Neuromuscular Disorders

NMDs Overview - Nerve-Muscle Maze

Neuromuscular disorders (NMDs) impair function of motor units: anterior horn cells, peripheral nerves, neuromuscular junction (NMJ), or muscles.

• Key Types:
  • Motor Neuron Diseases (e.g., ALS)
  • Peripheral Neuropathies (e.g., GBS)
  • NMJ Disorders (e.g., Myasthenia Gravis, LEMS)
  • Myopathies (e.g., Dystrophies, Myotonias)
• Anesthetic Focus:
  • Pre-op assessment of system involvement.
  • Careful airway & respiratory management.

  ⭐ General increased sensitivity to non-depolarizing NMBs and risk with succinylcholine in many NMDs.

Myasthenia Gravis & LEMS - Junction Function Fracas

• Myasthenia Gravis (MG):
  • Patho: Postsynaptic ACh receptor antibodies.
  • Clinical: Fluctuating, fatigable weakness (ocular, bulbar, limb). Improves with rest. Thymoma association.
  • Anesthesia:
    • NDMRs: Extreme sensitivity (use 1/10th - 1/20th dose). Titrate carefully.
    • Succinylcholine: Relative resistance (may need 1.5-2 mg/kg).
    • Avoid: Aminoglycosides, quinolones, $Mg^{2+}$, β-blockers.
    • Post-op: High risk of respiratory failure. Continue pyridostigmine.
• Lambert-Eaton Myasthenic Syndrome (LEMS):
  • Patho: Presynaptic P/Q-type $Ca^{2+}$ channel antibodies → ↓ACh release.
  • Clinical: Proximal weakness, improves with brief exercise. Autonomic features. Strong SCLC link.
  • Anesthesia:
    • NDMRs & Succinylcholine: Increased sensitivity to both (↓ doses).
    • Volatile-sparing techniques preferred.

⭐ Myasthenia Gravis patients show marked sensitivity to non-depolarizing muscle relaxants, requiring dose reduction to 1/10th to 1/20th of the usual dose.

Muscular Dystrophies - Muscle Under Siege

• Hereditary, progressive muscle weakness, degeneration.
• Duchenne Muscular Dystrophy (DMD):
  • X-linked; absent dystrophin. Onset 2-5 yrs. Gower's sign.
  • Systemic: Cardiomyopathy (dilated), respiratory failure (restrictive), scoliosis.
  • Anesthesia:
    • ⚠️ NO Succinylcholine (hyperkalemia, rhabdomyolysis).
    • ↑ Sensitivity to NDMRs; titrate carefully.
    • MH risk: Be prepared (controversial).
    • Aspiration risk. Regional preferred.

⭐ Succinylcholine is absolutely contraindicated in Duchenne Muscular Dystrophy due to risk of hyperkalemic cardiac arrest and rhabdomyolysis.

• Becker Muscular Dystrophy (BMD):
  • Milder, later onset. Dystrophin abnormal/reduced. Similar concerns to DMD, less severe.
• General Anesthetic Points:
  • Pre-op: Cardiac/Respiratory evaluation crucial.
  • Careful positioning. Post-op respiratory care.

Other NMDs & Peri-op Pearls - Navigating Neuro-Weakness

• General NMD Peri-op Goals: Maintain muscle strength, ensure adequate ventilation, prevent complications (e.g., aspiration, rhabdomyolysis).
• Lambert-Eaton (LEMS): ↑ Sensitivity to both NDMRs & SCh. Pre-synaptic VGCC Ab. Improvement with activity.
• Periodic Paralysis:
  • HypoKPP: Triggered by CHO, rest post-exercise. Avoid glucose loads, K⁺-wasting drugs. Acetazolamide.
  • HyperKPP: Triggered by K⁺, fasting, cold. Avoid K⁺, SCh. Glucose + insulin for acute attacks.
• Congenital Myopathies: Variable SCh response; potential MH risk (e.g., Central Core Disease). Biopsy key.

⭐ In Guillain-Barré Syndrome, autonomic dysfunction is common, requiring careful hemodynamic management and avoidance of succinylcholine due to upregulation of ACh receptors.

High‑Yield Points - ⚡ Biggest Takeaways

• Myasthenia Gravis: ↑ sensitivity to NDMRs, resistance to succinylcholine; risk of postoperative respiratory failure.
• LEMS: ↑ sensitivity to both succinylcholine and NDMRs; strength improves with activity.
• Muscular Dystrophies (e.g., Duchenne): Avoid succinylcholine (risk of hyperkalemia, rhabdomyolysis); ↑ MH susceptibility.
• Myotonic Dystrophy: Myotonic crisis with succinylcholine or neostigmine; avoid succinylcholine.
• Guillain-Barré Syndrome: Risk of autonomic instability and hyperkalemia with succinylcholine.
• Regional anesthesia is often preferred when possible in these patients.
• Vigilant postoperative respiratory monitoring is critical for all neuromuscular disorders.