Neuroanaesthesia · Core03 / 14
Myasthenia gravis for emergency laparotomy52-year-old woman, 66 kg, with a perforated duodenal ulcer for emergency laparotomy
Myasthenia gravis: relaxant dosing, full-stomach induction and cholinergic versus myasthenic crisis
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Know this first
- Myasthenia: resistant to suxamethonium, very sensitive to non-depolarisers.
- Ventilation predictors: myasthenia for more than 6 years, more than 750 mg pyridostigmine daily, vital capacity below 2.9 L, other chronic lung disease.
- Cholinergic crisis: muscarinic signs and worse weakness after edrophonium 1–2 mg.
- Lambert-Eaton: more sensitive to both suxamethonium and non-depolarisers.
The patient
52-year-old woman, 66 kg, with a perforated duodenal ulcer for emergency laparotomy; vomiting, full stomach.
Generalised myasthenia gravis for 7 years: mild dysphagia and nasal speech; vital capacity 2.1 L.
Drugs: pyridostigmine 60 mg five times daily (last dose 3 hours ago), prednisolone 15 mg daily, azathioprine.
Vitals: BP 102/62, HR 114, SpO2 95% on air, RR 24, temperature 38.2 °C.
The examiner asks
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Consider
What are your concerns?
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- A 52-year-old with generalised myasthenia and bulbar symptoms, for emergency laparotomy on a full stomach. Patient
- Dysphagia and nasal speech mean bulbar weakness, so her aspiration risk is high. Airway
- Sepsis and surgery can push her into a myasthenic crisis. Stem trigger
- Antibiotics for peritonitis: aminoglycosides and fluoroquinolones worsen her weakness. Disease
- Prednisolone 15 mg daily means she needs steroid cover for major surgery.
Prepare
How will you get her ready?
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- Ventilation predictors: 7 years of disease and VC 2.1 L both count, and she has bulbar symptoms. Disease
- Her pyridostigmine is 300 mg a day, under the 750 mg threshold.
- So I warn her she may wake ventilated, and book an ICU bed. After surgery
- She keeps taking pyridostigmine; doses resume by tube after surgery. Disease
- Major surgery, so hydrocortisone 100–150 mg IV that day, then a taper over 1–2 days.
- Quantitative twitch monitor ready, and sugammadex in the room.
Plan
How will you induce and maintain anaesthesia?
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- Rapid sequence with suxamethonium 1.5 mg/kg; the normal dose beats her resistance. Airway
- She's prone to phase II block, so I watch the twitch afterwards. Disease
- Any further rocuronium at a half to a third of the usual dose, titrated on the monitor.
- Volatile anaesthesia relaxes her muscle and cuts her relaxant need.
- Unlike Lambert-Eaton, where both relaxant types are potentiated, she resists suxamethonium.
- Reverse with sugammadex, 2 or 4 mg/kg by depth, not neostigmine.
- Extubate only with clear strength; her weakness can return hours later. After surgery
Crisis
In recovery, an hour after extubation, she is drooling, sweating and weaker; HR 46, SpO2 89%. A colleague gave neostigmine 2.5 mg. What do you do?
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- Cholinergic crisis until proven otherwise: neostigmine on top of her pyridostigmine. Crisis
- 100% oxygen, sit her up, suction; intubate if her breathing fails.
- Small pupils point to a cholinergic crisis; large pupils to a myasthenic one.
- Atropine 1 mg IV for the bradycardia and secretions; glycopyrrolate also helps.
- If still unsure, edrophonium 1–2 mg: worse weakness confirms cholinergic crisis.
- No more anticholinesterase; ventilate in ICU until it wears off. After surgery
Fatal mistakes
- Full-dose rocuronium without a twitch monitor.
- More anticholinesterase given into a cholinergic crisis.
- Extubating her while still weak.
- An aminoglycoside for her peritonitis.
Checked against: Stoelting 8e · Miller 10e · Barash 9e
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