Autonomic nervous system (ANS) testing checks for imbalances in the part of the body that controls many "autonomic" or automatic processes, including:
The ANS has two parts: the sympathetic nervous system and the parasympathetic nervous system. Blood pressure is primarily controlled by the sympathetic nervous system, while the parasympathetic system controls the heart rate. These two systems keep the cardiovascular system in balance. If it becomes unbalanced, a person may experience a variety of symptoms that are sometimes vague and can affect many bodily functions.
People suffering from disorders of the autonomic system may experience:
In some patients, these symptoms may be mild and occur only once in a while. For others, symptoms may occur more frequently and be more severe. It is important to exclude other medical conditions that have similar symptoms, such as diabetes or Parkinson's disease, before linking them to autonomic dysfunction.
One reason people commonly undergo ANS testing is because they faint. This is also called syncope. Fainting can occur while standing, after coughing, after a large meal or even while straining during bladder or bowel movements. Sometimes syncope can be caused by abnormal heart rhythms, or cardiac arrhythmia. A thorough cardiac evaluation is always indicated for someone suffering from syncope. There are cases, too, when people do not faint but instead feel lightheaded after standing for a long time. This is usually due to a drop in blood pressure and is a common reason why people seek medical attention.
Testing the autonomic system involves measuring heart rate, blood pressure and sweat function. Most testing sessions last from 60-90 minutes, depending upon the number of studies administered. The tests are non-invasive and painless. All are approved by the U.S. Food and Drug Administration and provide information about the nature and severity of autonomic disorders.
Heart rate is measured with a three-lead electrocardiograph (ECG) machine. Adhesive leads are placed on the chest and the heart rate is recorded throughout the test. The heart rate varies with breathing, and this variation is called the sinus arrhythmia. Sometimes this variation is diminished; sometimes it is exaggerated. These changes provide insight into nervous system control of heart function. Pacemakers limit the variation in heart rate, so this test is not indicated for people who use these devices.
Measuring blood pressure is an important part of any physical examination. Standard methods involve tightening a blood pressure cuff around the upper arm to calculate blood pressure at one moment in time. For those with possible ANS disorders, blood pressure is measured continuously using a recording sensor placed on the wrist over an artery. This provides a "real time" measure of blood pressure.
Tilt table testing helps determine how blood pressure and heart rate change in response to body position. The tilt table is a device that holds the body at various upright angles. This accentuates the force of gravity and lessens muscle tone. Together these factors lead to increased pooling of blood in the lower extremities. Results can show whether a patient is disposed to low blood pressure or to fainting during these physiologic events. During this test, blood pressure and heart rate are continuously recorded.
Sweat function is measured by looking at how much humidity is created on the skin when sweat glands are active. A small recording chamber is attached to the forearms and legs and a sweat measurement is taken in the resting state. A stimulant is then applied to the skin around the chamber. This activates sympathetic nerve fibers, leading to increased sweating and a change in the humidity of the skin, which is measured by the recording device. Chambers can be placed on any part of the body to determine the pattern and amount of sweat function. Too much or too little sweating can be linked to sympathetic nervous system problems.
Treating ANS disorders depends upon the nature of the problem and can include medications as well as physical therapy approaches. Since each patient is unique, the course of treatment is individualized.
Three examples include: