Testing sudomotor function can be a fast and accurate method for detecting early stages of distal small fiber neuropathy. Eccrine glands that are responsible for sweat response receive a rich supply of small blood vessels and are innervated by sympathetic C nerve fibers of the autonomic nervous system. These fibers are thin and long and so can be impaired at the very early stage of different metabolic diseases. “Sweat response may be the most sensitive test in detecting distal small fiber neuropathy” (Low PA and Gibbons CH publications).
SUDOSCAN is a medical device that has been cleared by the FDA for Galvanic Skin Response (GSR) and Sudorimetry. GSR is a change in the electrical properties of the skin in response to different kinds of stimuli. In GSR, changes in the voltage measured from the surface of the skin are recorded. The origin of the signal measured has been suggested to be the activation of sweat glands (sudomotor function).
SUDOSCAN evaluates sudomotor function on the palms of the hands and soles of the feet where the density of sweat glands is greatest.
The underlying technology is based on established principles of the electrochemical reaction between chloride (found in sweat) and the stainless steel electrodes of the SUDOSCAN. A low DC voltage ≤ 4 volts is applied generating a current relative to chloride ion flow supplied by the sweat glands and ducts. An Electrochemical Skin Conductance (ESC) is calculated for each hand and each foot on the basis of the current generated and the voltage applied.
Similar to cardiac stress tests for which exercise is used as a stimulus to capture specific information that is not assessable at rest, SUDOSCAN applies an electrical stimulus to the sweat glands to capture information about sweat dysfunction that could not be diagnosed in usual conditions. For a better understanding see the animation video on the web site.
Because the skin’s stratum corneum acts like an electrical capacitor when a low voltage stimulation is applied, the movement of chloride to the electrodes at the surface of the hands and feet can only occur via the sweat ducts, assuring that SUDOSCAN is targeting the underlying sweat gland and its sympathetic innervations.
This underlying theory has been confirmed in a comparative study performed on patients with Cystic Fibrosis (who have high sweat chloride concentration at the surface of the skin, basis of the sweat test) and controls (see publication by Hubert et al).
What information does SUDOSCAN provide?
SUDOSCAN test results are provided as hands and feet conductances (mean of left and right side). In studies high conductances have been found to correlate with normal sweat function and healthy nerve innervation (small C-fibers). Low conductances were associated with peripheral or autonomic neuropathy.
Physicians get simple and immediate information on small autonomic fiber status and use it in various medical settings:
Proactive monitoring: Medicine is trending towards prevention and early detection of disease and complications. The long, thin, and unmyelinated, sympathetic sweat gland nerves degenerate slowly yet have the potential to quickly regenerate with changes in a patient’s environment. Therefore physicians can use SUDOSCAN results to obtain an objective evaluation of sudomotor function in a patient with signs or symptoms suggestive of developing disease.
Assessing the level of intensive glycemic control for diabetics: It is important to determine whether or not a patient has autonomic neuropathy before prescribing intensive forms of glycemic treatment. Autonomic dysfunction detected with SUDOSCAN may alert a physician to refer a patient for further autonomic function testing. (Gerstein et al. AACORD study. N Engl J Med 2008)
Patient drug or lifestyle compliance: SUDOSCAN allows a physician to quickly determine sudomotor functioning as a biomarker for peripheral nerve integrity. 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. This information is complementary to traditional blood tests, but is extremely fast and offers immediate results. No subjectivity is introduced via patient verbal responses and quantitative results allow a precise follow-up.
Anyone can get tested on SUDOSCAN to assess their sudomotor function unless they have a specific contra-indication. As previously described, the potential uses for SUDOSCAN’s measure of sudomotor function include the evaluation and follow-up of peripheral autonomic and small fiber neuropathies, which can occur in a number of disease states (diabetes, amyloidosis, Fabry disease, Parkinson’s disease).
Sudomotor dysfunction is recognized by the American Academy of Neurology (AAN), American Diabetic Association (ADA) and American Academy of Clinical Endocrinologists (AACE) as a symptom of autonomic neuropathy, a common complication of diabetes, Parkinson Disease, and amyloidosis for example. Sudomotor tests such as SUDOSCAN are used in the evaluation of autonomic and small fiber neuropathies.
SUDOSCAN is a fast and non-invasive test that can be used to establish a patient’s baseline, detect sub-clinical nerve damage, then evaluate clinically significant changes in medical status. Research suggests that simple lifestyle changes can significantly improve sudomotor function as measured with SUDOSCAN and correlates 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.
Sweat rates vary from individual to individual and do not influence the results of SUDOSCAN. However, sweat gland physiology can vary by ethnicity and the effect this may have on SUDOSCAN results are under investigation. Ongoing studies of large populations should clarify normative ranges for different ethnicities.
A large database of healthy volunteers demonstrated that SUDOSCAN conductances do not depend on age.
Clinical tests run at different temperatures (varying by more than 5°C) demonstrate that temperature does not impact test results. Hands and feet have a limited role in thermoregulation due to their small area and thus their sweat rate is less temperature dependent than other body surfaces. However, It is recommended not to perform a SUDOSCAN test if the electrodes are too cold or on a patient immediately after coming in from an extremely cold environment.
Given that sweat rate changes with exercise it was important to evaluate the effects of exercise on SUDOSCAN tests. Test-retest reliability in 112 healthy controls before and after VO2max test for feet ESC showed a correlation coefficient of 0.814 (p<0.0001) and a mean percentage change of 1.15% (Casellini et al. Diab Tech & Ther 2013). These results confirm that SUDOSCAN measurements are not dependent on sweat rate.
Reproducibility is critical for any medical device to be useful. SUDOSCAN has been tested for reproducibility in several clinical studies; the coefficient of correlation among tests on 3 different SUDOSCAN devices by paired Spearman was > 0.96 (p<0.0001). (Schwarz et al. Br J Diab & Vasc Dis 2011), and the coefficient of correlation between measurements performed with 2 different devices in diabetic patients was 0.85 for hands and 0.93 for feet, p < 0.001 (Calvet et al. Exp Clin Endo Diab) 2012). The reproducibility between measurements performed before and after an exercise is also excellent (see above).
Analyzing dynamic chloride-ionic flow of the sweat glands (as compared to sweat volume) provides better information since chloride levels do not change with sweat rate (see article by Quinton et al on web site).
There is no safety concern for testing children on SUDOSCAN, and preliminary data obtained from clinical studies in pediatric age ranges are encouraging. However, outside of research trials, SUDOSCAN use remains restricted to the adult population until FDA clearance is obtained for pediatric age groups.
How does SUDOSCAN correlate with HbA1c?
SUDOSCAN focuses on assessment of sweat gland function and its sympathetic innervation, whereas HbA1C is a blood-based test. As such, SUDOSCAN is a complementary test to HbA1c. Small fiber neuropathy in type 2 diabetes depends not only on hyperglycemia but also on other metabolic disturbances such as hyperlipidemia. There is therefore no strict correlation evidenced between HbA1c and SUDOSCAN results. In research studies performed, cases have shown that there were significant decreases in ESC results over time while no difference in HbA1c occurred.
Nerve conduction studies (NCS, also known as EMG) measure the function of large, myelinated nerves. They can be used for evaluating large motor or sensory nerves, but not the sensory nerves that are thin and umyelinated – such as heat, cold, and pain perception.
SUDOSCAN measures the function of sweat gland sympathetic C-fibers. The function of these nerves correlates closely with small sensory nerves of the C-fiber variety. However SUDOSCAN and nerve conduction studies are not likely to correlate since they measure 2 different categories of nerves.
The need to retest will be based on a number of patient factors (e.g. initial SUDOSCAN results, specific disease, change in medical status or response to therapy) as well as the physician’s clinical judgment. Most often testing will occur no more than every 12 months.
One of the difficulties in evaluating and treating small and autonomic neuropathies is whether changes in symptoms signal a response to therapy or ongoing nerve loss. Sweat gland function as assessed with SUDOSCAN is quantitative and highly reproducible; changes in SUDOSCAN results may therefore help differentiate a progressive from a non-progressive peripheral small fiber or autonomic neuropathy.
Since only direct current is applied during a SUDOSCAN test, it does not interfere with pacemaker electronics; therefore SUDOSCAN should be safe to use in subjects with pacemaker. Safety tests have been performed to meet Electromagnetic Compatibility requirements according to ANSI/AAMI PC69:2007 : Active implantable medical devices— Electromagnetic compatibility— EMC test protocols for implantable cardiac pacemakers and implantable cardioverter guidelines.
Test conclusions performed by the Laboratoire Centrale des Industries Electriques (LCIE, Fontenay aux Roses, France) were that “SUDOSCAN is compliant according to ANSI/AAMI PC69: 2007 (Annexe M) standard”. (Report October, 2012).It is recommended, however, that patients with pacemakers perform SUDOSCAN testing in the presence of a medical doctor.
Cardioselective beta-blockers should not interfere with SUDOSCAN measurements. However additional testing is needed to confirm this and the potential effect of other cardiovascular drugs.
As SUDOSCAN is based on measurements of sweat glands innervated by cholinergic small C fibers, drugs with significant anti-cholinergic effects can influence SUDOSCAN results.
Tricyclic antidepressant agents, in particular amitriptyline, can significantly decrease ESC measured by SUDOSCAN. Therefore results must be interpreted with caution when anti-cholinergic agents are utilized. The effect of medications on ESC measurements is a critical factor and under continuous investigation in ongoing studies.
At the moment an observational study performed in an outpatient clinic in Germany on two groups of patients with type 2 diabetes, the first one receiving insulin, the second one other anti-diabetic drugs, demonstrated that after one year of follow-up patients receiving insulin had an improvement in their conductances while the second group experienced a small decrease. These preliminary results have to be confirmed in a larger study population.
Studies on patients with skin conditions affecting their palms and/or soles (e.g. atopic dermatitis) have not yet been completed and therefore it is not known how these disorders could affect SUDOSCAN results. Note: the skin of the palms and soles MUST be intact (without open sores, ulcers, or lacerations) for SUDOSCAN results to be valid.
Palms and soles must be free of dirt, ointments, or lotions prior to testing. Washing with soap and water is always the best option. The palms and soles must be DRY prior to placement on the electrodes. Note: the manufacturer-approved cleaning solution for the electrodes shall NOT be applied directly or indirectly to a patient’s skin prior to testing. The cleaning solution is intended for Sensor Plate cleaning only.
The size of a patient’s feet and hands do not impact test results. If, however, the surface area of a hand or foot is severely reduced (e.g. from an antecedent second or third degree burn or an amputation) with obliteration of the sweat glands, then SUDOSCAN results can be affected.
What is the shelf life of the electrodes?
SUDOSCAN sensor plates are made of high-grade stainless steel, which are more resistant to corrosive influences. Their shelf life depends on how often they are used and how well they are maintained. Cleaning must be performed after each patient using a manufacturer-approved cleaning solution only. This cleaning is necessary in order to: maintain hygiene and safety standards; stop the corrosion process initiated by the electrochemical reaction during each test; and ensure the highest test accuracy. Sensor plates are therefore replaced after every 100 successful tests. This ensures the physician that the results obtained for each test are of the highest quality.