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PATIENT 
Mr. M is a 23-year-old medical student who had an episode of syncope this morning after entering his anatomy lab for the first time. He is quite alarmed (and embarrassed).
What is the differential diagnosis of syncope? How would you frame the differential?
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CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
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Transient loss of consciousness may be caused by trauma, intoxication, hypoglycemia, or true syncope. Syncope refers to the abrupt, transient complete loss of consciousness and postural tone due to transient global cerebral hypoperfusion, usually due to transient profound hypotension. Since such profound hypotension can be catastrophic or fatal if prolonged, syncope may portend sudden cardiac death; therefore, a careful evaluation is critical to identify and treat patients with potentially life- threatening etiologies of syncope.
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Patients with syncope should be carefully evaluated to determine if they are at risk for sudden cardiac death.
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The differential diagnosis for syncope is best remembered by considering the 3 most common causes of syncope: reflex mediated syncope, cardiac syncope, and orthostatic hypotension (Figure 31-1).
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Each of these categories of syncope is associated with specific diseases/entities that cause syncope. Cardiac syncope is usually due to bradyarrhythmias or tachyarrhythmias but occasionally due to outflow obstruction (eg, aortic stenosis, hypertrophic cardiomyopathy [HCM]) or inadequate filling (pulmonary embolism [PE], tamponade). Orthostatic hypotension is usually due to inadequate preload (from dehydration or hemorrhage), but may also be secondary to autonomic dysfunction or drugs. Reflex syncope refers to a group of disorders that cause syncope due to increased vagal tone causing bradycardia and vasodilation and includes vasovagal syncope, carotid sinus syncope, and situational syncope. The full differential is listed below.
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Finally, cerebrovascular disease involving the posterior circulation is a rare cause of transient loss of consciousness but is almost invariably associated with other neurologic symptoms and is discussed briefly at the end of the chapter. On the other hand, seizures are easily confused with syncope and remain in the differential diagnosis of the patient who appears to have had syncope (see Figure 31-1).
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The evaluation of all patients with syncope must include a thorough history, physical exam, ECG, and blood glucose. A detailed history of the event is critical including the setting (warm, standing, sitting, during exertion, pain, anxiety, etc.), associated symptoms (nausea, chest pain, palpitations), and any signs observed by bystanders. Any significant persistence of confusion beyond a minute or two is critical, as this would suggest a postictal period and seizure. The past medical history and the patient’s medications should be carefully reviewed. The purpose of this evaluation is to uncover any findings that suggest cardiac disease, since patients with syncope and heart disease are at a markedly increased ...