1. Myasthenia gravis is an autoimmune disease with antibodies directed against the nicotinic acetylcholine receptor or other muscle membrane proteins (Table 23-3: Summary of the Different Presentations of Myasthenia Gravis). The majority of patients have abnormalities of the thymus (thymoma, thymic hyperplasia, thymic atrophy).
   1. The clinical hallmark of myasthenia gravis is skeletal muscle weakness (increased by repetitive muscle use) with periods of exacerbation and remission (Table 23-4: Osserman Staging System for Myasthenia Gravis).
      1. Neonatal myasthenia begins 12 to 48 hours after birth and reflects transplacental passage of antiacetylcholine antibodies.
      2. Focal myocarditis and atrioventricular heart block may be present.
   2. Treatment includes administration of anticholinesterase drugs, thymectomy, corticosteroids, and immunosuppressants. Whereas underdosage with anticholinesterase drugs results in skeletal muscle weakness, overdosage leads to a “cholinergic crisis.” The role of thymectomy for the treatment of myasthenia is not clearly established.
   3. Management of Anesthesia
      1. The primary concern is the potential interaction among the disease, treatment of the disease, and neuromuscular blocking drugs. Patients with uncontrolled or poorly controlled myasthenia are exquisitely sensitive to even small (defasciculating) doses of nondepolarizing muscle relaxants.
      2. The variability in response to different muscle relaxants warrants careful monitoring with a peripheral nerve stimulator and its correlation with clinical signs of recovery from neuromuscular blockade. Short- or intermediate-acting nondepolarizing muscle relaxants are usually recommended.
2. Myasthenic Syndrome (Lambert-Eaton Syndrome)
   1. The myasthenic syndrome is a disorder of neuromuscular transmission associated with carcinomas, particularly small cell carcinoma of the lung. (This syndrome should be suspected in patients undergoing diagnostic procedures, such as diagnostic bronchoscopy, mediastinoscopy, or exploratory thoracotomy for possible cancer) (Table 23-5: Comparison of Myasthenia Gravis and Myasthenic Syndrome).
   2. Management of Anesthesia
      1. Patients with myasthenic syndrome are sensitive to the effects of both depolarizing and nondepolarizing muscle relaxants.
      2. Administration of 3,4-diaminopyridine should be continued until the time of surgery.

Rare Coexisting Diseases

1. Musculoskeletal Diseases
2. The Myotonias
3. Familial Periodic Paralysis
4. Myasthenia Gravis
5. Guillain-Barré Syndrome (Polyaradiculoneuritis)
6. Central Nervous System Diseases
7. Inherited Disorders
8. Anemias
9. Collagen Vascular Diseases
10. Skin Disorders