neurologic disorders
Myasthenia Gravis
Diagnosis of Myasthenia Gravis
Bedside tests (ice pack test, rest test) AChR antibody levels, electromyography, or both The diagnosis of myasthenia gravis is suggested by symptoms and signs and confirmed by tests. The traditional anticholinesterase test, done at bedside and using the short-acting (< 5 minutes) drug edrophonium, is not used in the US and in many other countries, and edrophonium is no longer available in the US. Because weakness due to myasthenia lessens in cooler temperature, patients with ptosis can be tested using the ice pack test. For this test, an icepack is applied to a patient's closed eyes for 2 minutes, then removed. A positive result is full or partial resolution of ptosis. The ice pack test usually does not work if patients have ophthalmoparesis. Patients with opthalmoparesis can be tested using the rest test. For this test, patients are asked to lie quietly in a dark room for 5 minutes with their eyes closed. If ophthalmoparesis resolves after this rest, the result is positive. Even if a bedside test is unequivocally positive, one or both of the following are required to confirm the diagnosis: Serum AChR antibody levels Electromyography (EMG) AChR antibodies are present in 80 to 90% of patients with generalized myasthenia but in only 50% with the ocular form. Antibody levels do not correlate with disease severity. Up to 50% of patients without AChR antibodies test positive for anti-MuSK antibodies. EMG using repetitive stimuli (2 to 3/seconds) shows a > 10% decrease in amplitude of the compound muscle action potential response in 60% of patients. Single-fiber EMG can detect abnormal neuromuscular transmission in > 95%. Once myasthenia is diagnosed, CT or MRI of the thorax should be done to check for thymic hyperplasia and a thymoma. Other tests should be done to screen for autoimmune disorders frequently associated with myasthenia gravis (eg, Vitamin B12 Deficiency , autoimmune hyperthyroidism, Patients in myasthenic crisis should be evaluated for an infectious trigger. Bedside pulmonary function tests (eg, forced vital capacity) help detect impending respiratory failure. 