- Weakness associated with myasthenia gravis is due to autoimmune destruction or inactivation of postsynaptic acetylcholine receptors at the neuromuscular junction, leading to reduced numbers of receptors and degradation of their function, and to complement-mediated damage to the postsynaptic membrane.
- Patients who have myasthenia gravis with respiratory muscle or bulbar involvement are at increased risk for pulmonary aspiration.
- Many patients with myasthenia gravis are exquisitely sensitive to nondepolarizing neuromuscular blockers (NMBs).
- Patients who have myasthenia gravis are at risk for postoperative respiratory failure. Disease duration of more than 6 years, concomitant pulmonary disease, a peak inspiratory pressure of less than −25 cm H2O (ie, −20 cm H2O), a vital capacity less than 4 mL/kg, and a pyridostigmine dose greater than 750 mg/d are predictive of the need for postoperative ventilation following thymectomy.
- Patients with Lambert-Eaton myasthenic syndrome and other paraneoplastic neuromuscular syndromes are very sensitive to both depolarizing and nondepolarizing NMBs.
- Respiratory muscle degeneration in patients with muscular dystrophy interferes with an effective cough mechanism and leads to retention of secretions and frequent pulmonary infections.
- Degeneration of cardiac muscle in patients with muscular dystrophy is also common, but results in dilated or hypertrophic cardiomyopathy in only 10% of patients.
- Succinylcholine should be avoided in patients with Duchenne’s or Becker’s muscular dystrophies because of unpredictable response and the risk of inducing severe hyperkalemia or triggering malignant hyperthermia.
- Anesthetic management in patients with periodic paralysis is directed toward preventing attacks. Intraoperative management should include frequent determinations of plasma potassium concentrations and careful electrocardiographic monitoring to detect arrhythmias.
- In patients with periodic paralysis, the response to NMBs is unpredictable, and neuromuscular function should be carefully monitored during their use. Increased sensitivity to nondepolarizing NMBs is particularly apt to be encountered in patients with hypokalemic periodic paralysis.
Although neuromuscular diseases are relatively uncommon, patients with these conditions will present to the operating room and to non-operating room procedure areas for diagnostic studies, treatment of complications, or surgical management of related or unrelated disorders. Overall debility, with diminished respiratory muscle strength and increased sensitivity to neuromuscular blockers (NMBs), predisposes these patients to postoperative ventilatory failure and pulmonary aspiration, and may slow their post-procedure recovery because of difficulty with ambulation and increased risk of falling. A basic understanding of the major disorders and their potential interaction with anesthetic agents is necessary to minimize the risk of perioperative morbidity.
Myasthenia gravis is an autoimmune disorder characterized by weakness and easy fatigability of skeletal muscle. It is classified according to disease distribution and severity (Table 35-1). The prevalence is estimated at 50-200 per million population. The incidence is highest in women during their third decade, and men exhibit two peaks, one in the third decade and another in the sixth decade.
Table 35-1 Myasthenia Gravis Foundation of America Clinical Classification of Myasthenia Gravis.1