Thermoregulatory Sweat Test
In this video Dr. Singer discusses the thermoregulatory sweat test including several visual examples of normal vs. abnormal finding in TST testing.
Alright, this was the autonomic testing battery that we’ll be demonstrating here. Just a few words about the thermoregulatory sweat test. This is a test we utilize at our institution extensively, about 2,000 tests a year, because we find it that helpful. It is a very labor-intensive test, however…a costly test, and some really more reserved for centers that have a special interest in autonomic nervous system, but nevertheless, I want to demonstrate how helpful this test can be.
So, what we do is we have the patient lying and put indicator powder on the skin really from head to toe, the entire anterior body surface is covered, and then we put the patient in a heating chamber and heat the patient up so the body core temperature rises and we look at the pattern of sweat recruitment throughout the entire body, not just at the 4 sites that we are able to test with the QSART, but the entire body surface. By doing so, you will see progressively changing in color and you see where the patient sweats and does not. This patient happened to have a limited autonomic ganglionopathy that we would have probably never picked up without having that test available.
So, this is a test that was also implemented some 30 years ago. Robert Fealey was a key developer of that test along with Phillip Low, and it is really a way to assess not only the post ganglionic pseudomotor neuron like the QSART does, but really to assess the pseudomotor pathway from the hypothalamus all the way to the sweat gland. That needs to be intact. If there is a lesion anywhere along the way you have an abnormality, so it’s very sensitive.
The way it’s performed – I’m showing you here our sort of temperature settings and humidity settings. We keep a close eye on skin temperature for safety reasons obviously, and we want to raise the core temperature by a certain amount in order to have an adequate stimulus, and a number of studies have shown that we need to reach the body core temperature of at least 38 degrees Celsius and at least a 1 degree Celsius increment in order to have an adequate stimulus. The powder we use is Alizarin red mixed with cornstarch and desiccant sodium carbonate.
This is what one of our sweat chambers looks like, it’s kept quite open. This does have a curtain here that keeps the humidity and the temperature in, and so claustrophobic patients usually have no problems getting through it. Here’s some examples of normal. This is a diffuse heavy sweating pattern that we see typically in males. Females tend to have lighter sweating patterns. Those are shown here. Those are normal variants. Here are some patterns of abnormality. The patient with length-dependent pattern, little bit of hands, legs and feet involved. This is a patient with segmental pattern sweat loss. We see that sometimes in pure autonomic failure, sometimes in Multiple System Atrophy, but also in certain ganglionopathies. This is a patient with distal sweat loss in the feet and also demarcated areas about the abdomen. These are diabetic radiculopathies and length-dependent peripheral neuropathy. This is a patient with global anhidrosis. This is a normal control, and this is a regional pattern sweat loss as you can for example see in Multiple System Atrophy. Interestingly all those six here were diabetics. Now, we have a few examples here of a normal pattern, a distal small fiber neuropathy pattern and a global anhidrosis pattern with a little bit preserved sweating in hands and feet. That’s very classic for Multiple System Atrophy. Multiple System Atrophy is really where this test comes in so very handy and is precious for detecting that condition and confirming the condition because this is really one of the only conditions where you see impaired central pathways with completely normal peripheral pathways. So, if you do a QSART where you really just test the axon reflex, you have completely normal responses. Yet when you put this patient in the heating chamber and have a central stimulus applied that requires the central pathways to be intact, they can be globally anhidrotic. This pattern in someone with parkinsonism is diagnostic if I might say.
Wolfgang Singer, MD
Associate Professor of Neurology
Mayo Clinic Rochester, MN