Neurocardiogenic syncope is the most common cause of acute
loss of consciousness in adults. The present study attempted to identify neuroendocrine and hemodynamic changes before
syncope that could therefore play a pathophysiologic role. Twenty-five patients referred for chronic
orthostatic intolerance had plasma
catecholamines measured serially; 21 patients during tilt-table testing (evoking
syncope in 13) and 4 others with spontaneous
syncope while supine. Forearm blood flow was measured by impedance plethysmography. All 12 patients with blood sampled before tilt-induced
syncope had progressive, marked increases in plasma
epinephrine levels (mean 11 times baseline, p <0.0001) before
syncope. Simultaneously obtained
norepinephrine levels increased to a much smaller extent than did
epinephrine levels ("sympathoadrenal imbalance"). In the same patients, forearm vascular resistance decreased by 21% before
syncope. Proportionate changes in forearm vascular resistance before
syncope correlated negatively with those in the
epinephrine:
norepinephrine ratio (r = -0.75, p = 0.005). Patients without
syncope had forearm vasoconstriction and no sympathoadrenal imbalance during tilt. Patients with
syncope while supine also had sympathoadrenal imbalance before
loss of consciousness. Sympathoadrenal imbalance precedes tilt-evoked and spontaneous
neurocardiogenic syncope and correlates with concurrent skeletal muscle vasodilation. Sympathoadrenal imbalance may contribute to hemodynamic derangements precipitating
neurocardiogenic syncope.