Neurology · Topic 19 of 22

Myasthenia Gravis & Myasthenic Crisis

Neurology · neuromuscular emergency

Myasthenia Gravis & Myasthenic Crisis

Myasthenic crisis is weakness that needs ventilatory support: trend vital capacity, secure the airway early, and give plasma exchange or IVIG.

Key numbers

Trend vital capacity and inspiratory forceVC < 20 mL/kg

Plasma exchange or IVIG for crisisIVIG 0.4 g/kg × 5 days

01 · What it isCrisis: weakness requiring intubation or NIV

Myasthenic crisis (MGFA class V) is myasthenic weakness severe enough to need intubation or noninvasive ventilation to avoid intubation; it strikes 15–20% of patients with MG at least once.

02 · WatchTrend vital capacity and inspiratory force

Admit impending crisis for close respiratory and bulbar monitoring and trend FVC and negative inspiratory force: VC < 20 mL/kg (or < 1 L) or NIF weaker than −20 to −30 cmH₂O signals impending failure.

03 · GivePlasma exchange or IVIG for crisis

Plasma exchange or IVIG (0.4 g/kg daily for 5 days) is the mainstay of crisis treatment; experts consider plasma exchange faster and more effective, while trials show the two as equivalent.

Causes & risk factors

Why it happens

Look for the trigger, usually infectionCrisis is often triggered by infection; evaluate every crisis patient for an infectious source and review recent medications.

Presentation

What you'll see

Fatigable ptosis, diplopia and bulbar weaknessMG causes fatigable weakness — ptosis, diplopia, proximal limb, neck and bulbar weakness (dysphagia, nasal speech, choking) — with normal sensation and deep tendon reflexes.

Do right now

In this order

First moves

Trend vital capacity and inspiratory forceVC < 20 mL/kg

→

Plasma exchange or IVIG for crisisIVIG 0.4 g/kg × 5 days

→

Avoid fluoroquinolones, aminoglycosides, IV magnesium

→

Look for the trigger, usually infection

Why each step

Trend vital capacity and inspiratory forceAdmit impending crisis for close respiratory and bulbar monitoring and trend FVC and negative inspiratory force: VC < 20 mL/kg (or < 1 L) or NIF weaker than −20 to −30 cmH₂O signals impending failure.

Plasma exchange or IVIG for crisisPlasma exchange or IVIG (0.4 g/kg daily for 5 days) is the mainstay of crisis treatment; experts consider plasma exchange faster and more effective, while trials show the two as equivalent.

Avoid fluoroquinolones, aminoglycosides, IV magnesiumAvoid drugs that worsen myasthenia: fluoroquinolones (boxed warning), aminoglycosides, magnesium sulfate, procainamide, quinine, calcium channel blockers and immune checkpoint inhibitors.

Look for the trigger, usually infectionCrisis is often triggered by infection; evaluate every crisis patient for an infectious source and review recent medications.

Management

Orders

Steroids can transiently worsen weaknessHigh-dose corticosteroids can cause transient worsening (over half of patients), so in crisis start them alongside — or a few days after — plasma exchange or IVIG, for a sustained response.

Hold pyridostigmine while intubatedGuidelines commonly suggest stopping pyridostigmine during crisis on the ventilator because it increases airway secretions, then restarting it before extubation.

Confirm with AChR antibodies or EMGConfirm MG with acetylcholine receptor antibodies (80–90% in generalized MG; MuSK antibodies in many seronegative patients) or EMG with repetitive stimulation; single-fiber EMG is abnormal in over 95%.

Image the chest for thymomaOnce MG is diagnosed, get chest CT or MRI for thymoma; resect any thymoma, while thymectomy for MG itself is elective and done only when the patient is stable.

Pyridostigmine relieves symptoms; immunotherapy controls diseasePyridostigmine is first-line symptomatic therapy (neostigmine when parenteral is needed); corticosteroids or nonsteroidal immunosuppressants are added when treatment goals are not met.

The rules

Know these

Sepsis bars PLEX; renal failure bars IVIGChoose by comorbidity: plasma exchange cannot be used in sepsis and carries hemodynamic and venous-access risks; IVIG is contraindicated in hypercoagulable states, renal failure or immunoglobulin hypersensitivity.

Myasthenic or cholinergic crisis?

Commonly confused

Cause	MG exacerbation, often an infection
Fasciculations	Absent
Secretions and gut	No muscarinic excess
Management	Ventilatory support plus plasma exchange or IVIG

More detail

Reference

Show 6 reference facts

Avoid neuromuscular blockers when possible in MG; if one is needed, use rocuronium or vecuronium and reverse with sugammadex.

Plasma exchange may work better than IVIG in MuSK-antibody MG.

FcRn inhibitors (efgartigimod, rozanolixizumab, nipocalimab) and complement inhibitors (eculizumab, ravulizumab, zilucoplan) treat generalized MG; complement inhibitors need meningococcal vaccination or prophylactic antibiotics first.

Bedside ice-pack test: 2 minutes of ice over closed eyes improves myasthenic ptosis (sensitivity about 80%); edrophonium testing is no longer available in the US.

Maintenance prednisone starts low (10 mg daily) and rises by 10 mg weekly to about 60 mg to limit steroid-induced worsening, then tapers to the lowest effective dose.

Do not use magnesium sulfate for eclampsia in a patient with MG; barbiturates or phenytoin usually suffice.

Pearl

In crisis, the vital capacity is the vital sign — trend it, and treat the trigger.

Sources · Merck Manual Professional — Myasthenia Gravis · Sanders DB et al. International consensus guidance for management of myasthenia gravis: Executive summary. Neurology 2016;87:419–425 · Chen P et al. Summary of the best evidence for the prevention and management of myasthenic crisis after thymectomy. Gland Surg 2024;13:540–551 · Ciprofloxacin tablets prescribing information — DailyMed (boxed warning: exacerbation of myasthenia gravis) · Prado MB et al. Acetylcholinesterase Inhibitors in Myasthenic Crisis: A Systematic Review of Observational Studies. Neurocrit Care 2021;35:528–544 · Wendell LC, Levine JM. Myasthenic crisis. Neurohospitalist 2011;1:16–22 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).