In this video Dr. Goldstein introduces adult-onset dysautonomias including a discussion about sympathoadrenal imbalance and syncope.
Adults, I think in terms of autonomic system malfunction and a great example of this is fainting reaction. And as I have mentioned before, I discovered this phenomenon of sympathoadrenal imbalance, it’s illustrated here in contrast with Cannon’s idea of the emergency sympathoadrenal system. Here you can see in this patient that at first norepinephrine and epinephrine go up together, this is on a fraction of baseline and is on a log scale. So, they go up together, but then right about back here norepinephrine sort of goes up a little bit, but adrenaline is going up a lot and this happens long before there’s actually a fall in blood pressure and the patient faints. Because of stimulation of beta-2 receptors and skeletal muscle by adrenaline, the forearm vascular resistance falls in response to adrenaline. So, if you look at the forearm vascular resistance, at first it goes up, which it’s supposed to do, but then I think as the adrenaline is building up, the forearm vascular resistance is going down, and right here the forearm vascular resistance goes below baseline. When that happens, at least to my experience, that means that there’s going to be a fainting reaction and you can see the blood pressure plummets several minutes after that. So that’s sympathoadrenal imbalance. It’s a situation where the sympathetic noradrenergic system and sympathetic adrenergic system get kind of discombobulated. And I think it’s an evoked central pattern where if you can’t fight, you’re in a distressing situation, you can’t fight, and you can’t flee, and another possibility is faint. Ge Lamont is the process of writing a paper exactly on that topic. Here’s the concept, and it’s really quite simple, when you’re tilted up normally there are things that happen. The heart gets smaller, the stroke line is smaller because you’ve shifted your blood volume distribution but at the same time because of sympathetic noradrenergic system activation the blood vessels to skeletal muscle tighten, because of that you are able to maintain blood flow to the brain even though the heart is pumping out less blood. When somebody is going to faint, then I think because of adrenaline and sympathoadrenal imbalance, now there’s a shunting of blood away from the brain and towards skeletal muscle maybe teleologically getting ready for fight or flight response and that comes at the expense of blood flow to the brain. That’s my sort of simple concept for tilt-induced syncope. Two things are happening, the heart is smaller, cardiac output is smaller, and there’s an absence of the usual vasoconstriction and skeletal muscle instead, there is skeletal muscle vasodilation because of adrenaline.
David S. Goldstein, M.D., Ph.D
Chief, Autonomic Medicine Section
NINDS, National Institutes of Health