Myasthenia Gravis
Acquired autoimmune neuromuscular disorder in which serum IgG antibodies to acetylcholine receptors in the postsynaptic membrane of the neuromuscular junction lead to receptor loss, and skeletal muscle fatigue and weakness. Characterized by progressive muscular weakness on exertion, followed by recovery of strength after a period of rest.
Etiology and pathogenesis
- Antibodies to the ACH receptors
- Existence of pathological changes in the thymus in 70-80% of the patients – hyperplasia, neoplasm (thymoma). The autoimmune process ocurres in the thymus.
- Decrease in number of nicotinic Ach receptors in the postsynaptic membrane at neuromuscular junctions of skeletal muscle. The degree of reduction of Ach receptors generally correlates with the severity of MG
Classification
- Ocular myasthenia
- Mild generalized MG
- Severe generalized MG
- Crisis
Special forms
- Neonatal myasthenia; a transient illness, 1/8 babies of MG mothers
- Juvenile myasthenia; in patients younger than 18 years of age, similar to MG in young adults
Clinical features
- Insiduous onset, but it can be acute (febrile, allergic conditions, surgeries, emotional and physical stress, administration of some drugs, etc.)
- Exacerbations may also occur in pregnancy and before menses. Symptoms may be worsened by quinine, quinidine, procainamid, aminoglucosid antibiotics, lithium, phenitoin, which should be therefore avoided in such patients
- Follows slowly progressive course. There is a predilection for the external ocular muscles and other cranial muscles, including musticatory, facial, pharyngeal and laryngeal muscles.
- The clinical hallmarks are muscle weakness and fatigability
- The weakness tends to increase with repeated activity and improves with rest
- Weakness usually occurs in a characteristic distribution
Oculomotor disorders diplopoa and ptosis
- The eyelid and extraocular muscles are the first muscles to be involved in about 65% of cases and are affected at some stage of the disorder in > 90% of patients
- Symptoms consist of unilateral or bilateral ptosis and diplopia, which are typically asymmetric and toward the end of the day. The degree of ptosis is variable and may present on one side or the other at different times. Puppilary responses are not affected. Ocular MG – 10-15% of patients (weakness remains confined to the eyelid and extraocular muscles).
Facial and bulbar muscle weakness
- Leads to loss of facial expression ( a characteristic “flattened” or “snarling” smile), inability to whistle, and difficulty in speaking (“nasal” speech), chewing and swallowing; the face has typical myopathic appearance – facies myopathica