Dysautonomia is a condition in which abnormal autonomic function negatively affects a patient's health. Some conditions may be transient and mild, while others can result in progressive neurodegenerative diseases (see accompanying table). The autonomic system is the part of the nervous system mainly responsible for involuntary, unconscious functions within the body's organs, contrasting with voluntary, concise skeletal-muscle activity. The autonomic system has sympathetic and parasympathetic portions that maintain body homeostasis. The main chemical messenger of the sympathetic nervous system is norepinephrine (noradrenaline), while that of the parasympathetic system is acetylcholine. A Clinical Staff Conference held on May 31, 2000, at the National Institutes of Health and moderated by Goldstein reviewed current knowledge about the spectrum of dysautonomias.
Chronic autonomic failure is commonly characterized by orthostatic hypotension (a rapid decrease in blood pressure exceeding 20/10 mm Hg and resulting in an inability to stand for more than one to two minutes) not caused by fluid depletion or prolonged bed rest and is a manifestation of sympathetic neuro-circulatory failure. Diseases, toxic agents, and medications can cause this autonomic failure, while episodes without a cause are called primary chronic autonomic failure. The latter can manifest solely with orthostatic hypotension, but multiple system atrophy includes autonomic failure and progressive central neurodegeneration. Autonomic failure also can occur in association with Parkinson's disease.
Orthostatic intolerance differs from orthostatic hypotension by its potential for delayed onset and also may result from cerebral hypoperfusion secondary to carotid disease. Dysautonomia resulting in orthostatic pooling of blood in splanchnic and dependent areas, poor renin-angiotensin-aldosterone function, acute baroreflex failure, and excessive extravasation may cause orthostatic intolerance.
Three more common conditions associated with altered autonomic function include neurogenic essential hypertension, psychogenic ischemic heart disease, and congestive heart failure. About 40 percent of patients with untreated essential hypertension have increased norepinephrine levels that stimulate the heart, elevate cardiac output, and increase renin secretion in the kidney. Panic disorders increase sympathetic outflows that can trigger cardiac events in patients with fixed coronary artery stenosis. In heart failure, cardiac norepinephrine release is increased, augmenting cardiac hypertrophy and accelerating cardiac decompensation. This explains why the cautious use of beta blockade is recommended in these patients.
Chronic fatigue syndrome, characterized by unexplained fatigue for at least six months not relieved by rest and without any clear cause, may be related to dysautonomia. Many of these patients have orthostatic symptoms on prolonged head-up tilting, but the extent to which dysautonomia is responsible for symptoms in patients with chronic fatigue syndrome remains unclear.
The authors conclude that neurochemical, neuroimaging, and molecular genetics techniques will clarify hypofunctional dysautonomias in the future.