Remember that Patients with Myasthenia Gravis Exacerbation Usually Look Well Until Just Before they Require Intubation
Eliahu S. Feen MD
Jose I. Suarez MD
Myasthenia gravis is an autoimmune disease in which a patient primarily forms autoantibodies to the acetylcholine receptors on the postsynaptic membrane in the neuromuscular junction. Other kinds of autoantibodies also may play a role. In general for myasthenia gravis, the postsynaptic muscle membrane is distorted and the concentration of acetylcholine receptor proteins is reduced. As a result, the neurotransmitter acetylcholine released from the presynaptic nerve terminal—although it is released in normal concentration—has a muted effect. Themuscle weakness that results can take several forms. In ocularmyasthenia gravis, the weakness affects the extraocular muscles, with findings of diplopia and ptosis. In both bulbar myasthenia gravis (affecting primarily the bulbar muscles) and in generalized myasthenia gravis (affecting primarily the limb muscles), respiratory muscle involvement is common. Myasthenic crisis represents the exacerbation of myasthenia gravis to the point of requiring mechanical ventilation.
Factors Contributing to Mechanical Ventilation
Many myasthenia gravis patients will complain of dyspnea frequently, especially with exertion. Exacerbations of myasthenia gravis present with two important risk factors for respiratory compromise. The first is that the muscle weakness impairs diaphragmatic accessory respiratory muscle function. As a result breaths are shallow. Consequently, to maintain adequate oxygenation, the respiratory rate climbs, which further exacerbates muscle fatigue. The shallow breaths also limit tidal volume, which impairs gas exchange. The second mechanism is that with pharyngeal dysfunction, the risk of aspiration rises. The development of aspiration pneumonia will further compromise both oxygenation and gas exchange because of shunting. The ominous danger of this situation is emergent respiratory failure. Once a patient develops extreme muscle fatigue, respirations will very rapidly become insufficient to maintain oxygenation. The accumulation of carbon dioxide
(CO2) due to progressively inadequate gas exchange will cause hypercarbic respiratory failure. Careful monitoring of myasthenia gravis patients is necessary in order to avoid an emergent intubation as well as to avoid the complications of emergent respiratory failure and hypercarbia (such as hypoxia, hypotension, acidosis, and arrhythmias). As amyasthenia gravis patient shows signs of respiratory compromise, closer monitoring becomes essential. If such patients are not yet in an intensive care setting, it is necessary to place them in one.
(CO2) due to progressively inadequate gas exchange will cause hypercarbic respiratory failure. Careful monitoring of myasthenia gravis patients is necessary in order to avoid an emergent intubation as well as to avoid the complications of emergent respiratory failure and hypercarbia (such as hypoxia, hypotension, acidosis, and arrhythmias). As amyasthenia gravis patient shows signs of respiratory compromise, closer monitoring becomes essential. If such patients are not yet in an intensive care setting, it is necessary to place them in one.