Myasthenic Crisis

Myasthenia gravis is antibody-mediated failure of neuromuscular transmission, giving fatigable weakness — worse with repetition and at the end of the day. A crisis is weakness severe enough to threaten the airway or ventilation, and it is usually…

Fatigable weakness + falling FVC — find the precipitant

The pathway

1 · Recognise fatigability

Weakness worse with repetition and at the end of the day
Ptosis on sustained upgaze, diplopia, nasal speech, difficulty chewing towards the end of a meal
Limb weakness is proximal and fatigable
Reflexes are preserved — this is the discriminator from Guillain-Barré

2 · Look for the precipitant — it is usually a drug or an infection

Infection is the commonest
Drugs: aminoglycosides, macrolides, quinolones, beta-blockers, magnesium, some anaesthetic agents, and others
Missed or delayed pyridostigmine, including from being nil by mouth
Surgery, pregnancy, or a recent steroid increase (steroids transiently worsen before they help)

3 · Measure the FVC

Serial forced vital capacity, alongside the clinical picture — not saturations alone
20 ml/kg or below is a major escalation trigger; in myasthenia the decision rests on clinical features plus respiratory measurements, not a single threshold
Perform a single-breath count at the bedside as a usable proxy: under 20 is concerning, and the trend matters more than any single figure
Bulbar weakness with pooling secretions is an airway emergency regardless of the number

4 · Do not get lost in cholinergic crisis

Excess pyridostigmine can also cause weakness, with SLUDGE features — salivation, lacrimation, diarrhoea, cramps, small pupils
It is much rarer than myasthenic crisis at modern doses, but it is real and it does happen
Do not delay airway and ventilatory support while you work out which crisis this is — that assessment is identical either way
Once support is in place, the distinction matters: stop pyridostigmine while ventilated and let neurology sort the rest

5 · Treat the crisis

IVIG or plasma exchange — as in GBS, one or the other, not both
Treat the infection and stop the offending drug
Steroids are part of longer-term control but can worsen weakness transiently — a neurology decision, not a ward one
Withhold pyridostigmine in an intubated patient to reduce secretions

6 · Prescribe carefully from now on

Check every new drug against the list that precipitates crisis before prescribing
Flag it clearly in the notes and on the drug chart
Magnesium is the one most often given without thinking — in obstetrics, and for hypomagnesaemia

Drugs

Drug	Dose	Route	Notes
Immunoglobulin (IVIG)	0.4 g/kg/day for 5 days	IV	Or plasma exchange. Not both.
Pyridostigmine	Usual dose, at the usual times	PO	Time-critical. Nil by mouth is a reason to find an enteral or alternative route with neurology and pharmacy, not to omit it. The exception is the intubated patient in crisis, where it is often held because of secretions.
Magnesium	Avoid ROUTINE replacement	—	Magnesium can worsen myasthenia and is a common inadvertent trigger, particularly in obstetrics and in reflex electrolyte replacement. Significant deficiency may still need cautious correction — discuss it urgently with neurology, critical care or your senior rather than either giving it reflexively or withholding it absolutely.
Macrolides, quinolones, aminoglycosides	Can worsen myasthenia	—	Use alternatives where possible; discuss and monitor if one is clinically necessary — sometimes one of these is still the least-bad antimicrobial choice. Decide it with microbiology, do not simply refuse.

When to escalate

FVC at or below 20 ml/kg, falling, or a weak cough — critical care now,Bulbar weakness, wet voice or pooling secretions — anaesthetics immediately,Any patient with myasthenia needing a new antibiotic, or magnesium replacement — check it against the precipitant list and discuss it first,Known myasthenic admitted nil by mouth — pyridostigmine route needs solving today, not tomorrow

Reference: Association of British Neurologists myasthenia gravis management guidelines; NICE CKS myasthenia. Local neurology and critical care pathways take precedence.

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