B: Neurally mediated syncope
Diagnose neurally mediated syncope.
The most likely diagnosis is neurally mediated syncope. Neurally mediated syncope (also known as neurocardiogenic or reflex syncope) is the most common form of syncope and is seen primarily in younger adults. The underlying syncopal mechanism, termed the neurocardiogenic or vasodepressor reflex, is a response of vasodilation, bradycardia, and systemic hypotension, which leads to transient hypoperfusion of the brain. Neurally mediated syncope includes vasovagal syncope, which may be provoked by noxious stimuli, fear, stress, or heat overexposure; situational syncope, which is triggered by cough, micturition, defecation, or deglutition; and carotid sinus hypersensitivity, which is sometimes experienced during head rotation, shaving, or use of a tight-fitting neck collar. Prodromal symptoms, including nausea and diaphoresis, are classically present before the syncopal event, and fatigue and generalized weakness are typically present afterward.
Hypoglycemia in patients without diabetes mellitus is rare; therefore, evaluation for pathologic hypoglycemia should occur only in the presence of the Whipple triad: symptomatic hypoglycemia, documented plasma glucose level of 55 mg/dL (3.1 mmol/L) or lower, and prompt symptomatic relief with correction of hypoglycemia. This patient's quick recovery from the syncopal episode without any intervention is not compatible with hypoglycemia-induced syncope.
Orthostatic syncope is classically associated with rapid onset of syncope after positional changes. Prodromal symptoms (such as lightheadedness) are often present. Orthostatic syncope is most commonly caused by hypovolemia, medications, and alcohol intoxication. Less commonly, primary autonomic failure (Parkinson disease, multiple system atrophy, multiple sclerosis) or secondary autonomic failure (diabetes, amyloidosis, connective tissue disease, spinal cord injury) can lead to orthostatic syncope. The diagnosis is confirmed by a sustained reduction of 20 mm Hg or more in systolic blood pressure (or ≥10–mm Hg drop in diastolic blood pressure) within 3 minutes of assuming upright posture, which is not present in this patient.
Postural orthostatic tachycardia syndrome is characterized by (1) frequent symptoms that occur with standing (such as lightheadedness, palpitations, generalized weakness, blurred vision, and fatigue), (2) an increase in heart rate of more than 30/min during a positional change from supine to standing, and (3) the absence of orthostatic hypotension. The standing heart rate is often higher than 120/min. This patient's findings are not compatible with postural orthostatic tachycardia syndrome.
Neurally mediated syncope is the most common form of syncope and is seen primarily in younger adults; prodromal symptoms (nausea, diaphoresis) are classically present before the syncopal event, and fatigue and generalized weakness are typically present afterward.
Runser LA, Gauer RL, Houser A. Syncope: evaluation and differential diagnosis. Am Fam Physician. 2017;95:303-312. [PMID: 28290647]
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