Residency · Residency · Neurology
Myasthenic Crisis and Neuromuscular Respiratory Failure
Overview
Myasthenic crisis is a life-threatening exacerbation of myasthenia gravis causing respiratory failure that requires intubation or non-invasive ventilation. It occurs in approximately 15-20% of MG patients at some point during their disease. Mortality was historically high but has been reduced to approximately 5% with modern ICU care. Neuromuscular respiratory failure is not unique to MG and can also occur in Guillain-Barre syndrome, ALS, critical illness myopathy/neuropathy, and other NMJ or muscle disorders. Early recognition and proactive monitoring are the keys to preventing emergent intubation.
Triggers for Myasthenic Crisis
Infection is the most common trigger, accounting for approximately 40% of cases, with respiratory infections predominating. Medication changes, including reduction or withdrawal of immunosuppression or introduction of NMJ-impairing drugs, are another major trigger. Surgery and postoperative stress, including effects of anesthetic agents, can precipitate crisis. Medications known to worsen MG include aminoglycosides, fluoroquinolones, macrolides, beta-blockers, magnesium sulfate, D-penicillamine, immune checkpoint inhibitors, iodinated contrast (rarely), and botulinum toxin. Pregnancy, especially the postpartum period, and emotional or physical stress are additional triggers. Tapering immunotherapy too quickly is a preventable cause. In approximately 30% of cases, no identifiable trigger is found.
Bedside Respiratory Assessment
Forced Vital Capacity (FVC)
FVC is the single most important bedside measurement for monitoring neuromuscular respiratory failure. Normal FVC is approximately 60-70 mL/kg. An FVC below 30 mL/kg indicates significant respiratory compromise, while an FVC below 20 mL/kg or below 1 liter should prompt consideration of intubation even if oxygen saturation remains normal. The trend is more important than any single absolute value; a decline of more than 30-40% from baseline is alarming. FVC should be measured in both upright and supine positions, as a drop of more than 25% in the supine position indicates diaphragmatic weakness.
Negative Inspiratory Force (NIF) / Maximum Inspiratory Pressure (MIP)
Normal NIF is more negative than -70 cmH2O. A NIF less negative than -30 cmH2O indicates severe inspiratory weakness and should prompt consideration of intubation. NIF is less effort-dependent than FVC but requires good technique to be reliable.
The "20/30/40 Rule" for Intubation
| Parameter | Normal | Concerning | Intubation Threshold |
|---|---|---|---|
| FVC | 60-70 mL/kg | <30 mL/kg | <20 mL/kg (or <1 L) |
| NIF (MIP) | More negative than -70 cmH2O | Less negative than -40 cmH2O | Less negative than -30 cmH2O |
| FVC decline from baseline | — | >20% | >40% |
| Supine FVC drop | <10% from sitting | >15% | >25% (diaphragm weakness) |
This rule provides clear thresholds for considering intubation: FVC below 20 mL/kg, NIF less negative than -30 cmH2O, or greater than 40% decline in FVC from baseline. Any single one of these criteria should prompt serious consideration of intubation.
Other Bedside Assessments
The single-breath count, where the patient counts from 1 on a single breath, correlates with FVC: a count below 20 corresponds to an FVC below 1 liter. The head lift test assesses neck flexor strength; inability to sustain head lift for 5 seconds correlates with respiratory muscle weakness. Cough strength reflects the ability to clear secretions. Hypophonic or nasal voice suggests bulbar weakness and aspiration risk.
Do NOT Rely On
Pulse oximetry alone is inadequate because oxygen saturation drops late in neuromuscular respiratory failure. Hypoventilation leads to hypercarbia before significant hypoxemia, especially in patients receiving supplemental oxygen. Arterial blood gas showing hypercarbia is a late and ominous sign indicating that ventilatory failure is imminent or already occurring. Subjective dyspnea is unreliable because patients may not report breathlessness until respiratory failure is advanced.
<image>Diagram showing the progressive stages of neuromuscular respiratory failure from early diaphragmatic weakness through hypoventilation to respiratory arrest, with corresponding FVC, NIF, and ABG changes at each stage</image>
Management of Myasthenic Crisis
Acute Immunotherapy
IVIG (Intravenous Immunoglobulin)
IVIG is administered at a dose of 2 g/kg divided over 5 days (0.4 g/kg/day). Onset of effect occurs at 3-5 days with peak effect at 1-2 weeks. It is preferred when plasmapheresis access is limited or hemodynamic instability is a concern. Side effects include headache, infusion reactions, thrombosis, renal insufficiency, and hemolytic anemia.
Plasmapheresis (PLEX)
Plasmapheresis typically involves 5 exchanges over 10-14 days performed every other day. Onset of effect may be faster than IVIG at 2-3 days. The randomized trial by Barth et al. demonstrated that PLEX and IVIG are equivalent in myasthenic crisis. PLEX requires central venous access, and hemodynamic instability is a relative contraindication. The benefit is transient (lasting weeks) without concomitant immunosuppressive therapy.
Evidence Comparison
No definitive randomized controlled trial demonstrates superiority of one treatment over the other. The choice depends on institutional expertise, patient comorbidities, and access. Both are temporizing measures; definitive immunosuppression must follow.
Ventilatory Management
Non-invasive ventilation (BiPAP) may be used as a bridge in cooperative patients with intact airway protective reflexes, but close monitoring for deterioration is essential. Intubation should be performed when FVC falls below 15-20 mL/kg, NIF is less negative than -25 cmH2O, PaCO2 is rising, the patient cannot handle secretions, there is aspiration risk, or clinical deterioration occurs despite BiPAP. Succinylcholine should be avoided because MG patients may be resistant (requiring higher doses); non-depolarizing agents should be used at reduced doses (rocuronium at 50% dose with sugammadex available for reversal). Extubation readiness requires FVC above 15-20 mL/kg, NIF more negative than -30 cmH2O, adequate cough, manageable secretions, and ability to protect the airway.
Medication Management During Crisis
Immunosuppressants should be continued and not withdrawn during crisis. Pyridostigmine should be held during intubation because it increases secretions and complicates ventilator weaning; it is restarted when extubation is planned. Corticosteroids should be avoided or minimized during acute crisis if the patient is not already taking them due to the risk of early steroid-induced worsening; if initiating, start at a low dose of 10-20 mg prednisone. The trigger should be treated aggressively, including antibiotics for infection and discontinuation of offending medications.
Cholinergic Crisis vs. Myasthenic Crisis
Cholinergic crisis is weakness caused by excessive cholinesterase inhibitor (pyridostigmine overdose). It is distinguished from myasthenic crisis by prominent muscarinic symptoms including miosis, excessive salivation, lacrimation, urination, diarrhea, emesis (the SLUDGE mnemonic), bradycardia, and fasciculations. This was historically more common but is rare with modern dosing practices. Management consists of stopping pyridostigmine, providing supportive care, and administering atropine for muscarinic symptoms.
<image>Comparison table differentiating myasthenic crisis from cholinergic crisis based on pupil size, secretions, fasciculations, heart rate, and bowel sounds</image>
Other Causes of Neuromuscular Respiratory Failure
Guillain-Barre Syndrome
GBS is the most common non-MG cause of neuromuscular respiratory failure, with 20-30% of patients requiring intubation. The same respiratory monitoring principles apply, with FVC and NIF measured every 4-6 hours. Autonomic instability adds complexity to ventilator management.
Amyotrophic Lateral Sclerosis
ALS causes chronic progressive respiratory failure. FVC should be monitored at every clinic visit. BiPAP is initiated when FVC falls below 50% predicted or when orthopnea or nocturnal hypoventilation develops. Discussion of tracheostomy and long-term ventilation versus a palliative approach should occur early in the disease course.
Lambert-Eaton Myasthenic Syndrome (LEMS)
LEMS is a presynaptic NMJ disorder caused by anti-VGCC antibodies. Respiratory failure is less common than in MG but can occur. Unlike MG, LEMS produces proximal weakness that improves with brief exercise (facilitation). Repetitive nerve stimulation shows an incremental response at high-frequency stimulation, the opposite of the decremental pattern seen in MG.
Critical Illness Myopathy and Neuropathy
Critical illness myopathy and neuropathy is a common cause of difficulty weaning from the ventilator in ICU patients. Risk factors include sepsis, multi-organ failure, combined corticosteroid and neuromuscular blocking agent use, and prolonged ICU stay. Critical illness myopathy (CIM) shows a myopathic pattern on EMG with muscle membrane inexcitability and elevated CK. Critical illness neuropathy (CIN) presents as an axonal sensorimotor neuropathy on NCS/EMG. Combined CIM/CIN (critical illness neuromyopathy) is the most common presentation. Management focuses on minimizing steroids and paralytics, early mobilization, and supportive care.
Botulism
Botulism is caused by presynaptic NMJ blockade from Clostridium botulinum toxin. It produces descending paralysis affecting cranial nerves first, then limbs. Autonomic features include dilated unreactive pupils, dry mouth, and constipation. Repetitive nerve stimulation shows a decremental response at low frequency and incremental response at high frequency, similar to LEMS. Treatment consists of botulism antitoxin (heptavalent) and supportive care.
<image>Algorithm for management of acute neuromuscular respiratory failure showing serial FVC/NIF monitoring, intubation thresholds, and disease-specific treatments</image>
ICU Management Considerations
ICU management of neuromuscular respiratory failure includes telemetry for autonomic monitoring (especially in GBS), DVT prophylaxis given immobility and potential paralysis, stress ulcer prophylaxis, nutritional support with enteral feeding preferred if safe, early physical and occupational therapy, psychological support including ICU delirium prevention and communication aids for intubated patients, and consideration of tracheostomy if intubation is expected to exceed 2 weeks.
Clinical Pearls
A normal oxygen saturation does not mean respiratory function is adequate; neuromuscular respiratory failure causes hypoventilation and hypercarbia before hypoxemia. FVC should be measured every 4-6 hours in any patient admitted with suspected neuromuscular respiratory failure, with a declining trend being more important than any single value. The supine FVC drop (more than 25% from sitting) is a sensitive indicator of diaphragmatic weakness, and both positions should always be checked. During myasthenic crisis, pyridostigmine should be stopped while intubated because it increases secretions and complicates management. Magnesium sulfate should never be given to an MG patient without careful monitoring because magnesium blocks presynaptic calcium channels and can precipitate crisis. Fluoroquinolones carry a black box warning regarding NMJ blockade and should be avoided in MG patients when alternatives exist. If a patient with known MG develops weakness, infection should always be considered as the trigger before attributing it to disease flare alone, and the infection should be treated aggressively. Critical illness neuromyopathy is the most common cause of failure to wean from the ventilator in the ICU, and minimizing corticosteroids and neuromuscular blocking agents whenever possible is essential for prevention.
References
- Godoy DA, Mello LJ, Masotti L, Di Napoli M. The myasthenic patient in crisis: an update of the management in neurointensive care unit. Arq Neuropsiquiatr. 2013;71(9A):627-639.
- Barth D, Nabavi Nouri M, Ng E, et al. Comparison of IVIg and PLEX in patients with myasthenia gravis. Neurology. 2011;76(23):2017-2023.
- Rabinstein AA, Mueller-Kronast N. Risk of extubation failure in patients with myasthenic crisis. Neurocrit Care. 2005;3(3):213-215.
- Mehta S. Neuromuscular disease causing acute respiratory failure. Respir Care. 2006;51(9):1016-1021.
- Seneviratne J, Mandrekar J, Wijdicks EF, Rabinstein AA. Predictors of extubation failure in myasthenic crisis. Arch Neurol. 2008;65(7):929-933.
