Myasthenia gravis and also — anaesthetic mgmt for thymectomy for md anaesthesia exam . Short and crisp exam ready with subheadings and mnemonics to remember with possible viva questions .
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Myasthenia Gravis (MG) + Anaesthetic Management for Thymectomy
MD Anaesthesia Exam - Short & Crisp with Mnemonics + Viva Questions
PART 1 - MYASTHENIA GRAVIS
Definition
MG is an autoimmune postsynaptic neuromuscular junction (NMJ) disorder caused by IgG antibodies against nicotinic acetylcholine receptors (AChR) - leading to receptor destruction, complement-mediated end-plate damage, and fatigable skeletal muscle weakness.
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
Mnemonic: "PODS + R"
- Ptosis (most common initial sign)
- Ophthalmoplegia / diplopia
- Dysphagia / Dysarthria (bulbar involvement)
- Shortness of breath (respiratory muscles)
- Rest improves, exertion worsens (pathognomonic fatigability)
Ocular MG only (no generalization) = 15-20% of patients.
MGFA Clinical Classification (Jaretzki Classification)
| Class | Description |
|---|---|
| I | Ocular only |
| IIa | Mild, limb/axial predominant |
| IIb | Mild, bulbar/respiratory predominant |
| IIIa/b | Moderate, limb or bulbar/respiratory |
| IVa/b | Severe, limb or bulbar/respiratory |
| V | Requires intubation ± ventilation |
(Morgan & Mikhail's Clinical Anesthesiology, 7e)
Precipitants / Exacerbating Factors
Mnemonic: "MIST PEG"
- 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
| Test | Finding |
|---|---|
| Edrophonium (Tensilon) test | Transient improvement with short-acting anticholinesterase |
| Anti-AChR antibodies | Confirmatory in 85-90% |
| Repetitive nerve stimulation (RNS) | Decremental response (>10%) at 3 Hz |
| Single-fibre EMG (SFEMG) | Increased jitter - most sensitive test |
| CT chest | Thymoma screening (mandatory) |
| Ice pack test | For ocular MG - ptosis improves with cold |
Myasthenic Crisis vs Cholinergic Crisis
| Feature | Myasthenic Crisis | Cholinergic Crisis |
|---|---|---|
| Cause | Undertreated MG / precipitant | Excess anticholinesterase |
| Weakness | Yes | Yes |
| Pupils | Normal/dilated | Miotic (SLUDGE) |
| Secretions | Minimal | Excessive |
| Edrophonium | Improves | Worsens |
| Treatment | Increase anticholinesterase, IVIG, plasmapheresis | Stop anticholinesterase, atropine, ventilate |
SLUDGE mnemonic for cholinergic crisis: Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis
Treatment Summary
Mnemonic: "SIPS"
- 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
Mnemonic: "4 Rs"
- Respiratory function - FVC, FEV1 (FVC <2.9 L predicts need for postop ventilation)
- Regimen of medications - last dose of pyridostigmine, steroids
- Risk stratification - Osserman/MGFA class, bulbar involvement
- Review contraindicated drugs - aminoglycosides, Mg²+, succinylcholine carefully
Leventhal's Predictors of Post-op Ventilation (MGFA Score):
- 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)
Pre-op optimization:
- 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
Mnemonic: "PLAN - V"
- 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
Mnemonic: "No NMB if possible - TIVA/Volatile + TOF"
- 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
For VATS - one-lung ventilation (OLV) required:
- 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
Reversal:
- 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
Extubation criteria:
- TOF ratio >0.9
- Sustained head lift >5 seconds
- FVC >15 mL/kg
- Spontaneous breathing, cooperative patient
- Absence of bulbar symptoms / excess secretions
When to plan elective post-op ventilation:
- 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
| Drug Class | Examples | Effect on MG |
|---|---|---|
| Aminoglycosides | Gentamicin, tobramycin | Block NMJ → worsen MG |
| Fluoroquinolones | Ciprofloxacin | Worsen |
| Beta-blockers | Propranolol | Worsen |
| Ca²+ channel blockers | Verapamil | Worsen |
| Magnesium | MgSO4 | Profound NMJ block |
| Antiarrhythmics | Quinidine, procainamide | Worsen |
| Lithium | - | Worsen |
| Chloroquine | - | Worsen |
| Long-acting NMBDs | Pancuronium | Prolonged block |
| Opioids | Morphine | Respiratory depression |
MG vs Lambert-Eaton (LES) - Classic Exam Comparison
| Feature | MG | Lambert-Eaton (LES) |
|---|---|---|
| Site | Post-synaptic | Pre-synaptic |
| Antibody | Anti-AChR | Anti-VGCC (voltage-gated Ca²+ channels) |
| Muscles | Ocular > Bulbar > Limb | Proximal limb > Ocular |
| Autonomic | Rare | Yes (dry mouth, constipation) |
| Reflexes | Normal | Reduced but improve with exercise |
| RNS | Decremental (3 Hz) | Incremental (50 Hz) |
| Weakness | Worse with exertion | Improves briefly with exertion |
| Association | Thymus | SCLC (60%) |
| Sensitivity to NMBDs | ↑ to non-dep, resistance to dep | ↑↑ to both |
POSSIBLE VIVA QUESTIONS
Q1. What is the mechanism of MG?
A: IgG antibodies against nicotinic AChR at postsynaptic NMJ → receptor destruction + complement-mediated end-plate damage → fatigable weakness.
Q2. What is the MGFA Class V?
A: Requires intubation with or without mechanical ventilation (except routine post-op management).
Q3. Why is succinylcholine avoided in MG?
A: Reduced AChRs → resistance (need 1.5-2× dose). Risk of Phase II block. Unpredictable response. Preferred alternative is high-dose rocuronium + sugammadex.
Q4. What is your drug of choice for NMBD reversal in MG and why?
A: Sugammadex - encapsulates rocuronium, no anticholinesterase activity, avoids cholinergic side effects and drug interactions with pyridostigmine.
Q5. What are Leventhal's predictors for post-operative ventilation?
A: MG duration >6 years, COPD, pyridostigmine >750 mg/day, FVC <2.9 L. Score ≥10 predicts need for ventilation.
Q6. How do you differentiate myasthenic crisis from cholinergic crisis?
A: Edrophonium test - improves in myasthenic crisis, worsens in cholinergic crisis. Clinical: SLUDGE features (miosis, secretions, bradycardia) in cholinergic crisis.
Q7. Why is atracurium preferred in MG patients requiring NMBDs?
A: Hofmann elimination - organ-independent metabolism, predictable duration, not affected by anticholinesterase drugs.
Q8. What is the single-fibre EMG finding in MG?
A: Increased jitter - most sensitive test for NMJ dysfunction.
Q9. What is OLV and how does it affect MG patients undergoing VATS thymectomy?
A: One-lung ventilation required for VATS. MG patients with reduced FVC tolerate OLV poorly; double-lumen tube placed. Careful pre-op assessment of FVC and respiratory reserve is mandatory.
Q10. Which antibody predicts thymoma association in MG?
A: Anti-AChR antibodies. Thymoma-related MG almost invariably has AChR antibodies (not anti-MuSK or LRP4). Additional antibodies against titin and ryanodine receptors are also common in thymoma-MG.
Q11. What is the role of TIVA in MG thymectomy?
A: Propofol + remifentanil TIVA provides excellent conditions - avoids volatile-agent-induced NMJ effects, allows smooth titration, ideal for avoiding NMBDs altogether, and remifentanil's ultra-short action prevents respiratory depression at extubation.
Q12. Why is regional anaesthesia (thoracic epidural or paravertebral block) important in MG thymectomy?
A: Reduces systemic opioid requirement → less respiratory depression → facilitates early extubation. Particularly valuable in sternotomy approach.
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
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