Spatial extension of sudomotor axon reflex sweating in human skin

Acetylcholine (ACh) applied to human skin is known to elicit a sweat response, which consists of a direct muscarinergic (M3) activation of sweat glands and a nicotinic axon reflex response from sudomotor terminals. To visualize the extent of axon reflex sweating after ACh-iontophoresis, iodine starch staining was used. Iontophoresis was performed under occlusion at the peroneal aspects of the lower leg and the center of the foot dorsum of healthy volunteers (n = 10). Ten minutes after stimulation, the area of dark blue staining was recorded by a video camera. Control experiments were performed with saline, histamine, pilocarpine and nicotine iontophoresis. The stained area was measured and the maximum and minimum distance of its boundary from the edge of the iontophoresis probe was determined (maximum/minimum radius). Sizes of stained areas and maximum radii were significantly greater on the lower leg compared to the foot (P < 0.01). The median sizes of the stained areas on the leg were 14.6 cm2 and on the foot dorsum 8.0 cm2. The respective median maximum radii were 3.1 cm on the leg and 2.3 cm on the foot dorsum (median minimum, leg 1.1 cm, foot 0.8 cm). These results match microneurographic findings of innervation territories of sympathetic efferent units. Area sizes of stained skin showed a close correlation between both stimulation sites (R = 0.96, P < 0.01), i.e. the sizes of sweat responses on leg and foot show a constant relation (foot/leg = 0.57) in spite of their great interindividual variability. This novel technique of establishing sweat responses provides information on the size of sudomotor innervation territories and may be useful for clinical studies in patients with suspected impairment of sympathetic functions.