Somatotopic arrangement of sudomotor axon reflexes in human skin

Background: In neuropathies impaired sweating is often one of the first symptoms, therefore the evaluation of the sweating reaction is a valuable method for the early detection of neuropathies. Quantitative sudomotor axon reflex testing (QSART) has been employed for that purpose and measures the amount of sweating induced by iontophoretic stimulation with acetylcholine quantitativelly. For measuring the extent of the sudomotor axon reflex iodine starch staining (Minor testing) has been used. The aim of our study was to compare both tests on three different sites on the leg: foot, lower leg and upper leg.

Methods: QSART and iodine starch staining after iontophoretic stimulation with acetylcholine were measured on 15 male volunteers (mean age: 25; range 24–27 years) on the left resp. right leg during one single session.

Results: QSART response, measured as area under the curve (AUC), was maximal at the lower leg (789 RU) smaller at the dorsum of the foot (501 RU, p=0.3) and even smaller at the upper leg (417 RU, p=0.3). The difference between lower leg and foot was significant (p<0.05). The size of the sudomotor axon reflex and the QSART response correlated (p<0.01). The sudomotor flare size was also biggest on the lower leg (39cm²) followed by the foot dorsum (28cm², p<0.05), where the flare was smaller than on the upper leg (16cm², p=0.6). The difference between lower leg and foot again was significant (p<0.01).

Conclusions: Both tests showed the same somototopic arrangement. The sweating was maximal at the lower leg, of a medium extent at the foot, and smallest at the upper leg. In contrast to the vasodilator flare mediated by nociceptive c-fibers, the sudomotor flare had its maximal extent on the lower leg. Our results also indicate that a larger innervation territory of sudomotor nerves is accompanied by a higher sweat output.