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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?

Syncope is the abrupt, transient complete loss of consciousness and postural tone. Syncope may be a warning sign that the patient is at risk for sudden cardiac death; therefore, a careful evaluation is critical to identify and treat patients with potentially life- threatening etiologies of syncope. The differential diagnosis is easily remembered by considering the brain's requirements to maintain consciousness. Derangement of any of these requirements may result in syncope. Consciousness requires the following:

  1. Organized cortical electrical activity

  2. Glucose

  3. Oxygen

  4. A functional delivery system to deliver oxygen and glucose.

This in turn requires open vascular conduits and an adequate BP.

By far, most causes of syncope result from hypotension. Therefore, it is useful to look at the determinants of BP.

BP = cardiac output (CO) × total peripheral resistance (TPR)

CO = stroke volume (SV) × heart rate (HR)

Simple substitution: BP = SV × HR × TPR

SV = end-diastolic volume (EDV) – end-systolic volume (ESV)

Simple substitution: BP = (EDV – ESV) × HR × TPR

In summary, the differential diagnosis of syncope can be remembered by considering the requirements for consciousness (ie, BP [determined by EDV, ESV, HR, TPR], organized cortical electrical activity, glucose, oxygen, and open vascular conduits).

Differential Diagnosis of Syncope

  1. BP = (EDV − ESV) × HR × TPR

    1. Inadequate EDV (poor filling)

      1. Dehydration

      1. Hemorrhage

      1. Pulmonary embolism (PE)

      1. Cardiac tamponade

    1. Elevated ESV (inadequate emptying)

      1. Aortic stenosis (AS)

      1. Hypertrophic cardiomyopathy

    1. Heart rate disorders

      1. Tachycardias

        1. Ventricular tachycardia (VT)

        1. Supraventricular tachycardia associated with accessory pathway (Wolff-Parkinson-White [WPW] syndrome)

      1. Bradycardias

        1. Neurally mediated syncope (cardio-inhibitory type)

          1. Neurocardiogenic syncope

          1. Situational syncope

          1. Carotid sinus syndrome

        1. Sinus node disorders

          1. Sinus bradycardia (< 35 beats per minute)

          1. Sinus pauses (> 3 seconds or > 2 seconds with symptoms)

        1. Atrioventricular (AV) block (second- or third-degree)

    1. Decreased TPR (vasodilatation)

      1. Neurocardiogenic syncope (vasodepressor type)

      1. Drugs (α-blockers, vasodilators, nitrates, tricyclic antidepressants, and phenothiazines)

      1. Hypersensitive carotid (vasodepressor type)

      1. Sepsis (usually causes protracted hypotension rather than syncope)

      1. Addison disease (usually causes protracted hypotension rather than syncope)

  2. Disorganized electrical activity: Generalized seizures

  3. Hypoglycemia

    1. Iatrogenic (eg, insulin and sulfonylureas)

    1. Insulinomas (exceedingly rare)

  4. Hypoxemia (usually results in impaired consciousness or coma rather than syncope)

  5. Obstructed vascular conduits

    1. Vertebrobasilar insufficiency

    1. Subclavian steal

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Mr. M reports that he was in his usual state of health and felt perfectly well prior to entering the anatomy dissection room. Upon viewing the cadaver, he felt queasy and warm. He became diaphoretic and collapsed ...

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