Myasthenia gravis and lambert-eaton myasthenic syndrome
Myasthenia gravis
The incidence of myasthenia gravis (MG), an autoimmune disease of the neuromuscular junction, is 1:20,000 in the adult population and higher in patients with other autoimmune diseases, such as thyroiditis and rheumatoid arthritis. Characterized by weakness of voluntary skeletal muscle, MG can be classified by age, etiology (Table 172-1), or the presence or absence of bulbar signs and symptoms because the muscles innervated by the cranial nerves are most frequently involved (Table 172-2).
Table 172-1
Subgroups or Types of Myasthenia Gravis
| Type | Description |
| Nonimmune | |
| Congenital myasthenic syndrome | Defects in proteins at the neuromuscular junction |
| Immune | |
| Neonatal myasthenia gravis | Transplacental passage of AChR antibodies |
| Juvenile myasthenia gravis | Onset before 18 years of age |
| Early-onset myasthenia gravis | Onset at 18 to 50 years of age |
| Late-onset myasthenia gravis | Onset after 50 years of age |
| Seronegative myasthenia gravis | No detectable AChR antibodies |
Table 172-2
Signs and Symptoms Associated with the Classes of Myasthenia Gravis
| Class | Signs and Symptoms |
| I | Any ocular muscle weakness |
| May have weakness of eye closure All other muscle strength is normal |
|
| II | Mild weakness affecting other than ocular muscles May also have ocular muscle weakness of any severity |
| III | Moderate weakness affecting other than ocular muscles May also have ocular muscle weakness of any severity |
| IV | Severe weakness affecting other than ocular muscles May also have ocular muscle weakness of any severity |
| V | Defined by requirement for intubation, with or without mechanical ventilation, except when used during routine postoperative management |
Pathophysiology
Circulating antibodies to acetylcholine (ACh) receptors (AChRs), which are found in 70% to 90% of patients with MG, are thought to reduce the number of functional AChRs via three different mechanisms: competitive blockade of AChRs; complement-mediated lysis of the receptor and muscle end plate, resulting in blocked access to the receptor; and increased destruction and decreased production of AChRs. The net effect of these mechanisms is a motor end plate with decreased surface area and decreased functional AChRs (Figure 172-1).
