Myasthenia gravis

Introduction
Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction, in which antibodies affect neuromuscular transmission, leading to weakness.

Myasthenic crisis
This term refers to patients in respiratory failure due to weakness from myasthenia gravis. Most commonly this term refers to patients who require intubation for this disorder, but terminology is variable, as some include patients with severe weakness.

Epidemiology
Up to 15-20% of patients with MG develop a myasthenic crisis within two years of diagnosis of the disorder. Mortality ranges from 5-12% in most reports, with the largest study showing a mortality rate of 12%. This mortality rate is a significant improvement from prior decades, with old reports suggesting mortality in excess of 40%. It most commonly affects young women and old men, and the proportion of older patients appears to be increasing.

Classification
In 2000, a classification of MG was published by the Myasthenia Gravis Foundation of America (MGFA), as follows:

Complications
Pneumonia is more common in patients requiring mechanical ventilation, although this is confounded by the fact that presence of infection is also associated with a need for mechanical ventilation.

Treatment
It is reasonable to initiate plasmapheresis or immunoadsorption as a first-line treatment in patients with myasthenic crisis, as it is associated with a decreased probability of requiring intubation (although this has not been proven in a prospective trial). In the largest study of patients in crisis (including 250 episodes in 223 patients, at 10 different centers in Germany), treatment with plasmapheresis or immunoadsorption as initial therapy, rather than IVIG, was associated with a higher probability of not requiring intubation (OR 4.49, p=0.003). Treatment with either plasmepheresis or IVIG is a Neurocritical Care Society Clinical Performance Measure.

Noninvasive vs invasive ventilation
Noninvasive ventilation may be is sufficient in up to 38% of cases.

Prognosis
In the largest study of myasthenic crisis (including 250 episodes in 223 patients, at 10 different centers in Germany), mortality was 12%. Median duration of mechanical ventilation was 12 days, with a median 16-day ICU stay and median 26-day hospitalization. 20% of patients required prolonged mechanical ventilation after discharge. Risk factors for prolonged mechanical ventilation (>15 days) include advanced age, medical comorbidities (cardiac disease, diabetes mellitus), late-onset myasthenia, and pneumonia.