Incipient respiratory failure is usually marked by a reduction of vital capacity, often accompanied by restlessness, anxiety, diaphoresis, or tremor. Once the diaphragm fails, movements of the chest wall and abdomen become paradoxical (the abdomen moves inward during inspiration) or there may be shallow excursions of the chest, alternating with paradoxical movements as discussed in
Chap. 25 where the characteristics of neuromuscular respiratory failure are described. In an emergency, after clearing of the airway, such a patient can be supported briefly by a tight-fitting face mask and manual bag (Ambu) breathing. The chest wall will be very compliant as a result of muscular weakness.
Management of the crisis entails timely and careful intubation followed by mechanical ventilation in a critical care unit that is equipped to attend to the medical and neurologic needs of such patients. Respiratory failure in a few patients can be managed by the use of bilevel positive airway pressure (BIPAP) according to Rabinstein and Wijdicks, but in our experience, this has not been consistently effective in avoiding endotracheal intubation. One must cope with both the oropharyngeal weakness and secretions that endanger the airway, and the diaphragmatic weakness. Anticholinesterase drugs, which exaggerate secretions, are best withdrawn at the time of intubation. A useful maneuver is to allow the patient to remain off cholinergic drugs for several days while on a ventilator; there is often a heightened response to the reinstitution of medications after this period. The use of plasma exchange or intravenous gamma globulin as described earlier, is equivalently effective in hastening improvement and weaning from the ventilator. Some of our colleagues have used high-dose corticosteroid infusions in these circumstances but this measure has not been particularly successful in our unit and, in the short run, carries the risk of inducing worsening of the weakness (Panegyres et al).
Patients with myasthenic crisis generally respond to plasma exchange or to immunoglobulin infusions in 1 or 2 days, but more often a week or more is required for recovery after a full course of 4 to 5 exchanges or 3 to 5 g/kg IVIg given in divided daily doses. Whether the previously mentioned studies (e.g., Gajdos et al) comparing the 2 treatments and comparing doses of IVIg in myasthenic exacerbations are pertinent to crisis is unknown but we almost always institute one or the other soon after it as it is apparent that respiratory failure is imminent or worsening.
It is generally best to wait 2 or 3 weeks before committing a patient to tracheostomy. When weaning from the ventilator is anticipated, anticholinesterase agents are reintroduced slowly, and treatment with corticosteroids can be instituted if necessary. Oral doses of 60 mg pyridostigmine or 15 mg neostigmine are roughly equivalent to 0.5 to 1 mg neostigmine intravenously and 1.5 to 2 mg intramuscularly, as noted in Table 46-1. The management of the critically ill patient with myasthenia is reviewed in the monograph by Ropper and colleagues.