Physiology · Year 1 · from Physiology

Case 3: Myotonic Dystrophy - Chloride Channel Dysfunction

Clinical Image

Source: Clinical illustration of myotonia and muscle physiology

Patient Presentation

A 35-year-old male presents to the neurology clinic with a 10-year history of difficulty releasing his grip after shaking hands or opening jars. He also complains of weakness, particularly in his hands and lower legs. He notes that symptoms are worse in cold weather but improve after repeated muscle use ("warming up"). His father and grandfather had similar symptoms, and his father required a pacemaker for heart block at age 50. On questioning, he reports excessive daytime sleepiness, swallowing difficulties, and recent diagnosis of cataracts at an unusually young age.

Demographics

  • Age: 35 years
  • Sex: Male
  • Family history: Father and grandfather affected (autosomal dominant pattern), father with cardiac conduction disease

Chief Complaint

Difficulty releasing grip (myotonia), progressive weakness in hands and lower legs

Physical Examination

  • General: Hatchet-shaped facies (temporalis and masseter wasting), frontal balding
  • Cranial nerves: Ptosis, weakness of facial muscles, dysarthria, bilateral cataracts
  • Motor:
  • Grip myotonia: Cannot release handshake for several seconds (demonstrates prolonged muscle contraction)
  • Percussion myotonia: Tapping thenar eminence causes sustained thumb adduction
  • Weakness: Distal > proximal, finger extensors and ankle dorsiflexors most affected
  • Reflexes: Diminished throughout
  • EMG at bedside: "Dive bomber" sound characteristic of myotonic discharges

Workup

  • Electromyography: Myotonic discharges - waxing and waning amplitude and frequency producing characteristic "dive bomber" sound
  • Genetic testing: Expanded CTG trinucleotide repeat in DMPK gene (>1000 repeats; normal <35)
  • ECG: First-degree AV block, prolonged QRS
  • Echocardiogram: Mild LV dysfunction
  • Sleep study: Central sleep apnea, excessive daytime sleepiness
  • Glucose tolerance test: Impaired glucose tolerance (insulin resistance common)

Diagnosis

Myotonic Dystrophy Type 1 (DM1, Steinert disease)

Treatment

  1. Cardiac monitoring: Annual ECG, Holter monitor; consider pacemaker/ICD for conduction disease
  2. Myotonia treatment (if symptomatic): Mexiletine (sodium channel blocker), avoid triggering drugs
  3. Excessive daytime sleepiness: Modafinil
  4. Cataract surgery when visually significant
  5. Physical and occupational therapy
  6. Monitor for respiratory insufficiency: PFTs, consider BiPAP
  7. Avoid depolarizing muscle relaxants (succinylcholine) - can trigger prolonged myotonia
  8. Anesthetic precautions: Increased sensitivity to sedatives and respiratory depressants
  9. Genetic counseling: Anticipation (earlier onset and increased severity in successive generations due to repeat expansion)

Physiological Principles Demonstrated

  • Normal muscle relaxation: After an action potential, muscle relaxation requires calcium reuptake by SERCA and termination of electrical activity. Chloride channels (ClC-1) stabilize the membrane potential near the chloride equilibrium potential, preventing spontaneous activity.
  • Myotonia mechanism: In myotonic dystrophy, abnormal RNA accumulation from the expanded repeat sequesters splicing factors, causing mis-splicing of chloride channel (CLCN1) mRNA. Reduced chloride conductance destabilizes the resting membrane potential.
  • Repetitive firing: With reduced chloride conductance, the membrane is hyperexcitable. A single action potential triggers repetitive depolarizations, causing sustained muscle contraction (myotonia) even after voluntary effort ceases.
  • Warm-up phenomenon: Repeated muscle activation eventually depletes sodium channel availability through inactivation, reducing hyperexcitability. This is why myotonia improves with repeated use.
  • Cold worsening: Cold temperatures slow sodium channel recovery from inactivation, but the membrane instability from chloride channel dysfunction persists, worsening myotonia.
  • RNA toxicity: Unlike DMD where the protein is absent, DM1 is caused by toxic gain-of-function of expanded CUG repeat RNA, which affects splicing of multiple genes, explaining the multi-system involvement (cardiac, CNS, endocrine, cataracts).

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