A 4-year-old child is observed to hold his eyelids open with his fingers and to close one eye periodically, especially in the evening. He has some trouble swallowing his food. He usually appears sad, although he laughs often enough. He can throw a ball, and he runs well. Which of the following is likely to result in the diagnosis?

Myasthenia gravis is an autoimmune disorder in which circulating acetylcholine receptor- binding antibodies are found in approximately 90% of cases, less commonly, when the patient demonstrates isolated eye disease. Other nonspecific evidence of autoimmune disease includes elevated ANAs, presence of abnormal immune complexes, and an association with the hypothyroidism related to lymphocytic thyroiditis, also an autoimmune disorder. The earliest signs of myasthenia gravis are ptosis and weakness of the extraocular muscles, followed by dysphagia and facial muscle weakness. The distinguishing hallmark of this disease is rapid fatiguing of the involved muscles. The conduction velocity of the motor nerve is normal in this condition. When the involved muscle is repetitively stimulated for diagnostic purposes, the EMG shows a decremental response, which can be reversed by the administration of cholinesterase inhibitors. In older children this test is accomplished with edrophonium chloride, but it should be avoided in young infants, as cardiac arrhythmias may result (neostigmine would be used instead). Cholinesterase inhibitors are the primary therapeutic agents, and intramuscular neostigmine methylsulfate, oral neostigmine bromide, and oral pyridostigmine are available. Other therapeutic modalities for myasthenia gravis include immunosuppression, plasmapheresis, thymectomy (an enlarged thymus is frequently seen on chest x-ray), and treatment of hypothyroidism. CPK should be normal.