Thermal imaging of microhemodynamics and integrated mapping of sweat gland activity as a method for diagnosing autonomic neuropathy

Background and Objectives: Dynamic infrared thermography is a promising method for imaging physiological processes, including not only microhemodynamics but also the activity of eccrine sweat glands. Impaired sweat gland activity in the examined areas may primarily indicate damage to cholinergic nerve fibers. This study aims to develop and apply a novel approach for analysing dynamic thermograms, which is based on the separation of the original temperature signal into two independent components–vasomotor (microhemodynamics) and sudomotor (sweat gland activity)–for the detection of neuropathy in patients with type 2 diabetes mellitus. Materials and Methods: Dynamic thermograms were recorded using a cooled camera with a temperature sensitivity of 0.02°C during a breathing test, which involved three sharp inhalations performed at 2-minute intervals. Thermograms were obtained from 11 healthy subjects and 11 patients with type 2 diabetes mellitus; the mean age of the participants was 58 ± 7 years. Thermogram pixels were classified into two categories: in the first category, temperature dynamics were influenced solely by microhemodynamics, while in the second category, they were influenced by the combined effect of microhemodynamics and sweat gland activity. To classify thermogram points, the values of the modulus of the derivative of temperature fluctuations with a threshold above 0.03°C/s were used. Visualization of regions with active sweat glands was achieved by integrating the information obtained over the entire duration of the experiment. Results: The use of the obtained integrated maps has made it possible to determine the relative area occupied by active sweat glands in the group of patients with type 2 diabetes mellitus (5.25%) and in the group of healthy subjects (22.25%). The results indicate a statistically significant decrease in the area of active sweat glands in the group of patients with diabetes mellitus compared to the control group. This finding may be a sign of impaired cholinergic sympathetic innervation and the presence of diabetic neuropathy in the upper limbs. Conclusions: The integrated map of functioning sweat glands enables a clinician to identify the body surface area with impaired autonomic function in the limbs, particularly for assessing the severity of peripheral neuropathy in patients with type 2 diabetes mellitus.

thermography, diabetes mellitus, hemodynamic, sweat glands, neuropathy, autonomic function, imaging, derivative, temperature