Myasthenic Crisis

A myasthenic crisis (MC) is an acute exacerbation of MG leading to:

• Weakness of the upper airway muscles resulting in obstruction and/or aspiration

and/or

• Weakness of the respiratory muscles causing reduced tidal volumes

Incidence

About one-fifth of patients with MG experience a crisis, usually within the first two years of diagnosis [7].

Such crises should not be fatal, as long as patients receive timely respiratory support and appropriate immunotherapy to reduce the myasthenic weakness of the upper airway and respiratory muscles [8].

MC is more likely in patients whose history includes previous crisis, oropharyngeal weakness, or thymoma [9].

Learning bite

Approximately 20% of patients will experience a myasthenic crisis, characterised by airway compromise or respiratory failure.

Presentation

In MC, the patient may look anxious with rapid and shallow breathing due to fatigue of respiratory muscles.
Paradoxical breathing and recruitment of accessory muscles with inspiration are important clinical signs.
Dysarthric speech and stridor indicate imminent airway obstruction (bulbar myasthenia) [10].

Cardiac arrhythmias, occur in about a fifth of patients with an MC [11].

Common precipitating factors for MC include:

• Respiratory tract infection
• Sepsis
• Stress, surgery, trauma and pregnancy
• Inadequate treatment
• Drugs
• Initial high dose steroid therapy: some patients (approximately 10%) [11] have a paradoxical worsening of MG if steroids are started at high dose. This is referred to as a ‘steroid dip’ [1,10]
• Idiopathic: approximately 30% [10]

Medications that may trigger MC include:

• Aminoglycoside and quinolone antibiotics
• Antiarrhythmics such as quinidine and procainamide
• Antihypertensives including B-blockers and calcium channel blockers
• Neuromuscular blocking agents
• Neuropsychiatric drugs such as phenytoin and lithium
• Magnesium containing compounds

Bulbar myasthenia

Findings of bulbar myasthenia associated with upper airway compromise include:

• Flaccid dysarthria with hypernasal, staccato or hoarse speech
• Dysphagia
• Tongue weakness
• Vocal cord abductor paralysis which may produce laryngeal obstruction with stridor [8]

Learning bite

MC is a life-threatening complication of MG. Respiratory failure requires early recognition and prompt management.