How are autonomic disorders diagnosed?
The first step in trying to establish if there is a problem with the autonomic nervous system is through a thorough history and physical examination. Certain aspects of the history are very important such as whether or not you are suffering from diabetes or a disorder such as Parkinson’s disease. This is because both of these conditions have a high association with problems with the autonomic nervous system. Then your neurologist will establish through the history which parts of your autonomic nervous system are potentially the problem. The physical examination will also help. For example, if you have diabetes and the physical examination demonstrates a neuropathy (see neuropathy page) then there is a high probability that you also have damage to your autonomic nerves. Or if the examination picks up the findings of Parkinson disease this may also suggest involvement of the autonomic nerves.
Sometimes patients will have a drop in blood pressure on standing up and the cause will be simple such as getting too high a dose of their blood pressure medications. This should become apparent in the history. Also, if you are unwell for any other reason, such as with a gastrointestinal illness for example, you may have a drop in blood pressure on standing that is a reflection of “volume depletion” which can occur if you have been vomiting, having diarrhoea or are simply unable to take in enough fluids because you are ill. In this scenario, treating the infection and rehydrating with fluids should significantly help the problem- again this will become apparent in the history.
The next step may be blood tests such as the test for diabetes (HbA1c), also checking vitamin B12 levels, for any markers of infection, thyroid function, HIV testing and others depending on the differential diagnosis that your neurologist has formulated based on the history and examination above. You may then go on to have some more specific autonomic function testing as described below.
One of the ways to diagnose a problem with the autonomic nervous system is through a tilt table test. This tests the cardiovascular portion of the autonomic nervous system but as described above this is the largest component. This simply involves measuring blood pressure and heart rate in the lying down and tilted up (to 60 degrees) position for about 40 minutes. At the end of the tilt test the heart rate and blood pressure are measured in the standing position. This part of the test only takes 5 minutes.
The blood pressure and heart rate may change in several different patterns and that is what the test will determine. For example, the blood pressure may gradually decrease throughout the 40 minutes or it may remain stable throughout and then suddenly drop. The heart rate may increase gradually throughout the test and this may or may not lead to a fainting episode. In a fainting episode it may just be the blood pressure that drops or it may be the blood pressure and heart rate. Also, the heart rate may remain high and stay like that throughout the test without any real fluctuation. Therefore, a lot of information regarding the functioning of your autonomic nervous system is obtained from this test.
How the heart rate and blood pressure vary in response to two breathing tests is also part of the test. One of the breathing tests is measuring how your heart rate varies with deep breathing. For this part of the test you simply breathe in and out and the heart monitor measures how your heart rate varies. How your heart rate varies with deep breathing is an important marker of functioning of your autonomic nervous system. When we breathe in our heart rate increases and when we breathe out it decreases. If your heart rate does not vary very much this is a sign of dysfunction of your autonomic nervous system. It suggests that your heart rate cannot vary based on the particular requirements of your body. The situation where this becomes most obvious is through exercise however even something like fighting an infection requires you to have adequate heart rate variability. The second breathing test involves breathing in to a tube against pressure- this is called a Valsalva manoeuvre. In this test both blood pressure and heart rate response to the test are measured.
Having these tests will firstly help establish whether or not your symptoms relate to autonomic dysfunction. The test will also help categorise your autonomic dysfunction. Broadly speaking you may be suffering from orthostatic hypotension (OH), postural orthostatic tachycardia syndrome (POTS) or you may have what is referred to as autonomic failure which is where you have abnormalities on the breathing tests, tilt test and stand test. In OH the blood pressure drops on assuming the upright position to a level that may not be sufficient to maintain adequate blood supply to the brain. In POTS the problem is not the blood pressure dropping but the heart rate increasing excessively on standing up in order to maintain a normal blood pressure. In POTS the issue is really that the autonomic nervous system is out of kilter as opposed to abnormal. Both can lead to symptoms of orthostatic intolerance. When there is autonomic failure it suggests that your autonomic nervous system cannot adjust for any changes in your environment.
In general, younger people tend to suffer from POTS and older people from OH.
There are several other ways of testing different parts of the autonomic nervous system but they are very specialised and individualised tests.