Crisis Miastenica

CME Topic

Myasthenia Gravis Crisis
Eric M. Bershad,
MD,

Eliahu S. Feen,

MD,

and Jose I. Suarez,
Risk Factors

MD

Abstract: Myasthenia gravis (MG) is an autoimmune disorder resulting from the production of antibodies against acetylcholine receptors leading to the destruction of the postsynaptic membrane at the neuromuscular junction. In the US there are about 18,000 people withMG. Myasthenia gravis crisis (MGC) is defined as any MG exacerbation necessitating mechanical ventilation. Most patients presenting with MGC have an identifiable risk factor. The diagnosis of MGC should be suspected in all patients with respiratory failure, particularly those with unclear etiology. Acute management of MGC requires supportive general and ventilatory therapy and institution ofmeasures to improve the neuromuscular blockade. The latter includes plasma exchange or IV immunoglobulin, and removal of the offending trigger. The outcome of patients with MGC has improved significantly and the current mortality rate is about 4 to 8%. Key Words: autoimmune disorders, mechanical ventilation, myasthenia gravis, myasthenia gravis crisis, respiratory failure

Epidemiology
he incidenceof MG is 3 to 4 per million per year, and the prevalence is about 60 per million.1 Thus, in the United States there are about 18,000 people with MG. The incidence of MG is twice as high in women compared with men. The peak age of onset in women is in the childbearing years. In men there is no clear peak age of onset.1 Overall, 15 to 20% of patients with MG will experience a myasthenic crisis, andit usually occurs within the first 2 years after diagnosis of MG in most patients (74%).2

T

Most patients that develop MGC have an identifiable precipitating event; however, in 30 to 40% of patients, no triggering factor can be found (Table 1).2,3 Furthermore, in some patients, MGC may be the initial manifestation of MG.2 The most common identifiable precipitant of MGC is an infection(40%), usually in the upper airway (such as pneumonia). Another 10% of patients have aspiration pneumonitis as the triggering event. Other important predisposing factors include the initiation of new medications or a change in medications, recent surgery, trauma, botulinum injections, and thymoma.2–5 The presence of thymoma is higher among patients with MG who have a MGC than patients with MG but nohistory of MGC (30% versus 15%).6 The most common medications that may trigger MGC include the aminoglycoside and quinolone antibiotics, antiarrhythmics such as quinidine and procainamide, antihypertensives including -blockers and calcium channel blockers, magnesium sulfate, and neuromuscular blocking agents such as succinylcholine and curare derivatives.7 The initiation of high-dose steroids inpatients with MG leads to a paradoxical worsening of muscle weakness in almost 50% of patients (Table 2).8

Pathophysiology
MG is an autoimmune disease of the nicotinic acetylcholine receptor of skeletal muscle at the neuromuscular junc-

Key Points
• Myasthenia gravis crisis (MGC) is defined as any myasthenia gravis (MG) exacerbation necessitating mechanical ventilation. • About 15–20% ofpatients with myasthenia gravis will experience MGC, typically within the first 2 years of MG diagnosis. • Most patients with myasthenia gravis crisis have an identifiable precipitating event; however in 30 – 40% of patients, no triggering factor is found. • Plasma exchange or intravenous immunoglobulins are indicated to help facilitate recovery of neuromuscular blockade. • The overall outcome forpatients with myasthenia gravis crisis is excellent if therapeutic and supportive measures are instituted promptly.

From the Division of Neurocritical Care, The Neurological Institute, University Hospitals Case Medical Center, Departments of Neurology and Neurological Surgery, Case Western Reserve University, Cleveland, Ohio; and the Department of Neurology, Baylor College of Medicine, Houston,...