What is the management of myotonia congenita?

Management of Myotonia Congenita

Myotonia congenita can be effectively managed with sodium channel blockers like mexiletine as first-line pharmacological treatment, along with lifestyle modifications to minimize myotonic symptoms.

Pathophysiology and Clinical Presentation

Myotonia congenita is a non-dystrophic muscle disorder characterized by:

• Muscle stiffness that improves with repeated use ("warm-up phenomenon")
• Muscle hypertrophy
• Delayed muscle relaxation after voluntary contraction
• Two inheritance patterns:
  • Thomsen disease (autosomal dominant)
  • Becker disease (autosomal recessive)
• Caused by mutations in the CLCN1 gene encoding the skeletal muscle chloride channel

Pharmacological Management

First-line Treatment

• Sodium channel blockers:
  • Mexiletine: Most effective first-line agent for reducing myotonic symptoms 1
  • Dosing: Start at low doses and titrate based on symptom response
  • Mechanism: Stabilizes muscle cell membranes by blocking sodium channels

Alternative Medications

1. Carbamazepine:

   • Effective in some patients, particularly those with specific CLCN1 mutations 2
   • Can decrease muscle stiffness and improve quality of life

2. Acetazolamide:

   • Can be effective for myotonia at low doses 3
   • Caution: May cause weakness at higher doses
   • Start at very low doses and monitor closely

3. Other membrane-stabilizing agents:

   • Phenytoin
   • Procainamide
   • Tocainide (limited use due to side effect profile)

Non-Pharmacological Management

Lifestyle Modifications

• Regular, gentle exercise to reduce muscle stiffness
• Avoid cold exposure which can trigger or worsen symptoms
• Warm-up exercises before physical activity
• Avoid prolonged muscle inactivity

Physical Therapy

• Stretching exercises to improve muscle flexibility
• Techniques to manage muscle stiffness
• Activity pacing strategies

Special Considerations

Anesthesia Precautions

• High risk during general anesthesia due to potential diaphragmatic myotonia 4
• Anesthesiologists should be informed of the diagnosis before procedures
• Avoid depolarizing muscle relaxants (e.g., succinylcholine)
• Monitor closely for respiratory complications

Pregnancy and Family Planning

• Genetic counseling for affected individuals planning pregnancy
• Discussion of inheritance patterns and recurrence risk
• Management of symptoms may need adjustment during pregnancy

Monitoring and Follow-up

• Regular neurological assessment to monitor symptom progression
• EMG studies to assess myotonic activity when clinically indicated
• Adjustment of medication dosage based on symptom control and side effects

Prognosis

• Generally non-progressive condition 5
• Most patients have normal life expectancy
• Treatment focuses on symptom management rather than disease modification
• Many patients do not require pharmacological treatment unless symptoms are severe

Treatment Algorithm

1. Assessment: Determine severity of myotonia and impact on daily functioning
2. Mild symptoms: Consider lifestyle modifications alone
3. Moderate to severe symptoms:
   • Start with mexiletine at low dose
   • If ineffective or not tolerated, try carbamazepine or acetazolamide
4. Refractory symptoms: Consider combination therapy or alternative membrane stabilizers
5. Ongoing management: Regular follow-up to assess symptom control and medication side effects

Unlike myotonic dystrophy, myotonia congenita does not typically require surveillance for systemic complications, as it primarily affects skeletal muscle without significant cardiac or respiratory involvement 5.