What is the recommended treatment for Thomson's disease (myotonic congenital dystrophy)?

Treatment of Thomsen's Disease (Myotonia Congenita)

The first-line treatment for Thomsen's disease (myotonia congenita) is sodium channel blockers, particularly mexiletine, which effectively reduces muscle stiffness and improves quality of life.

Disease Overview

Thomsen's disease is an autosomal dominant non-dystrophic skeletal muscle disorder characterized by:

• Muscle stiffness and inability to relax after voluntary contraction
• Caused by mutations in the CLCN1 gene encoding the skeletal muscle chloride channel-1
• Typically presents at birth or in early childhood 1
• Often features muscle hypertrophy
• Non-progressive or slowly progressive course
• No significant impact on life expectancy

Diagnostic Considerations

Before initiating treatment, confirm diagnosis through:

• Clinical presentation of myotonia (delayed muscle relaxation after contraction)
• Family history (autosomal dominant inheritance pattern)
• Electromyography showing myotonic discharges
• Genetic testing for CLCN1 mutations (particularly p.P480H and others) 2
• Muscle biopsy (if necessary) showing reduced ClC-1 expression on sarcolemma

Treatment Algorithm

First-line Pharmacological Treatment

1. Sodium channel blockers:

   • Mexiletine (start at 150-200 mg twice daily, titrate as needed)
   • Monitor for cardiac side effects (ECG monitoring recommended)
   • Alternative: tocainide (if mexiletine unavailable)

2. If sodium channel blockers are ineffective or not tolerated:

   • Anticonvulsants:
     • Phenytoin (300-400 mg/day)
     • Carbamazepine (400-1200 mg/day)

3. Other medication options:

   • Acetazolamide (250 mg 2-3 times daily)
   • Thiazide diuretics
   • Propranolol (for patients with prominent adrenergic triggers)

Non-pharmacological Management

• Regular, gentle exercise program to maintain muscle function
• Avoid triggers that worsen myotonia:
  • Cold exposure
  • Sudden exertion after rest
  • Potassium-rich foods (in some patients)
  • Stress and fatigue

Special Considerations

Monitoring

• Annual follow-up is recommended for patients with muscular dystrophies and myotonic disorders, even during asymptomatic phases 3
• Regular assessment of:
  • Muscle strength and function
  • Cardiac function (ECG)
  • Respiratory function
  • Medication side effects

Cardiac Complications

• While less common in Thomsen's disease compared to myotonic dystrophy type 1 (Steinert's disease), cardiac monitoring is still important
• Consider ECG screening to rule out conduction abnormalities 3

Respiratory Management

• Respiratory complications are rare in Thomsen's disease but may include:
  • Breathing difficulties in infants 1
  • Eye closure myotonia

Genetic Counseling

• Genetic counseling is recommended for patients and families
• Autosomal dominant inheritance pattern (50% chance of transmission to offspring)
• Genetic testing can confirm diagnosis and inform family planning

Important Pitfalls to Avoid

1. Misdiagnosis: Do not confuse with recessive generalized myotonia (Becker's disease), which has later onset and transient muscle weakness after exertion 4

2. Overlooking progression: Some cases initially diagnosed as Thomsen's disease may evolve to resemble myotonic dystrophy (Steinert's disease) later in life 5

3. Medication side effects: Monitor for cardiac side effects of sodium channel blockers, particularly in patients with pre-existing cardiac conditions

4. Anesthesia risks: Patients with myotonia congenita may have increased sensitivity to certain anesthetics and muscle relaxants; inform anesthesiologists before any surgical procedures

5. Ptosis and other atypical features: Some patients may present with ptosis or other unusual manifestations that can complicate diagnosis 2

By following this treatment approach, most patients with Thomsen's disease can achieve significant improvement in myotonia symptoms and maintain good quality of life.