24.3 Myasthenic Crisis (03.O.2)

Crisis is an airway diagnosis, not a titer

Myasthenia gravis (MG) is an autoimmune attack on the neuromuscular junction, most often acetylcholine-receptor (AChR) antibodies, less often muscle-specific kinase (MuSK) or LRP4. Myasthenic crisis is MG severe enough that ventilatory or bulbar failure threatens life—typically needing intubation or noninvasive ventilation. About 10% of generalized MG patients have a crisis, often in the first 2 years, after infection, surgery, pregnancy, or a new medication. Mortality with modern ICU care is far lower than in the era of the iron lung, but crash intubation of a patient with a weak neck and pooled secretions still kills.

The examination wants you to separate crisis from cholinergic crisis, intubate before the crash, choose IVIG or PLEX, hold pyridostigmine when the patient is ventilated, avoid a short list of drugs, and not send an unstable patient to the operating room for thymectomy on hospital day 1.

Crisis versus cholinergic crisis

If the picture is mixed and the patient is in extremis, secure the airway first. The historical edrophonium (Tensilon) test can precipitate either worse weakness or a cholinergic storm and is not an ICU procedure. The ice-pack test (ptosis improving after a minute of ice over the eyelid) is a clinic trick for suspected MG, not a crisis tool.

Intubate before the crash

Respiratory failure in MG is fatigable. Morning FVC can look acceptable and evening FVC can be a rapid-sequence emergency. Watch for staccato speech, weak neck flexion (the patient cannot lift the head off the pillow), a weak cough, paradoxical abdominal motion, and pooling of secretions. Arterial blood gas is a late monitor.

Many services use numbers similar to GBS: FVC under 20 mL/kg or a rapidly falling NIF. Some teach a myasthenia-specific 20/20/20 frame (FVC under 20 mL/kg, NIF worse than about −20 cm H2O, rising carbon dioxide). The exact mnemonic matters less than the principle: intubate electively when bulbar function or the trajectory of spirometry is unsafe. Noninvasive ventilation can rescue selected patients without heavy secretion burden or hypercapnic coma; it is not a plan for someone who cannot swallow.

Avoid succinylcholine (unpredictable duration and hyperkalemia risk in some neuromuscular patients). If a paralytic is required, a reduced-dose nondepolarizing agent with neuromuscular monitoring is the usual approach; anticipate prolonged block.

Pyridostigmine when the tube is in

Pyridostigmine should not be stopped in a worsening but still extubated patient—missed doses are a cause of crisis. Once the patient is invasively ventilated, most neurointensivists hold pyridostigmine. The modest benefit on strength is outweighed by bronchorrhea and mucus plugging, and the drug muddies the examination. Restart it before a weaning trial so that junctional transmission is supported when the tube comes out. Intravenous pyridostigmine (about 1 mg IV for 30 mg oral) can cause arrhythmias and is generally avoided.

IVIG or PLEX, then steroids with eyes open

International consensus guidance treats PLEX and IVIG as the short-term treatments for impending or manifest crisis. Expert consensus often favors PLEX when a faster effect is needed; randomized comparisons suggest similar overall efficacy. IVIG is typically 2 g/kg divided over 2–5 days. PLEX is usually 5 exchanges over 7–14 days. Choose PLEX with caution in sepsis and hemodynamic instability; choose IVIG with caution in hypercoagulability, significant renal failure, or immunoglobulin hypersensitivity.

Corticosteroids are part of bridging to longer-term control, but they can worsen weakness 5–10 days after initiation in a large fraction of patients. Start them in the ICU after rescue therapy is underway, not as the sole emergency drug on the ward. A steroid dip is a reason the patient is still in the unit, not a reason to abandon steroids forever. Steroid-sparing immunosuppressants (azathioprine, mycophenolate, tacrolimus, rituximab—especially in MuSK MG) are not same-day crisis drugs.

MuSK MG is more bulbar, more crisis-prone, and often responds better to PLEX and rituximab than to acetylcholinesterase inhibitors, which may worsen MuSK symptoms.

Drugs that unmask the junction

Build a stop-list into every crisis admission medication review:

Infection still needs treatment. Pick a different antibiotic class rather than leaving pneumonia untreated.

Thymoma and thymectomy are not day-1 ICU procedures

A chest CT looking for thymoma belongs in the workup of new MG, and thymoma is over-represented among patients who reach crisis. Thymectomy for thymoma, or for non-thymomatous AChR generalized MG in the indicated age window, is disease-modifying—but it is planned after crisis rescue, when the patient is extubated or at least immunologically pretreated. Sending a crashing myasthenic to the operating room on arrival is the wrong sequence unless there is a separate surgical emergency (for example an obstructing mass with airway compromise that anesthesia and surgery have already planned around).

Examination traps

• Performing edrophonium at the bedside of a hypercapnic patient.
• Stopping pyridostigmine in a still-extubated patient who was missing doses.
• Continuing high-dose pyridostigmine after intubation and then blaming the disease for mucus plugging.
• Starting 80 mg of prednisone on the ward without IVIG or PLEX and without an ICU bed.
• Prescribing levofloxacin and intravenous magnesium for a COPD flare in known MG.
• Booking thymectomy as the 2 a.m. solution to ventilator dependence.