In this video Dr. Singer describes the Quantitative Sudomotor Axon Reflex Test (QSART) for assessing autonomic function.
So, let’s dive into the first one of those tests: the sudomotor axon reflex test. In order to understand the test, you should really go to its basis which is the multi-compartmental sweat cell. This is basically a capsule that has two separated components to it. An outer compartment that is being filled with acetylcholine, and then iontophoresed into the skin, where a sweat response is being induced. But we are not interested in a sweat response right under that outer rim, which is a direct sweat response, we are simply stimulating sweat glands. What we’re really interested in is the response in the center compartment of that capsule because that’s separated from the outer rim and requires another form of stimulation to occur in order to see a response, and that’s an axon reflex. So we are basically stimulating a peripheral axon that then travels a distance to a different set of sweat glands where it induces the sweat response and that sweat response requires that the post ganglionic sympathetic fiber is intact and that’s where the axon reflex test is coming from and superior to direct stimulation of the sweat gland.
We record that response at 4 standardized sites, one at the forearm, one at the distal leg, one at the proximal leg and one at the foot, we will demonstrate all that in a little while. Here is an example of a normal sweat response on the left with this rather homogeneous response at all 4 sites, and then the right example of the patient with distal small fiber neuropathy and virtually absent response in the foot. Here on top again, a normal example, and on the bottom, the patient with a severe form of autoimmune autonomic ganglionopathy who has absent responses at all 4 sites. Now if someone comes in on 20 mg of Amitriptyline the response could look just like that. So that is something to always keep in mind. It is not always pathology that results in abnormal tests.
Now a brief word about QSWEAT. You may have heard that and now wonder what’s QSART, what’s QSWEAT? Really QSWEAT is the commercial version of QSART. It is still a QSART, it just has its own brand name. There’re a few differences from the original Mayo sudorometer, certain sensors, the type of gas used, etc. to the desiccant and so on. There’re a few differences but overall the principal is the same. And what we found out, our big surprise, is that QSWEAT responses tend to be smaller than QSART responses. They have still not figured out why that is in spite of extensive search. We have also tried to find conversion factors that allow us to convert one response to the other but that didn’t seem to work out, so we decided we need normative data specifically for QSWEAT and we derive those.
Now as I mentioned before there are factors other than disease that can affect an abnormal sweat response, and I mentioned the medications already, so this is always something to keep in mind. Age and gender clearly play a role and then of course autonomic disease. Gender plays a big role in sudomotor responses. Responses from women, on average, markedly lower than the responses we get from men. Age is not as big of a factor. There’s a wide age range between 15 and 75 where there’s really no significant change occurring, it’s just the very young and the very old where there’s a drop off. Then, on the right here, I am showing you a little example of a recent study we did on subjects where there was a spuriously absent response. So otherwise healthy people we wouldn’t expect an absent response of the forearm. And we intensified our skin preparation and voila, they all had a normal response. So, skin preparation is really important, and we will demonstrate that again in some detail here how to obtain a good response with the right skin preparation.
Wolfgang Singer, MD
Associate Professor of Neurology
Mayo Clinic Rochester, MN