The Quantitative Sudomotor Axon Reflex Test (QSART) is a specialized, non-invasive diagnostic tool used to evaluate the integrity of the peripheral nervous system. It focuses on the nerves that control sweating, which are part of the autonomic nervous system. The test helps clinicians investigate symptoms related to nerve damage or dysfunction. Understanding a positive QSART result is the first step toward clarifying a medical concern and guiding further diagnostic efforts.
Understanding the QSART Test
The QSART specifically measures the function of sudomotor nerves, which are the small, unmyelinated sympathetic C-fibers responsible for regulating sweat gland activity. These fibers are often among the first to be damaged in various forms of peripheral neuropathy. The test evaluates the postganglionic sympathetic sudomotor system.
The procedure involves controlled chemical stimulation of the sweat glands. This is typically done using iontophoresis, a process that pushes acetylcholine into the skin at four different sites, such as the forearm and three locations on the leg. Acetylcholine acts as a neurotransmitter, triggering the nerve endings to initiate an axon reflex. A humidity sensor then measures the quantity and timing of the sweat produced in response to this stimulus.
The measurements provide data on three main parameters: the onset latency (how quickly sweating begins), the peak sweat production, and the total sweat volume. Analyzing the sweat response at various anatomical locations offers a quantitative assessment of the function of these small nerve fibers. This determines whether the nerves controlling the sweat glands are working as expected.
Interpreting a Positive Result
A positive QSART result signifies compromised sudomotor function, most often due to damage to the postganglionic sympathetic small nerve fibers. This abnormality is typically seen as a reduced or absent sweat volume compared to established normal values. The deficit suggests the nerve failed to propagate the signal and stimulate the eccrine sweat glands to produce adequate sweat.
The pattern of the abnormal sweat response offers insight into the nature of the nerve damage. A length-dependent reduction, where the sweat response is diminished or absent in the feet but less affected proximally, suggests a distal small fiber neuropathy. This pattern is common in neuropathies that start at the longest nerves. Abnormality is also identified if the distal site produces less than one-third of the sweat volume compared to the more proximal sites.
While the most common and clinically significant positive finding is the quantitative reduction in sweat output, a positive result may also involve an excessively prolonged sweat response or a reduced latency, seen in certain painful neuropathies. This reduction directly reflects the degree of small fiber damage. A positive QSART result identifies the presence of nerve dysfunction but does not, on its own, determine the underlying medical cause.
Conditions Indicated by Autonomic Dysfunction
A positive QSART test indicates small fiber damage, pointing toward a diagnosis of autonomic neuropathy, a malfunction of the involuntary nervous system. A wide range of underlying medical conditions can cause this nerve damage. The most frequent cause worldwide is diabetes mellitus, where high blood sugar levels progressively injure the small nerve fibers over time.
Autoimmune disorders are another significant group of causes, including Sjögren’s syndrome, systemic lupus erythematosus, and rheumatoid arthritis. In these instances, the immune system mistakenly attacks the small nerve fibers. Infections like Lyme disease and HIV, or exposure to specific neurotoxic medications, can also trigger autonomic neuropathy.
In many cases, the neuropathy is categorized as idiopathic, meaning a specific cause cannot be identified despite a medical workup. Conditions like Postural Orthostatic Tachycardia Syndrome (POTS) and other forms of dysautonomia are often associated with small fiber neuropathy detected by QSART. The QSART findings help objectify the physiological impairment present in these syndromes. Rarely, genetic disorders or systemic diseases like amyloidosis can also cause sudomotor dysfunction.
Next Steps Following a Positive Result
Following a positive QSART test, a comprehensive medical workup is required to identify the specific etiology of the autonomic dysfunction. Blood tests are typically ordered to screen for common causes, such as checking for diabetes indicators like hemoglobin A1c and evaluating for autoimmune markers. This focused laboratory investigation helps narrow the list of potential diseases.
The physician may also recommend confirmatory testing to further characterize the small fiber neuropathy. A skin biopsy, which measures the density of nerve fibers within the epidermis, is often considered the gold standard for confirmation. This test provides structural evidence of nerve loss, complementing the functional data provided by the QSART.
Treatment following the diagnosis focuses on two main strategies: managing the underlying condition and providing symptom relief. If a primary cause like diabetes or an autoimmune disorder is identified, controlling that disease is important to slowing the progression of nerve damage. Symptomatic relief involves medications to address specific issues related to the autonomic dysfunction, such as pain, digestive issues, or blood pressure regulation.