Neuromuscular transmission disorder characterized by fluctuating weakness and fatigability of bulbar and other voluntary muscles without loss of reflexes or impairment of sensation or other neurologic function.
Diagnostic criteria:
A. Characteristic signs and symptoms
One or more of the following:
- Diplopia, ptosis, dysarthria, weakness in chewing, difficulty in swallowing, muscle weakness with preserved deep tendon reflexes, and, less commonly, weakness of neck extension and flexion, and weakness of trunk muscles
- Increased weakness during exercise and repetitive use with at least partially restored strength after periods of rest
- Dramatic improvement in strength following administration of anticholinesterase drug (edrophonium (Tensilón®) and neostigmine);
and one or more of the following:
B. EMG and repetitive stimulation of a peripheral nerve: In myasthenia gravis repetitive stimulation at a rate of 2 per second shows characteristic decremental response which is reversed by edrophonium or neostigmine. Single fiber studies show increased jitter.
C. Antibodies to Acetylcholine Receptors
Exclusions:
- Congenital myasthenic syndrome, progressive restricted myopathies, steroid and inflammatory myopathies, motor neuron disease
- Multiple sclerosis, variants of Guillain-Barré syndrome (e.g., Miller-Fisher syndrome)
- Organophosphate toxicity, botulism, black widow spider venom
- Eaton-Lambert syndrome
- Stroke
- Medications: neuromuscular blocking agents, aminoglycosides, penicillamine, antimalarial drugs, colistin, streptomycin, polymyxin B, tetracycline
- Hypokalemia; hypophosphatemia
Severity (Osserman classification):
- I: Ocular myasthenia
- IIA: Mild generalized myasthenia with slow progression: no crises, responsive to drugs
- IIB. : Moderately severe generalized myasthenia : severe skeletal and bulbar involvement but no crises; drug response less than satisfactory
- III: Acute fulminating myasthenia, rapid progression of severe symptoms, with respiratory crises and poor drug response
- IV: Late severe myasthenia, same as III but progression over 2 years from class I to II
References:
- Ossermann KE. Myasthenia gravis. New York: Grune and Stratton; 1958.
- Engel AG. Myasthenic syndromes. In: Engel AG, Franzini Armstrong C, editors. Myology. 2nd ed. New York: McGraw Hill; 1994. p. 1798-835.
Created: Oct 11, 2006