CPAP or bilevel ventilation for hypoventilation.

• Cataract surgery, diabetes treatment, endocrine care, orthoses for foot drop, dysphagia care.

• Anaesthesia: avoid suxamethonium and respiratory depressants; postoperative monitoring.

• Genetic counselling, prenatal and preimplantation testing.