How is MG diagnosed?
The diagnosis of MG is made from a history of skeletal muscle weakness and easy fatigability that improves with rest. Any muscle group may be involved, but the ocular muscles are the most frequently affected. Bulbar muscle weakness predisposes to impaired breathing, swallowing, and airway control. Periods of exacerbation and remission are common.
Confirmatory tests include the following:
· Edrophonium (Tensilon) test: A small (2–5 mg) dose of edrophonium may elicit an improvement in strength in the myasthenic. Edrophonium is an acetylcholinesterase inhibitor. It increases the availability of acetylcholine to the receptor by inhibiting its degradation. The increased availability of acetylcholine enhances the likelihood of agonist–receptor interaction and therefore endplate depolarization, leading to improved neuromuscular transmission in the myasthenic. Strength will not improve in the normal patient. In fact, lingual fascicula-tions may be noted from an excess of acetylcholine.
· Electromyography: A motor nerve is stimulated three times per second, and the electrical activity of the muscle is recorded. A decrement of response of at least 10% (fade) by the fifth stimulation is usually present in the myasthenic.
· Regional curare test: A forearm tourniquet is applied and inflated. Curare 0.2–0.5 mg is then injected intravenously into the isolated forearm (distal to the tourniquet), and electromyography is performed on that limb. The myas-thenic will show marked decrement of response to repeated stimuli, whereas no change should occur in the normal individual. This test is employed when the edro-phonium test and simple electromyography are equivocal.
· Acetylcholine receptor antibodies: These antibodies are detectable in 85–90% of myasthenics and are diagnostic for MG. The titer of antibodies does not correlate with the severity of the disease.
· Computed tomography (CT) scan or magnetic resonance imaging (MRI): These radiologic tests may be used to confirm the presence of an abnormal thymus gland.
Once diagnosed, MG is clinically classified according to the Osserman Classification System:
· Osserman I: Confined to the extraocular muscles and presents with diplopia.
· Osserman II: Generalized weakness is present.
· Osserman III: Bulbar muscles and severe skeletal weakness are present along with respiratory compromise.
· Osserman IV: Late severe myasthenia, with severe disease developing 2 years after initial diagnosis. The response to therapy in stage IV is poor.