Somatotopic arrangement of sudomotor axon reflex sweating in humans

Background Impaired sweating may be one of the first symptoms in neuropathies, and therefore the evaluation of sweating might facilitate their early detection. Sudomotor axon reflexes can be quantified by two different methods: quantitative sudomotor axon reflex testing (QSART) measures the amount of local sweating, whereas staining with the iodine starch reaction assesses the extension of the sudomotor axon reflex area. The aim of our study was to compare both tests at three different sites on the leg: foot, lower leg and thigh. Methods QSART and iodine starch staining after iontophoretic stimulation with acetylcholine were performed on 15 male volunteers (mean age: 25; range 24–27 years) on the left resp. the right leg during a single session. Results QSART response, measured as area under the curve (AUC), was maximal at the lower leg (911 AUC), smaller at the dorsum of the foot (585 AUC) and even smaller at the thigh (480 AUC). The difference between lower leg and thigh was significant (p < 0.02). The sudomotor axon reflex area was also biggest on the lower leg (39 cm2) followed by the foot dorsum (28 cm2), and then the thigh (16 cm2). The differences between lower leg and thigh as well as between lower leg and foot were significant (p < 0.01, resp. p < 0.04). The size of the sudomotor axon reflex areas and QSART responses were correlated (p < 0.01). Conclusions QSART and sudomotor axon reflex areas had similar somatotopic arrangements in human skin. The bigger the axon reflex area was the stronger the QSART response was. This indicates that the size of the innervation territories of sudomotor fibres covaries with the amount of local sweat production. The latter is a surrogate for increased sweat gland density or capacity in skin areas of dense sudomotor innervation.