🧠 MOCK OSCE · FCPS, MCPS, MD PAEDIATRICS ⏱ 10 min · MYOTONIA

Myotonia · Short Case

Candidate task: perform focused neurological examination on a child with myotonia.
Then discuss differential diagnosis, investigations, management & follow‑up.
Pre‑exam Protocol
⏤ · Wash, Warm, Introduce, Position, Expose, Approach ▾

Standard pre‑examination protocol – must be demonstrated:

🖐 Wash hands with sterilizing solution.
🔥 Warm hands and stethoscope.
👋 Introduce yourself to child & parent.
🧍 Position child: seated, then standing for gait.
👕 Exposure — allow full neurological exam.
➡️ Approach from the right side.
✔ CPSP marker: Pre‑exam Protocol is observed and scored.
1. Clinical Examination (≈6 min)
02 General Look (Inspection from end of bed) ▾

Key observations:

  • Facial features: myopathic facies (long, thin face, ptosis, tented mouth – myotonic dystrophy).
  • Muscle bulk: muscle hypertrophy (myotonia congenita – "Hercules" appearance), wasting (myotonic dystrophy).
  • Gait: stiff, difficulty initiating movement, difficulty rising from chair.
  • Hands: difficulty releasing grip (myotonia).
  • Speech: dysarthria (myotonic dystrophy).
  • Eye signs: ptosis, cataracts (myotonic dystrophy).
  • Neck: muscle wasting (myotonic dystrophy).
  • General: frontal balding (myotonic dystrophy).
👁 Red flags: neonatal hypotonia, respiratory distress, feeding difficulties (congenital myotonic dystrophy).
03 Focused Neurological Examination ▾

Systematic motor exam for myotonia:

  • Grip myotonia: ask child to make a tight fist, hold for 3‑5 seconds, then open quickly. Observe delayed relaxation.
  • Percussion myotonia: tap thenar eminence (thumb adduction), tongue (dimpling), deltoid, quadriceps → observe sustained contraction (dimple/muscle bulge).
  • Warm‑up phenomenon: myotonia improves with repeated muscle contractions (classic in myotonia congenita).
  • Paradoxical myotonia: worsens with repeated contractions (paramyotonia congenita).
  • Muscle strength: may be normal (myotonia congenita) or reduced (myotonic dystrophy).
  • Muscle tone: may be normal or reduced.
  • Reflexes: usually normal; may be reduced (myotonic dystrophy).
  • Gait: observe walking, heel‑toe, toe‑walking.
  • Cranial nerves: ptosis, facial weakness, dysarthria, dysphagia (myotonic dystrophy).
  • Cataracts: check for lens opacities (myotonic dystrophy).
🔍 Key: myotonia = delayed relaxation after voluntary contraction or percussion. Warm‑up phenomenon = improvement with repetition.
04 General Physical Exam (Hands → Face → Chest → Abdomen → Limbs) ▾

Systematic examination:

  • Hands: grip myotonia, muscle hypertrophy/wasting.
  • Face: myopathic facies, ptosis, tented mouth, frontal balding (myotonic dystrophy).
  • Eyes: cataracts (myotonic dystrophy), ptosis.
  • Chest: respiratory effort (myotonic dystrophy – diaphragmatic weakness).
  • Heart: arrhythmias, conduction defects (myotonic dystrophy, myotonia congenita – rare).
  • Abdomen: hernias (myotonic dystrophy).
  • Testes: testicular atrophy (myotonic dystrophy).
  • Endocrine: diabetes, hypothyroidism (myotonic dystrophy).
📏 Anthropometry: weight, height – plot on growth charts.
05 Developmental Assessment & Associated Signs ▾

Assess:

  • Motor milestones: delayed (myotonic dystrophy).
  • Cognitive: intellectual disability (myotonic dystrophy – especially congenital).
  • Speech: dysarthria, language delay.
  • Behaviour: autism, ADHD (myotonic dystrophy).
  • Feeding: difficulties (congenital myotonic dystrophy).
🧠 Red flags: neonatal hypotonia, respiratory distress, feeding difficulties (congenital myotonic dystrophy).

📋 Case Presentation – (fill in during exam)

This is a _____-year-old _____ child, referred for _____ (difficulty releasing grip / stiff muscles / muscle stiffness). On examination, the child appears _____ (well/unwell), with _____ (myopathic facies / muscle hypertrophy / wasting). Myotonia: grip myotonia _____ (present/absent), percussion myotonia _____ (present/absent), warm‑up phenomenon _____ (present/absent). Muscle strength: _____ (normal / reduced), reflexes _____ (normal / reduced). Eye findings: cataracts _____ (present/absent), ptosis _____ (present/absent). Growth: weight _____ percentile, height _____ percentile. Associated signs: _____ (frontal balding, testicular atrophy, cardiac arrhythmias).

2. Viva Discussion (≈4 min)
06 Viva · Differential, Investigations, Management, Follow‑up ▾
🔹 Differential Diagnosis

• Myotonic Dystrophy Type 1 (DM1) – CTG repeat, multisystem.
• Myotonic Dystrophy Type 2 (DM2) – CCTG repeat, proximal weakness.
• Myotonia Congenita (Thomsen) – chloride channel (CLCN1), AD, muscle hypertrophy.
• Myotonia Congenita (Becker) – chloride channel (CLCN1), AR, muscle hypertrophy.
• Paramyotonia Congenita – sodium channel (SCN4A), cold‑induced, paradoxical myotonia.
• Sodium Channel Myotonia – SCN4A, warm‑up, no cold sensitivity.
• Schwartz‑Jampel syndrome – myotonia, chondrodysplasia, blepharophimosis.
• Brody disease – exercise‑induced stiffness (no electrical myotonia).
• Hyperkalemic periodic paralysis – myotonia, episodic weakness.
• Drug‑induced myotonia – statins, chloroquine, colchicine.
• Myotonia in hypothyroidism – rare.
• Isaacs syndrome (neuromyotonia) – continuous muscle activity (not true myotonia).

🔹 Investigations – Diagnosis

• EMG – myotonic discharges (dive‑bomber sound) – diagnostic.
• Genetic testing – DMPK (DM1), CNBP (DM2), CLCN1, SCN4A.
• CK – normal or mildly elevated.
• Muscle biopsy – may show myopathic changes, type 1 fiber atrophy (DM1).

🔹 Investigations – Aetiology

• Genetic testing – confirmatory for specific myotonic disorders.
• Family history – AD inheritance (DM1, DM2, Thomsen).
• ECG – cardiac conduction defects (DM1).
• Ophthalmology – cataracts (DM1).
• Endocrine – glucose tolerance, thyroid (DM1).
• Parental examination – to identify affected parent (especially DM1).

🔹 Investigations – Exclude Others

• EMG – differentiates myotonia from myopathy/neuropathy.
• CK – elevated in myopathies, normal in most myotonias.
• Thyroid function – hypothyroidism (rare myotonia).
• ECG – arrhythmias (DM1).
• Genetic testing – to differentiate subtypes.
• Muscle biopsy – if diagnosis unclear.

🔹 Investigations – Rule Out Complications

• ECG / Holter – cardiac conduction defects (DM1).
• Pulmonary function tests – respiratory muscle weakness (DM1).
• Ophthalmology – cataracts (DM1).
• Endocrine – diabetes, hypothyroidism (DM1).
• Swallow study – dysphagia (DM1).
• Cognitive assessment – intellectual disability (DM1).

🔹 Management – Across Organ Systems

Neurological

Symptomatic: mexiletine (sodium channel blocker) – reduces myotonia. Carbamazepine, phenytoin (alternative).

Cardiac

DM1: regular ECG/Holter (conduction defects, arrhythmias). Pacemaker if indicated.

Respiratory

DM1: pulmonary function tests; N‑IV if respiratory failure.

Ophthalmology

Cataracts: surgery if visually significant.

Endocrine

Diabetes: screening and management. Hypothyroidism: thyroid replacement.

Rehabilitation

Physiotherapy, OT – maintain function, prevent falls.

Genetic Counselling

AD inheritance (DM1, DM2, Thomsen). Prenatal diagnosis, genetic testing for at‑risk relatives.

Psychosocial

Support groups, counselling, educational support (learning difficulties).

📈 Prognosis

  • Myotonia congenita (Thomsen/Becker): non‑progressive; normal life expectancy.
  • Paramyotonia congenita: non‑progressive; normal life expectancy.
  • Myotonic dystrophy type 1 (DM1): progressive; reduced life expectancy (cardiac/respiratory).
  • Congenital DM1: severe; high neonatal mortality; intellectual disability common.
  • Myotonic dystrophy type 2 (DM2): milder than DM1; later onset.
  • Sodium channel myotonia: non‑progressive; normal life expectancy.

📋 Follow‑up Schedule

  • DM1: annual neurology, cardiology (ECG/Holter), respiratory, ophthalmology, endocrinology.
  • DM2: similar to DM1 but milder course.
  • Myotonia congenita: as needed; reassure benign nature.
  • Paramyotonia: as needed; avoid cold.
  • Genetic: counselling for family members.
  • Transition: plan for adult care (DM1 requires lifelong multidisciplinary care).
💡 Examiner expectation: ability to identify myotonia (grip and percussion), differentiate myotonic dystrophy (multisystem, cataracts, frontal balding, cardiac) from myotonia congenita (isolated myotonia, muscle hypertrophy), and discuss genetic testing and multidisciplinary management. Know the red flags: cardiac conduction defects (DM1), respiratory failure (DM1), neonatal hypotonia (congenital DM1).
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Mock OSCE · Myotonia · Based on Wyne‑Harris, Nelson & Pediatric Clinical Advisor