Firing properties of sudomotor neurones in hyperhidrosis and thermal sweating

Objectives: Idiopathic palmar-plantar hyperhidrosis is characterized by excessive sweating of the palms and feet, and is commonly treated by transthoracic regional sympathicotomy. As the condition is believed to be due to a high sudomotor drive, we wanted to assess the firing properties of individual sudomotor neurones in this state of sympathoexcitation, extending our recent work on other pathologies associated with high sympathetic nerve activity. Methods: Single-unit recordings were made from eight sudomotor neurones supplying the fingers via tungsten microelectrodes inserted percutaneously into the median nerve at the wrist or upper arm. Results: Typical of sudomotor, muscle vasoconstrictor and cutaneous vasoconstrictor neurones recorded in healthy individuals in states of high sympathetic drive, all units had low firing probabilities (active in only 30.0 ± 6.7 (SE) % of cardiac intervals) and primarily fired only once per heart beat. The percentage of cardiac intervals in which the neurones generated 1, 2, 3 or 4 spikes was 60.4 ± 6.3, 22.9 ± 3.9, 9.7 ± 2.1 and 3.4 ± 1.3%, respectively. For comparison, these values were 77.6 ± 7.7, 15.0 ± 4.1, 4.6 ± 2.3 and 1.8 ± 1.3% for eight sudomotor neurones innervating the hairy skin of the foot during thermally-induced sweating in normal subjects. Interpretation: We conclude that the firing properties of spontaneously active sudomotor neurones in subjects with hyperhidrosis are similar to those of sudomotor neurones active during thermal sweating, reflecting an increase in central sympathetic drive to the sweat glands in hyperhidrosis.