The Quantitative Sudomotor Axon Reflex Test (QSART) is a non-invasive diagnostic tool used to assess the function of the sudomotor system, the peripheral nerves that control sweating. A positive QSART result indicates that the nerves responsible for regulating sweat production are not functioning correctly. This test checks a specific part of the autonomic nervous system, which manages involuntary bodily functions like heart rate, blood pressure, and body temperature regulation.
How the QSART Test Works
The QSART procedure is designed to isolate and test the integrity of the sweat-controlling nerve pathway. The test uses a mild electrical current (iontophoresis) to introduce acetylcholine, a naturally occurring neurotransmitter, into the skin. Acetylcholine triggers the sweat glands to produce sweat. The test is typically performed at four sites, such as the forearm and three locations on the leg and foot.
The apparatus measures the volume of sweat produced in response to this stimulation and the time it takes for the sweat to appear, which is called the latency. By directly stimulating the sweat gland and the nerve endings, the QSART specifically evaluates the postganglionic sympathetic sudomotor axon. These axons are small, unmyelinated C-fibers, and their function reflects the health of the small nerve fibers throughout the body.
Interpreting an Abnormal Result
A positive QSART result means the measured sweat output or response time falls outside the established normal range. This finding points to impaired sudomotor function, reflecting damage to the small nerve fibers. The most common abnormal finding is a significantly reduced or absent sweat output, indicating that the nerves are damaged and cannot effectively signal the sweat glands to activate. This output is compared against the normal sweat volume expected for a person’s age, sex, and the specific location being tested.
Another significant finding is delayed latency, meaning the nerve signal is taking too long to reach the sweat gland. This suggests that while the nerve may still be functional, the transmission of the signal is slowed down due to damage. The results are analyzed for a length-dependent pattern, where the sweat response is more impaired in the feet than in the forearm, a typical sign of a distal neuropathy.
Conditions Associated with Autonomic Dysfunction
A positive QSART test strongly suggests the presence of an autonomic neuropathy. The test is particularly useful for identifying Small Fiber Neuropathy (SFN) because the sudomotor fibers evaluated are the small-diameter nerves affected by this condition. SFN symptoms often involve burning pain, numbness, and tingling, and the compromised sudomotor function measured by QSART provides an objective sign of the underlying pathology.
Several common underlying medical conditions can lead to this type of nerve damage and an abnormal QSART result. Diabetes is a frequent cause, where chronically high blood sugar levels gradually injure the small nerve fibers, leading to diabetic autonomic neuropathy. Autoimmune disorders, such as Sjögren’s syndrome, can also cause nerve inflammation and damage, resulting in impaired sweating. A positive QSART can also be seen in cases of generalized dysautonomia or specific conditions like Postural Orthostatic Tachycardia Syndrome (POTS).
Impaired sudomotor function means the body struggles to regulate its temperature effectively through sweating. This can lead to symptoms like heat intolerance or, conversely, excessive sweating in areas that are less affected. Identifying the nature and distribution of the sweat loss helps clinicians categorize the type and severity of the underlying neuropathy.
What Happens After a Positive Test
A positive QSART result is a significant finding that directs the subsequent diagnostic and management strategy, but it is not a final diagnosis on its own. The immediate next step involves searching for the underlying cause of the nerve damage. This may include blood tests to check for common etiologies like diabetes, vitamin deficiencies, or markers for autoimmune diseases.
Confirmatory testing is often pursued to solidify the diagnosis suggested by the QSART. For example, if Small Fiber Neuropathy is suspected, a skin biopsy is frequently performed to quantify the density of intraepidermal nerve fibers (IENFD). Management focuses on two simultaneous paths: addressing the primary disease, if one is identified, and managing the symptoms of the autonomic dysfunction. Treatment for the underlying condition, such as optimizing blood sugar control in diabetes, aims to slow or halt the progression of nerve damage, while symptom management helps the individual cope with issues like abnormal sweating or temperature regulation difficulties.