A 37-year-old woman comes to you for recurrent episodes of double vision and drooping of her eyelids for the last month. Such episodes occur, last for hours, and resolve spontaneously. She also repos occasional hoarseness, which also come and go. Vital signs and physical examination are normal. Most appropriate next step in diagnosis is?

The clinical manifestations are highly characteristic of myasthenia gravis. This disorder has three general features: the fluctuating nature of muscle weakness, predominant involvement of ocular muscles (with diplopia and ptosis), and positive clinical response to administration of cholinergic agents. Crisis of weakness involving respiratory muscles was the most frequent cause of death before the advent of positive pressure respirators. The disease is autoimmune-mediated and results from autoantibodies to the muscular nicotinic receptors. Besides the pharmacologic test, a progressive decrease in the amplitude of muscle potential is the diagnostic feature of myasthenia gravis. Electromyography is therefore very useful in the diagnosis of this condition. Blood, urine, and CSF analysis are entirely within normal limits in myasthenia gravis, although they are indeed frequently performed in the initial screening. MRI of the head and EEG recording would be entirely useless in this setting. Since the disorder is due to impaired cholinergic transmission at the neuromuscular junction, skeletal muscle biopsy is within normal limits at the light microscopic level. Occasionally muscle biopsy is performed to rule out other causes of muscle weakness, such as myopathic processes.