Myasthenia Gravis (MG): Anaesthetic Management for Thymectomy & Myasthenic Crisis

What is Myasthenic Crisis?

• Occurs in 15–20% of MG patients, usually within the first 2 years of disease onset
• A progression in severity of the underlying disease, often amplified by a trigger

• Cardiovascular: beta-blockers, calcium channel blockers, quinidine, procainamide, lidocaine
• Antibiotics: aminoglycosides, tetracyclines, clindamycin, fluoroquinolones, polymyxin B
• Others: neuromuscular blockers, corticosteroids (paradoxical worsening), phenytoin, thyroid replacement

1. Secure airway — respiratory failure arises from muscle weakness, not hypoxia; supplemental O₂ alone is inadequate
2. Plasma exchange — first-line for severe crisis; effective in up to 95% of cases; multiple exchanges over 1–2 weeks
3. IVIG — 2 g/kg in divided doses over several days; equivalent efficacy, easier to administer
4. Both are combined with high-dose oral prednisolone (60 mg/day) — but steroids can transiently worsen weakness, so always give alongside PE or IVIG
5. Reinstate or adjust pyridostigmine (60–120 mg orally q4–6h) if crisis was precipitated by dose reduction

Anaesthetic Management for Thymectomy

Preoperative Assessment

• Document disease class (MGFA Classification I–V) — class, affected muscle groups, bulbar/respiratory involvement
• Identify predictors of postoperative ventilation requirement:
  • Disease duration > 6 years
  • Vital capacity < 4 mL/kg (or < 2–2.9 L)
  • Peak inspiratory pressure < –25 cmH₂O
  • Pyridostigmine dose > 750 mg/day
  • Coexisting pulmonary disease
• Optimise before surgery: patients with respiratory or oropharyngeal weakness should receive preoperative IVIG or plasmapheresis; if strength normalises, postoperative complication risk approaches that of a non-myasthenic patient
• Screen for coexisting autoimmune disorders (hypothyroidism, rheumatoid arthritis, SLE)
• Review all drugs for potential MG exacerbation

Premedication

• Avoid or use with great caution: opioids and benzodiazepines — MG patients are hypersensitive to respiratory depressant effects
• Consider H₂-blocker or proton pump inhibitor (aspiration risk with bulbar involvement)
• Continue anticholinesterase medications up to the morning of surgery in most patients; discuss with the surgical/neurological team

Induction & Airway

• Standard induction agents can be used, but even moderate doses of propofol or opioids may cause marked respiratory depression
• Volatile agent-based anaesthesia is preferred — deep inhalational anaesthesia alone (sevoflurane/desflurane) frequently provides sufficient relaxation for intubation and surgery without any NMB

Neuromuscular Blocking Agents (NMBs)

• Neuromuscular monitoring is mandatory — train-of-four throughout; confirm full recovery before extubation
• Sugammadex is the preferred reversal agent if rocuronium is used (avoids anticholinesterase accumulation)
• Neostigmine reversal may be used cautiously; remember it also inhibits plasma cholinesterase, theoretically prolonging succinylcholine and ester local anaesthetic action

Maintenance

• Volatile agents (sevoflurane/isoflurane) provide dose-dependent relaxation and are the backbone of MG anaesthesia
• Supplement with short-acting opioids titrated carefully
• Target multimodal analgesia to minimise opioid requirement (NSAIDs, paracetamol, regional techniques where feasible)
• Thoracic epidural analgesia is an excellent option for open transsternal thymectomy — reduces opioid requirement and facilitates early extubation

Surgical Note

• Preserve phrenic nerves — critical; inadvertent phrenic nerve injury is catastrophic in an already weak patient
• Complete resection of all mediastinal fatty areolar tissue is required to remove ectopic thymic tissue

Extubation & Postoperative Care

• Extubate only after confirming adequate neuromuscular recovery (TOF ratio > 0.9) and ventilatory function
• Predictors of need for postoperative mechanical ventilation (see preoperative assessment above) must guide planning
• Have ICU/HDU bed available for all MG thymectomy patients
• Restart anticholinesterase agents as soon as oral intake resumes postoperatively; if oral route unavailable, give parenterally at 1/30th the oral dose
• Intensive pulmonary physiotherapy, early mobilisation, avoid respiratory depressants
• Watch for postoperative myasthenic crisis — infection, residual NMB, pain-limited respiration, and medication delays are all triggers

• Morgan and Mikhail's Clinical Anesthesiology, 7e — Anaesthetic Considerations, Myasthenia Gravis
• ROSEN's Emergency Medicine — Myasthenic Crisis Management
• Sabiston Textbook of Surgery — Perioperative Management for Thymectomy
• Miller's Anesthesia, 10e — Postoperative Myasthenic Crisis Predictors