Medical Tests for Measuring Dysautonomia

Some degree of dysfunction with the autonomic nervous system is probably very widespread, especially as we get older. For example, more than 25 percent of people over the age of 75 suffer from mild orthostatic hypotension, in which it may be harder for people to stand without feeling dizzy due to the autonomic nervous system's inability to appropriately adjust blood pressure.

Almost any medical problem—or even treatment—can impact the autonomic nervous system either directly or indirectly. A problem with the autonomic nervous system is called dysautonomia. Before correcting the problem, though, it's important to properly test to make sure that the nature of the dysautonomia is correctly understood.

Orthostatic Blood Pressure Measurement

The most common method of testing the autonomic nervous system can be done with a blood pressure cuff, a watch, and a bed. The blood pressure is measured and the pulse is taken when the patient is lying flat, sitting, and standing up, with about two minutes in between positions. In normal people, the blood pressure should not vary by more than about 10 diastolic (the bottom blood pressure number) or 20 systolic (the top number), though these guidelines vary from place to place.

If the blood pressure falls, it may not be a problem with the autonomic nervous system: there may just not be enough blood to maintain adequate pressure. The usual reason for this is dehydration, which is why we also check the pulse. If the blood pressure does fall, the pulse should increase as the body tries to increase blood pressure and get blood to the brain. If it doesn't, there may be a problem with the reflex arc involving the vagus nerve, which has autonomic nerve fibers that control heart rate.

Other Bedside Tests

Using an electrocardiogram (ECG or EKG) while doing some simple maneuvers can increase the sensitivity of tests for dysautonomia. For example, the ratio of the distance between two electrical waves on the 15th and 30th heartbeats after standing from a seated position can indicate a problem with the vagus nerve. This can also be done during deep breathing. Up to age 40, an exhalation to inspiration ratio of less than 1:2 is abnormal. This ratio is expected to decrease as we age and also decreases with even a very mild diabetic neuropathy.

The Valsalva ratio is another simple, noninvasive bedside test that can be used to assess for dysautonomia. The patient bears down by exhaling with her mouth closed so that no air actually escapes. This normally causes the heart rate to increase until after the breath is released, at which point the parasympathetics tend to overshoot, causing a brief moment of bradycardia, when the heart rate drops below normal. If the heart rate does not increase during the Valsalva, there is likely sympathetic dysfunction. If it fails to slow afterward, it suggests parasympathetic dysfunction.

Other techniques measure changes in blood pressure after muscular contraction for a few minutes, or after keeping a limb immersed in cold water.

Advanced Autonomic Testing

When bedside tests are insufficient, there are more involved diagnostic procedures available at some institutions. These may involve the patient's being placed on a tilt table, which allows the patient's position to be changed rapidly and in a way that can be easily measured.

Skin conductance can be measured after a chemical is infused to make just that patch sweat in order to evaluate subtle differences between different regions of the body.

Sometimes serum levels of hormones like norepinephrine can be measured in response to a systemic stress, but such testing is unusual.

Sweat Tests

The sympathetic nervous system is responsible for causing secretion from sweat glands. Think of it as a way to ensure that our body stays cool enough to successfully run away from an attacking tiger.

Sometimes the sympathetic innervation to part of the body is lost, and this part no longer sweats. This is not always obvious, since perspiration may run from another region of the body to cover the part that no longer perspires. In a sweat test, the body is covered with a powder that changes color when sweating, making regional lack of perspiration more obvious. The downside is that this test is very messy.

Testing of Separate Body Parts

Because the autonomic nervous system involves almost every part of the body, it may be necessary to check how the autonomic nerves are working in one particular part rather than just the cardiovascular system.

A variety of eye drops might be used to assess the autonomic innervation of the eyes. Tearing of the eyes may be assessed by inserting a thin slip of soft paper at the corner of the eye to see how much moisture the paper absorbs. Bladder function can be assessed by a cisternogram, and the motility of gastrointestinal systems might be assessed by radiographic studies.

We've only described some of the many tests used to assess the autonomic nervous system. The truth is that dysautonomias are commonly under-recognized, and many institutions do not have any more than the basic bedside tests. This may partially be because most dysautonomias are due to problems that also affect other parts of the body in more obvious ways, which then limits the usefulness of further testing. For example, diabetes is a common cause of dysautonomia that is diagnosed by standardized blood tests for diabetes, rather than starting with the autonomic nervous system.

If a problem with the autonomic nervous system is suspected and confirmed, it is likely that more testing will be needed to determine the cause. Rather than trying to just treat the dysautonomic symptoms, addressing the core cause of the illness is the best way to get the autonomic nervous system back in balance.