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What is myasthenia gravis crisis (MC)
Myasthenia gravis crisis is defined as myasthenia gravis (MG) exacerbation leading to respiratory failure requiring assisted ventilation.
What is the incidence of Myasthenia gravis crisis?
Overall, 15% to 20% of patients with MG will experience an MC crisis, occurring within the first 2 years after diagnosis of MG in most patients (74%).
What are the predisposing factors for Myasthenia gravis crisis?
The most common identifiable precipitant of MC is an infection (40%), usually respiratory, such as pneumonia
Triggers of Myasthenia Gravis Crisis
| Infections (40%)—pneumonia, upper airway infection |
| Aspiration (10%) |
| Medications—botulinum toxins, prednisone |
| Stress—surgery, trauma |
| Idiopathic (30%) |
What are the arterial blood gas findings in Myasthenia gravis crisis?
Arterial blood gas results can be misleading because patients with MC usually become hypoxemic only at a late stage.
Earlier findings may include initial respiratory alkalosis with mild tachypnea, progressing later to respiratory acidosis as the diaphragm weakens further.
Normal PaCO 2 in a tachypneic patient means impending respiratory failure.
When should an Myasthenia gravis crisis patient be intubated?
The most common reasons for intubation include the following:
• A wet, gurgling voice, dysarthric/staccato speech, and stridor indicating poor airway protection;
• Abdominal paradoxical breathing indicating diaphragmatic weakness;
• Failure to manage secretions;
• Arterial blood gases showing a PaO 2 <60 mm Hg or PaCO 2 >60 mm Hg; and
• Increased work of breathing.The 20/30/40 rule (forced vital capacity <20 mL/kg; negative inspiratory pressure <30 cm H 2 O; and positive expiratory pressure <40 cm H 2 O) is probably the most helpful guide to decide when to intubate.
When should a Myasthenia gravis crisis patient be weaned off the ventilator?
General criteria have already been mentioned in the general management section of this chapter. For patients with MC the following criteria should additionally be considered:
• Improving strength (caution: degree of limb weakness does not correlate with diaphragm weakness);
• Improving bedside pulmonary function test (vital capacity >15 mL/kg, negative inspiratory pressure >30, and positive expiratory pressure >40);
• Absence of either severe atelectasis or pleural effusion; and
• Ability to manage secretions.
What is the ventilatory test in MC?
Ventilatory Test in Myasthenia Gravis Crisis (MC)
| Parameters | Normal Values | Threshold for Intubation | Initiation of Weaning Off Ventilator | Consideration for Extubation |
|---|---|---|---|---|
| FVC (mL/kg) | >60 | <20 | >15 | >25 |
| NIP (cm H 2 O) | >−70 | <−30 | >20 | >40 |
| PEP (cm H 2 O) | >100 | <40 | >40 | >50 |
FVC , Forced vital capacity; NIP , negative inspiratory pressure; PEP , positive expiratory pressure.
Modified from Godoy DA, Mello LJ, Masotti L, et al.: The myasthenic patient in crisis: an update of management in neurointensive care unit. Arq Neuropsiquiatr 71(9A):627-639, 2013.
What is the role of NIVPPV?
NIVPPV may be useful in either preventing intubation or reintubation except when PaCO 2 ≥45 mm Hg as the failure rate is higher.
What should be done with home anticholinesterase inhibitors when the patient is intubated?
Stopping pyridostigmine upon intubation is recommended as it causes increased secretions.
Pyridostigmine should be started as soon as the patient improves and weaning is initiated.
What is the role of steroids in Myasthenia gravis crisis?
Patients who are taking steroids should not stop them. Initiation of high-dose steroids in patients with MG may lead to a paradoxical worsening of muscle weakness.
What is the first line of treatment for Myasthenia gravis crisis?
Either plasma exchange (PLEX) 1.5 L of plasma (20 to 25 mL/kg) per session for approximately five sessions or until the patient experiences clinical improvement or intravenous immunoglobulin (IVIG) 0.4 g/kg for 5 days are the first-line options for treatment of patients with MC.
They are equally effective in crisis. Although never compared in a controlled study, PLEX seems to work faster after two to three session days compared to 5 days after IVIG initiation and might be preferable for managing MC.
Check IgA level prior to IVIG administration as anaphylaxis may be a serious adverse event in IgA-deficient individuals.
