Sudomotor skin responses to brain stimulation do not depend on nerve sensory fiber functionality

Evaluation of sudomotor innervation to the limbs was performed via electrical stimulation of the median nerve at the wrist and transcranial magnetic stimulation of the brain. Sympathetic skin responses (SSRs) from the palms and the soles were recorded in a patient suffering from a form of predominantly sensory neuropathy. Motor responses to nerve stimulation were present, even if with slower than normal conduction velocity. However, median, ulnar and sural nerve sensory potentials were absent. SSRs to median nerve stimulation at the wrist were missing on palms and soles. By contrast, during brain stimulation of the hand motor area with magnetic impulses, SSRs were reliably identifiable at all recording sites. It is hypothesized that SSRs to mixed nerve stimulation need normal functionality of afferent fibers, while those to brain stimulation can be elicited without any sensory information from the limbs.