The most appropriate diagnostic study is tilt-table testing. Tilt-table testing may be helpful in patients with reflex syncope triggered by standing, patients in high-risk settings (for example, construction workers, surgeons) with unexplained episodes of syncope, patients with recurrent syncopal episodes in the absence of organic heart disease, or patients with recurrent episodes in the presence of heart disease when cardiac causes of syncope are excluded. Given that this patient's syncope is triggered by prolonged standing in the operating room, occurs in a high-risk setting (posing a significant occupational hazard), and occurs in the absence of organic heart disease, she should undergo tilt-table testing. Although the sensitivity, specificity, and reproducibility of the tilt-table test are low, in this setting it may be helpful in elucidating the diagnosis and in differentiating orthostatic hypotension from neurocardiogenic syncope.

The American College of Physicians and other groups do not recommend brain imaging, either with CT or MRI, in the setting of witnessed syncope without seizure activity or other neurologic symptoms because the likelihood of a central nervous system cause of the syncope is extremely low and patient outcomes are not improved with further neurologic testing. This patient did not exhibit seizure activity, and her neurologic examination is normal; therefore, brain MRI is not indicated.

Echocardiography is recommended in the evaluation of syncope if structural heart disease is suspected. However, a cardiac cause of syncope is extremely low in this patient who has no history of structural or ischemic heart disease, no symptoms suggesting heart disease, no murmur on examination, and a normal 12-lead electrocardiogram.

The diagnostic yield of 24- to 48-hour electrocardiographic monitoring is very low (1%-2%) unless there are frequent syncopal episodes over a short period of time. Because this patient has already undergone 24-hour ambulatory electrocardiographic monitoring with normal results and her episodes occur every few months, it is unlikely that 48-hour ambulatory electrocardiographic monitoring will be diagnostically helpful. If arrhythmias were strongly suspected, an implantable loop recorder would be a more appropriate diagnostic option for this patient.

Since this patient has had three separate episodes of syncope that occur in high-risk situations, further diagnostic evaluation to determine the cause of the syncope and guide therapy should be pursued.