Myasthenia Gravis (MG) + Anaesthetic Management for Thymectomy

MD Anaesthesia Exam - Short & Crisp with Mnemonics + Viva Questions

PART 1 - MYASTHENIA GRAVIS

Definition

Quick Mnemonic: "MG = Missing Gates" - the AChR "gates" at the NMJ are destroyed by autoantibodies.

Pathophysiology

• IgG anti-AChR antibodies (found in 85-90% of generalized MG, 50-70% of ocular MG)
• Anti-MuSK antibodies (muscle-specific tyrosine kinase) - in AChR-negative patients; more severe facial/bulbar involvement
• Anti-LRP4 antibodies - 1-3%; milder disease
• Thymoma present in 10-15% of MG patients; ~70% show thymic lymphoid follicular hyperplasia
• Complement-mediated destruction of postsynaptic membrane → reduced functional AChRs
• (Morgan & Mikhail's Clinical Anesthesiology, 7e, p.1181)

Clinical Features

• Ptosis (most common initial sign)
• Ophthalmoplegia / diplopia
• Dysphagia / Dysarthria (bulbar involvement)
• Shortness of breath (respiratory muscles)
• Rest improves, exertion worsens (pathognomonic fatigability)

MGFA Clinical Classification (Jaretzki Classification)

Precipitants / Exacerbating Factors

• Medications (aminoglycosides, fluoroquinolones, beta-blockers, Ca²+ channel blockers, lithium, chloroquine, neuromuscular blockers, magnesium)
• Infection (most common precipitant)
• Surgery / Stress
• Thyroid disease (hyperthyroid worsens; hypothyroid also worsens and increases pyridostigmine need)
• Pregnancy (unpredictable)
• Electrophysiological / Emotional stress
• General anaesthesia

Diagnosis

Myasthenic Crisis vs Cholinergic Crisis

Treatment Summary

• Symptomatic - Pyridostigmine (AChE inhibitor), 60 mg TDS-QID
• Immunomodulation - IVIG, Plasma exchange (fastest acting, for crisis)
• Pharmacological immunosuppression - Steroids, Azathioprine, Mycophenolate, Rituximab, Eculizumab
• Surgical - Thymectomy (especially if thymoma; also beneficial in generalized MG without thymoma)

PART 2 - ANAESTHETIC MANAGEMENT FOR THYMECTOMY

Why Thymectomy?

• Thymoma found in 10-15% of MG patients
• 70% have thymic hyperplasia
• Thymectomy leads to remission/improvement in MG in >50% of non-thymoma patients
• Indications: Thymoma (mandatory), generalized MG (Class II-IV), age <60, AChR-antibody positive

Pre-operative Assessment

1. Respiratory function - FVC, FEV1 (FVC <2.9 L predicts need for postop ventilation)
2. Regimen of medications - last dose of pyridostigmine, steroids
3. Risk stratification - Osserman/MGFA class, bulbar involvement
4. Review contraindicated drugs - aminoglycosides, Mg²+, succinylcholine carefully

• Duration of MG >6 years
• History of COPD
• Dose of pyridostigmine >750 mg/day
• Vital capacity <2.9 L (Score ≥10 → likely to need post-op ventilation)

• Pyridostigmine: continue or hold morning dose (institution-specific; many anaesthetists omit on day of surgery to reduce secretions and sensitivity to NMBDs)
• Plasma exchange or IVIG if patient is in poor condition (Class III/IV)
• Steroids: continue peri-operatively (stress dose if on long-term therapy)
• Optimize pulmonary function

Anaesthetic Goals for Thymectomy

• Prevent crisis (avoid precipitants: infection, stress, offending drugs)
• Limit NMBDs (profound sensitivity to non-depolarizing agents)
• Avoid respiratory depression (opioids and sedatives used carefully)
• Neuromuscular monitoring (mandatory - TOF throughout)
• Ventilation post-op planning

Premedication

• Avoid benzodiazepines and opioids (respiratory depressants - use cautiously or avoid premedication)
• Glycopyrrolate/Atropine: to manage excessive secretions if pyridostigmine is continued
• Continue steroids
• Antacid prophylaxis: metoclopramide + H2 blocker (risk of aspiration from bulbar MG)
• Anti-anxiety: Small dose oral midazolam may be used cautiously with monitoring available

Induction

• IV induction preferred: Propofol (drug of choice - attenuates airway reflexes, no epileptogenic effect, short duration)
• Airway: Consider RSI if bulbar symptoms present (aspiration risk)
• Succinylcholine: Avoid if possible - resistance to succinylcholine (need higher doses due to reduced AChRs); risk of phase II block unpredictable
  • If RSI absolutely needed: use high-dose succinylcholine (1.5-2 mg/kg) or prefer rocuronium 1.2 mg/kg + sugammadex reversal
• Opioids: Short-acting agents preferred (remifentanil, fentanyl) - titrate carefully

Maintenance

• Volatile agents (sevoflurane, isoflurane, desflurane): Provide good muscle relaxation inherently; reduce NMBD requirement significantly
• TIVA (Propofol + Remifentanil): Excellent option - avoids volatile-related muscle relaxation unpredictability
• NMBDs:
  • If required: use short-/intermediate-acting non-depolarizing NMBDs (rocuronium, atracurium, mivacurium)
  • Dose: 1/10th to 1/4th of normal dose (profound sensitivity)
  • Mandatory TOF monitoring throughout
  • Avoid vecuronium/pancuronium (longer duration)
  • Prefer atracurium (organ-independent Hofmann elimination - safe in all patients)
• Avoid long-acting NMBDs, aminoglycosides, magnesium, high-dose volatile agents without TOF monitoring

Surgical Approach

• Sternotomy (median): traditional, good exposure, longer recovery
• VATS (Video-Assisted Thoracoscopic Surgery): preferred now; less pain, faster recovery, less post-op ventilation need
• Robotic: emerging

• Double-lumen tube (DLT) or bronchial blocker
• MG patients tolerate OLV poorly if respiratory reserve is compromised - careful pre-op FVC assessment critical

Reversal and Extubation

• Neostigmine: Use cautiously (anticholinesterase + glycopyrrolate). Risk: may precipitate cholinergic crisis + increase secretions
• Sugammadex: Drug of choice for reversal if rocuronium used - reverses without cholinergic side effects, avoids interaction with MG medications
• Confirm TOF ratio >0.9 before extubation

• TOF ratio >0.9
• Sustained head lift >5 seconds
• FVC >15 mL/kg
• Spontaneous breathing, cooperative patient
• Absence of bulbar symptoms / excess secretions

• Leventhal score ≥10
• Class IV/V MG
• Prolonged surgery
• Intraoperative complications
• Poor baseline FVC

Post-operative Management

• ICU/HDU monitoring mandatory
• Restart pyridostigmine via NG tube if extubated, or hold if ventilated (may increase secretions)
• Watch for myasthenic crisis (most common 24-72 hrs post-op)
• Analgesia: Avoid opioids if possible; use multimodal - paracetamol, NSAIDs, regional (thoracic epidural, paravertebral block excellent for sternotomy/VATS)
• Physiotherapy and early mobilization

Drug Interactions Table - What to AVOID

MG vs Lambert-Eaton (LES) - Classic Exam Comparison

POSSIBLE VIVA QUESTIONS

Sources: Morgan & Mikhail's Clinical Anesthesiology, 7e (p. 1180-1184); Bradley & Daroff's Neurology in Clinical Practice (blocks 3, 18, 23); Miller's Anesthesia, 10e; Adams & Victor's Principles of Neurology, 12e

MG Patient for Lower-Limb (Non-Thymectomy) Surgery

Anaesthetic Management - MD Exam Ready

KEY CONTEXT: Why Lower Limb is Different from Thymectomy

• No thoracic access needed → regional anaesthesia is strongly preferred
• No one-lung ventilation
• Shorter surgery, less physiological stress → crisis risk lower but NOT zero
• Regional techniques (spinal/epidural/peripheral nerve blocks) can completely avoid NMBDs, intubation, and volatile agents - ideal for MG

PRE-OPERATIVE ASSESSMENT

History

• MGFA Class (I-V) - severity of disease
• Bulbar symptoms: dysphagia, dysarthria, aspiration risk
• Respiratory symptoms: shortness of breath, exercise tolerance
• History of myasthenic/cholinergic crisis and prior intubation
• Duration of MG (>6 years = higher risk)
• Current medications: pyridostigmine dose (>750 mg/day = high risk), steroids, immunosuppressants
• Associated autoimmune disease: thyroiditis, RA, SLE

Examination

• Airway assessment (for GA backup plan)
• Respiratory: auscultation, check for respiratory muscle weakness
• Neurological: baseline power, bulbar signs

Investigations

• Pulmonary Function Tests: FVC, FEV1 (FVC <2.9 L → risk of post-op ventilation)
• ECG (associated myocarditis, arrhythmias rare)
• Serum anti-AChR antibody titre (>100 nmol/mL = higher crisis risk)
• Chest X-ray / CT (thymic mass - airway compromise?)
• Routine bloods, coagulation (before neuraxial block)

Risk Factors for Post-operative Myasthenic Crisis

• Duration of MG >6 years
• Antibody titre >100 nmol/mL
• Medication dose (pyridostigmine >750 mg/day)
• Pulmonary capacity (FVC <2.9 L / chronic pulmonary disease)
• Bulbar symptoms / previous crisis

PRE-OPERATIVE OPTIMIZATION

Mnemonic for pre-op: "CAPS" - Continue all MG meds, Aspiration prophylaxis, Plasma exchange if needed, Steroids (stress dose)

ANAESTHETIC TECHNIQUE - PREFERRED APPROACH

1st Choice: REGIONAL ANAESTHESIA (Neuraxial or Peripheral)

• Completely avoids NMBDs
• Avoids volatile/IV anaesthetic agents → no post-op sedation/respiratory depression
• Avoids tracheal intubation → no airway manipulation
• Reduces stress response
• Excellent post-op analgesia → minimal opioid requirement

Caution with local anaesthetics: Lidocaine (lignocaine) appears in the list of drugs that may worsen MG (Morgan & Mikhail, Table 29-5). Use bupivacaine or ropivacaine preferentially for neuraxial/peripheral blocks. If lidocaine must be used, use lowest effective dose.

2nd Choice: GENERAL ANAESTHESIA (if regional fails/refused/contraindicated)

• Propofol induction (drug of choice)
• Laryngeal mask airway (LMA) if airway permits - avoids full intubation
• Avoid or minimize NMBDs
• Neuromuscular monitoring (TOF) mandatory
• No NMB if possible - use deep volatile/propofol + LMA

Induction

• Propofol IV - drug of choice (short-acting, attenuates airway reflexes, no trigger for NMJ issues)
• LMA strongly preferred over ETT for lower-limb procedures - avoids need for muscle relaxation for intubation
• If ETT needed (full stomach, aspiration risk, poor airway):
  • Rocuronium 1.2 mg/kg for RSI + Sugammadex for reversal (not succinylcholine)
  • If standard intubation: use low-dose rocuronium with TOF guide + sugammadex

NMBDs in MG - Summary

Maintenance

• Volatile agents (sevoflurane preferred): inherent muscle relaxation reduces NMBD requirement; careful titration
• TIVA (Propofol + Remifentanil): excellent choice - avoids volatile effects, ultra-short opioid
• Short-acting opioids: remifentanil > fentanyl (avoid morphine)
• Multimodal analgesia: paracetamol, NSAIDs to reduce opioid load

Monitoring

• Standard ASA + TOF (Train-of-Four) monitoring mandatory if any NMBD given
• TOF ratio must be >0.9 before extubation
• SpO2, EtCO2 (especially important in MG for early respiratory compromise detection)

REVERSAL AND EXTUBATION

• 5 seconds sustained head lift
• 3 = TOF ratio >0.9 (T4/T1)
• Tidal volume adequate (FVC >15 mL/kg)
• Awake, cooperative, no excess secretions

• Sugammadex (if rocuronium used): 2 mg/kg at TOF count 2; 4 mg/kg for profound block
• Neostigmine (if atracurium used): 0.04-0.07 mg/kg + glycopyrrolate - monitor carefully for cholinergic signs

POST-OPERATIVE MANAGEMENT

• Watch for myasthenic crisis (first 24-72 hrs; signs = increasing RR with shallow breaths, dysphagia, drooling, weak cough, paradoxical breathing)
• Analgesia - multimodal (paracetamol + NSAIDs + peripheral nerve block catheter) to minimize opioids
• Timely restart of pyridostigmine (oral or NG)
• Check and continue steroids/immunosuppressants
• HDU/ICU if poorly controlled MG, high-risk features (Leventhal score ≥10, Class III/IV)

• Continuous SpO2, RR monitoring
• Outpatient surgery is safe in well-controlled MG (Class I/II) with short procedures and complete recovery before discharge (Miller's Anesthesia 10e)

• Increasing respiratory rate with shallow breaths (first sign)
• Dysphagia, drooling, change in phonation
• Weak cough, paradoxical breathing
• Hypoxia / hypercarbia (late sign)

DRUGS TO AVOID / USE CAUTIOUSLY

SPINAL vs GA FOR LOWER LIMB MG - QUICK DECISION GUIDE

MG for lower-limb surgery
        |
Is regional feasible? (No contraindication, cooperative patient, FVC adequate)
        |
       YES                          NO
        |                            |
Spinal/Epidural/PNB              GA with LMA
(best choice)                    (avoid ETT if possible)
        |                            |
No NMBD needed               No NMBD or rocuronium + sugammadex
Bupivacaine/Ropivacaine         TOF monitoring throughout
Monitor SpO2 post-block         Extubate only when fully reversed

POSSIBLE VIVA QUESTIONS

Sources: Miller's Anesthesia, 10e (pp. 4204-4207); Morgan & Mikhail's Clinical Anesthesiology, 7e (pp. 1180-1186); Barash Clinical Anesthesia, 9e