What is the treatment for myotonia congenita?

Treatment of Myotonia Congenita

Sodium channel blockers, particularly mexiletine, are the first-line pharmacological treatment for myotonia congenita when symptoms are severe enough to warrant intervention.

Understanding Myotonia Congenita

Myotonia congenita is a non-dystrophic myotonia characterized by delayed muscle relaxation after voluntary contraction (myotonia). Unlike myotonic dystrophy, it is not associated with progressive muscle wasting or multisystem involvement.

Two forms exist:

• Thomsen disease (autosomal dominant)
• Becker disease (autosomal recessive)

Both forms result from mutations in the CLCN1 gene encoding the skeletal muscle chloride channel.

Treatment Approach

When to Treat

Most patients with myotonia congenita do not require pharmacological treatment unless symptoms are severe 1. Treatment is indicated when:

• Myotonia significantly impacts quality of life
• Severe muscle stiffness affects daily activities
• Transient weakness episodes are debilitating

First-Line Pharmacological Treatment

Mexiletine is the medication with the strongest evidence for treating non-dystrophic myotonias including myotonia congenita:

• Mechanism: Sodium channel blocker that stabilizes cell membranes
• Dosing: Typically started at lower doses and titrated up as needed
• Evidence: Has demonstrated significant reduction in myotonia symptoms 2
• Monitoring: Requires ECG monitoring due to potential cardiac effects

Alternative Pharmacological Options

When mexiletine is unavailable, expensive, or poorly tolerated, alternatives include:

1. Lamotrigine:

   • Recent evidence shows effectiveness in non-dystrophic myotonias
   • Demonstrated significant improvement in myotonia severity scores
   • More widely available and less expensive than mexiletine
   • Standardized effect size of 1.5 (95% CI: 1.2-1.8) with number needed to treat of 2.6 3
   • Dosing: Gradually increased from 25mg to 300mg daily

2. Other sodium channel blockers:

   • Phenytoin
   • Carbamazepine
   • Flecainide

3. Acetazolamide (carbonic anhydrase inhibitor) may be beneficial in some cases 4

Non-Pharmacological Management

1. Lifestyle modifications:

   • Avoid cold exposure (can trigger or worsen symptoms)
   • Warm-up exercises before physical activity
   • Avoid prolonged inactivity

2. Physical therapy:

   • Gentle stretching exercises
   • Gradual warm-up routines

Special Considerations

Anesthesia Risk

Patients with myotonia congenita have increased risk during anesthesia due to potential diaphragmatic myotonia 1, 4. Anesthesiologists should be informed of the diagnosis before any surgical procedure.

Monitoring

Unlike myotonic dystrophy, myotonia congenita does not typically require extensive systemic monitoring as it does not affect multiple organ systems.

Treatment Algorithm

1. Assess symptom severity:

   • If mild with minimal impact on daily life → non-pharmacological management
   • If moderate to severe with functional impairment → proceed to pharmacological treatment

2. First-line pharmacological treatment:

   • Mexiletine (starting at lower doses, titrating up as needed)
   • Monitor for side effects and ECG changes

3. If mexiletine is unavailable or poorly tolerated:

   • Try lamotrigine (gradually increasing from 25mg to 300mg daily)
   • Alternative sodium channel blockers if lamotrigine fails

4. Concurrent non-pharmacological approaches:

   • Lifestyle modifications
   • Physical therapy

Common Pitfalls to Avoid

• Confusing myotonia congenita with myotonic dystrophy (which requires multisystem surveillance)
• Failing to warn patients about anesthesia risks
• Not recognizing that most patients with mild symptoms may not need pharmacological treatment
• Overlooking the potential for allergic reactions to medications (as reported with mexiletine 5)