Muscular Dystrophy (Myotonic Dystrophy)

There are over 30 different gene-linked muscular dystrophies. This page covers myotonic dystrophy type 1 (DM1), the most common — an autosomal dominant multisystem disorder (50:50 inheritance chance per child, affects males and females equally). Severity can worsen across generations.

Severity spectrum

• Mild DM1 — cataract and mild myotonia; normal life span.
• Classic DM1 — muscle weakness/wasting, myotonia, cataract, cardiac conduction abnormalities; increasing disability, possibly shortened life span.
• Congenital DM1 — hypotonia and severe weakness at birth, often with respiratory insufficiency and early death; intellectual disability.

Genetic testing — a CTG triplet-repeat expansion in the DMPK gene. A normal result rules out myotonic dystrophy; an expansion confirms the diagnosis if symptomatic.

Assessment

Signs:

• Unexplained cataracts — may be the only sign in patients >60 years.
• Myotonia — ask the patient to grip your fingers firmly then release rapidly; delayed relaxation confirms myotonia.
• Muscle weakness, particularly facial and distal (compromised finger dexterity); ankle dorsiflexor weakness can cause bilateral foot drop; neck weakness can cause a “dropped head” posture. Proximal muscles are typically spared initially.
• Frontal balding; mild learning difficulties.

Ask about: cardiac conduction disturbance symptoms (blackouts, syncope, palpitations, dyspnoea — heart block, bundle branch block, atrial flutter; AF/ventricular arrhythmias less common; can be an early disease manifestation); sleep problems (OSA symptoms, daytime sleepiness); GI/urinary symptoms. Take a family history and sketch a 3-generation family tree.

Management

If there is a personal/family history of myotonic dystrophy, request a non-acute genetic health assessment for counselling/testing.

Once diagnosed:

• Regular GP follow-up: HbA1c and thyroid tests; monitor for cataracts; ECG for conduction disturbance; screen for sleep apnoea (arrange a sleep study if indicated); monitor weight (increased insulin resistance risk); prompt cardiology assessment for cardiac symptoms.
• Symptomatic myotonia (muscle stiffness) can be treated if needed: phenytoin (epilepsy-range doses), mexiletine 150–200mg TDS, or acetazolamide 250–500mg BD–TDS.
• Consider a MedicAlert.