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The assessment of sudomotor function can serve as a quick and precise means of identifying initial stages of distal small fiber neuropathy. Small blood vessels extensively supply the eccrine glands responsible for sweat response, which receive innervation from the autonomic nervous system's sympathetic C nerve fibers. As these fibers are thin and lengthy, they may become impaired during the early stages of various metabolic disorders. Research publications suggest that "sweat response may be the most sensitive test in detecting distal small fiber neuropathy" (Low PA and Gibbons CH).
SUDOSCAN® is a non-invasive test that measures sudomotor function on the palms of the hands and soles of the feet, which have the highest density of sweat glands. The technology works by applying a low DC voltage ≤ 4 volts to generate a current relative to the chloride ion flow in the sweat glands and ducts, which is then used to calculate an Electrochemical Skin Conductance (ESC) for each hand and foot. This electrical stimulus captures information about sweat dysfunction that cannot be diagnosed under normal conditions. The skin's stratum corneum acts as an electrical capacitor, ensuring that the movement of chloride to the electrodes only occurs via the sweat ducts, targeting the underlying sweat gland and its sympathetic innervations. This theory is supported by a comparative study on patients with Cystic Fibrosis and controls. For a better understanding, an animation video is available on the website.
SUDOSCAN® provides test results for both the hands and feet, with the average conductance of both the left and right sides being reported. Various studies have found that higher conductances are indicative of normal sweat function and healthy nerve innervation, particularly in small C-fibers. In contrast, lower conductances have been associated with peripheral or autonomic neuropathy.
SUDOSCAN® provides physicians with simple and immediate information on small autonomic fiber status, which can be used in various medical settings. With the trend towards prevention and early detection of disease and complications, physicians can use SUDOSCAN® results to obtain an objective evaluation of sudomotor function in a patient with signs or symptoms suggestive of developing disease. Additionally, SUDOSCAN® can be used to assess the level of intensive glycemic control for diabetics before prescribing treatment. By detecting autonomic dysfunction, SUDOSCAN® may alert physicians to refer a patient for further autonomic function testing. Moreover, SUDOSCAN® allows physicians to quickly determine sudomotor functioning as a biomarker for peripheral nerve integrity, which can help in evaluating patient drug or lifestyle compliance. Worsening sudomotor function may signal poor patient response to treatment or lack of compliance, and initiate a discussion between physician and patient on a change in therapy. The quantitative results obtained from SUDOSCAN® offer a precise follow-up with no subjectivity introduced via patient verbal responses, making it an excellent complementary tool to traditional blood tests, while providing extremely fast and immediate results.
With the exception of specific contra-indications, anyone is eligible for SUDOSCAN testing to evaluate their sudomotor function. As mentioned earlier, the assessment of sudomotor function using SUDOSCAN is valuable for monitoring and follow-up of peripheral autonomic and small fiber neuropathies that may arise from various medical conditions such as diabetes, amyloidosis, Fabry disease, and Parkinson's disease.
The American Academy of Neurology (AAN), American Diabetic Association (ADA), and American Academy of Clinical Endocrinologists (AACE) recognize sudomotor dysfunction as a symptom of autonomic neuropathy, a common complication of diseases such as diabetes, Parkinson's disease, and amyloidosis. Sudomotor tests like SUDOSCAN are utilized to evaluate autonomic and small fiber neuropathies. SUDOSCAN is a rapid and non-invasive test that can establish a patient's baseline, detect sub-clinical nerve damage, and evaluate clinically significant changes in medical status. Studies suggest that simple lifestyle changes can substantially improve sudomotor function as measured with SUDOSCAN and are associated with cardio-metabolic improvement (Raisanen et al., Int J Environ Res Public Health, 2014).
Evaluation of sudomotor function using other tests such as Quantitative Sudomotor Axon Reflex Test (QSART) has revealed that women generally have lower results (sweat output) than men. This can be explained by lower sweat rates in women. Measurements performed by SUDOSCAN do not depend on sweat rate. A study performed on more than 500 women and more than 200 men showed no significant differences in hand or feet ESC. This will be confirmed in future pending studies on larger group populations.
The variability of sweat rates among individuals does not impact the accuracy of SUDOSCAN results. Nonetheless, sweat gland physiology can differ by ethnicity, and research is being conducted to examine the potential impact of this on SUDOSCAN outcomes. Future studies involving large and diverse populations are expected to establish normative ranges for different ethnic groups.
A substantial dataset of healthy volunteers has indicated that SUDOSCAN® conductances are not correlated with age.
Clinical tests conducted at different temperatures, varying by more than 5°C, have shown that SUDOSCAN® test results are not affected by temperature. As the hands and feet have a limited role in thermoregulation due to their small surface area, their sweat rate is less temperature dependent than other areas of the body. Nevertheless, it is advisable not to conduct a SUDOSCAN® test if the electrodes are too cold or if the patient has just come in from an extremely cold environment.
To assess the impact of exercise on SUDOSCAN® tests, a study was conducted on 112 healthy individuals before and after a VO2max test. The study found a correlation coefficient of 0.814 (p<0.0001) and a mean percentage change of 1.15% for feet ESC, indicating good test-retest reliability (Casellini et al. Diab Tech & Ther 2013). These results suggest that SUDOSCAN® measurements are not influenced by changes in sweat rate due to exercise.
To ensure its usefulness, a medical device must be reproducible. Several clinical studies have evaluated the reproducibility of SUDOSCAN®, and the results have been positive. Paired Spearman correlation coefficients among tests on three different SUDOSCAN® devices were >0.96 (p<0.0001), and the correlation coefficients between measurements performed with two different devices in diabetic patients were 0.85 for hands and 0.93 for feet, with p < 0.001. (Schwarz et al., Br J Diab & Vasc Dis 2011; Calvet et al., Exp Clin Endo Diab 2012). As mentioned earlier, the reproducibility between measurements performed before and after exercise is also excellent. Dynamic analysis of chloride-ionic flow in sweat glands (as opposed to sweat volume) provides more informative results, as chloride levels remain constant regardless of sweat rate (see Quinton et al. article on the website).
Testing children on SUDOSCAN® is considered safe and initial data from clinical studies conducted on pediatric patients have shown promising results. However, until FDA clearance is obtained for pediatric age groups, the use of SUDOSCAN in clinical practice is limited to the adult population.
SUDOSCAN® and HbA1c are complementary tests as they focus on different aspects of diabetes assessment. While HbA1c is a blood-based test, SUDOSCAN® assesses sweat gland function and its sympathetic innervation. Small fiber neuropathy in type 2 diabetes is influenced by various metabolic disturbances, including hyperlipidemia, in addition to hyperglycemia. Thus, there is no strict correlation between HbA1c and SUDOSCAN® results. In research studies, it has been observed that ESC results decreased significantly over time in some cases, while no significant difference in HbA1c was observed.
The frequency of retesting with SUDOSCAN depends on various factors, including the patient's initial test results, the underlying medical condition, changes in the patient's medical status or response to treatment, and the clinical judgment of the physician. In general, SUDOSCAN retesting is usually done no more than once every 12 months.
Differentiating between a progressive and a non-progressive peripheral small fiber or autonomic neuropathy can be challenging due to changes in symptoms that may indicate either a response to therapy or ongoing nerve loss. However, the quantitative and highly reproducible assessment of sweat gland function using SUDOSCAN® may aid in this differentiation. Changes in SUDOSCAN results may indicate whether the neuropathy is progressive or non-progressive.
SUDOSCAN® applies only direct current during the test, which means it does not interfere with pacemaker electronics. Therefore, it is considered safe to use in subjects with pacemakers. Safety tests were performed to meet the Electromagnetic Compatibility requirements specified in the ANSI/AAMI PC69:2007 guidelines for implantable cardiac pacemakers and implantable cardioverter. The Laboratoire Centrale des Industries Electriques (LCIE, Fontenay aux Roses, France) performed these tests and concluded that "SUDOSCAN® is compliant according to ANSI/AAMI PC69:2007 (Annexe M) standard" (Report October, 2012). However, it is recommended that patients with pacemakers undergo SUDOSCAN® testing under the supervision of a medical doctor.
Further studies are necessary to verify whether other cardiovascular medications affect SUDOSCAN® measurements; however, cardioselective beta-blockers are not expected to interfere with the test.
Since SUDOSCAN® measures sweat gland activity through cholinergic small C fibers, medications that have significant anti-cholinergic effects can impact the test results. Tricyclic antidepressants, especially amitriptyline, have been found to significantly reduce ESC measurements obtained by SUDOSCAN®. Therefore, caution must be exercised when interpreting results in patients using anti-cholinergic agents. The effect of medications on ESC measurements is a crucial factor and is being continuously investigated in ongoing studies.
Preliminary findings from an observational study conducted in a German outpatient clinic on two groups of patients with type 2 diabetes, one receiving insulin and the other receiving other anti-diabetic drugs, suggest that after one year of follow-up, patients receiving insulin showed an improvement in their conductances, while the second group experienced a slight decrease. However, these results are preliminary, and further research is needed with a larger study population to confirm these findings.
SUDOSCAN® results in patients with skin conditions that affect their palms and/or soles, such as atopic dermatitis, have not been fully investigated, so it is uncertain how these conditions may affect test outcomes. It should be noted that for SUDOSCAN® results to be accurate, the skin on the palms and soles must be intact, without any open sores, ulcers, or lacerations.
Before testing with SUDOSCAN®, it is necessary to ensure that the palms and soles are clean and free of any dirt, ointments, or lotions. The recommended method of cleaning is to wash the skin with soap and water. It is important that the palms and soles are thoroughly dried before placing them on the electrodes. It is also important to note that the manufacturer-approved cleaning solution for the electrodes is intended for Sensor Plate cleaning only and should not be applied directly or indirectly to a patient's skin before testing.
SUDOSCAN® test results are not affected by the size of a patient's hands or feet. However, if a hand or foot has a significantly reduced surface area due to an injury such as a second or third-degree burn or amputation, with the loss of sweat glands, it can impact SUDOSCAN® results.
The sensor plates used in SUDOSCAN® are manufactured from high-grade stainless steel, making them resistant to corrosive influences. The lifespan of the plates is dependent on how frequently they are used and how well they are maintained. After each patient, the plates must be cleaned with a manufacturer-approved solution to ensure hygiene and safety standards are maintained, to halt the electrochemical reaction that occurs during each test, and to guarantee the most accurate test results. As a result, the sensor plates are replaced after every 100 successful tests to ensure the physician that the test results obtained are of the highest quality.