To understand autonomic dysfunction and, perhaps more importantly, how to manage it, one must understand a few keys parts of the autonomic nervous system. This system of nerves is really two main subsystems of nerves; the parasympathetic nervous system and the sympathetic nervous system (see chart).
When doctors speak of dysautonomia, they are usually referring to a dysfunction in the parasympathetic nervous system. (Dysfunctions can also occur in the sympathetic nervous system as well and this causes a whole other collection of signs and symptoms. Raynaud’s Syndrome is an example of this).
At least for purposes of understanding dysautonomia, the most important nerve in the parasympathetic nervous system is the vagus nerve.
The vagus nerve is a nerve leading from the brain to the heart and most blood vessels and contributes in an important way to helping determine the heart rate and blood vessel diameter. That is why many times these symptoms are referred to as vasovagal symptoms (such as vasovagal syncope); “vaso” referring to the vascular system and “vagal” referring to the vagus nerve.
The importance of the vagus nerve really comes into play when a person changes positions such as getting up from lying to sitting or sitting to standing. When a person stands up, gravity pulls the body’s blood downward. The healthy response to this is for rapid blood vessel contraction and an increase in heart rate to force blood back upward. The combination of heart rate and diameter of the veins and arteries determines the blood pressure.
When the rate is too low and/or the blood vessels are too dilated for the situation, blood pressure drops and blood pools in the lower parts of the body.
For those with dysautonomia, the vagus nerve does not act quickly or sufficiently enough and blood pools in the lower half of the body and blood pressure drops in the remainder of the body.
In reality, those with significant dysautonomia have a very complex array of changes in the body that go beyond a simple dysfunction of the vagus nerve. One dysfunction that is particularly important is the body’s response to low fluid volume. In those with severe dysautonomia, the kidneys and adrenals do not increase their secretion of renin and aldosterone.
This leads to a failure to sense and respond to low plasma volume. This is important in management.
Diagram of blood volume distribution in a healthy person and a person with dysautonomia while standing. In a health person, contraction of blood vessels reduces the size of the vascular ’tank’. Coupled with an increase in heart rate, these actions maintain sufficient blood pressure and forces blood to the upper part of the body. Dysautonomia prevents these actions and blood pools to the lower half of the body.