Depressive effects of segmental and heterotopic application of transcutaneous electrical nerve stimulation and piezo-electric current on lower limb nociceptive flexion reflex in human subjects.

OBJECTIVES: To evaluate quantitatively the effectiveness and clinical relevance of various parameters of electrical stimulation used to relieve acute and chronic pain and to further knowledge of the mechanisms that may underlie the physiologic response produced by the transcutaneous application of each type of current.

DESIGN: A nociceptive flexion reflex (RIII reflex) elicited in the lower limb by electrical stimulation of the sural nerve at the ankle was studied before, during, and after application of the following conditioning stimuli: (1) non-noxious transcutaneous electrical nerve stimulation (TENS)--low-intensity (2mA), short-duration (0.1 msec), and high-frequency (100Hz) rectangular pulses (TENS1); (2) noxious TENS--high-intensity (20mA), long-duration (2 msec), and low-frequency (3Hz) rectangular pulses (TENS2); (3) noxious piezo-electric current (PEC)--high-voltage, low-charge, low-intensity, and low-frequency rectangular pulses delivered by a piezo-electric ceramic device (PECs1); (4) a sham PEC situation in which the piezo-electric device did not produce any electric current (Sham PEC). Each conditioning stimulus (TENS1, TENS2, PECs1, Sham PEC) was applied for a 2-minute period either segmentally on the sural nerve itself or heterotopically on the skin overlying the first interosseous space of the contralateral hand.

PARTICIPANTS: Twenty-four healthy volunteers (14 women, 10 men, 19 to 52 years of age), who were briefed and familiarized with the experimental procedure. During the experimental sessions, subjects were prone in bed to ensure muscular relaxation.

MEASURES: Value of the nociceptive RIII reflex before, during, and after application of conditioning stimuli in the four procedures described above. This reflex was selected because it has been shown to be an objective and physiologic correlate of pain.

RESULTS: Non-noxious TENS1 stimulation applied segmentally produced inhibitions of the RIII reflex only during the 2-minute conditioning period. When given segmentally, the noxious TENS2 stimulation produced a facilitatory effect during the 2 minutes of application, followed by significant inhibitory after-effects. The administration of TENS2 heterotopically resulted in inhibitions of the RIII reflex both during and after the 2-minute conditioning period. Application of PECs1, whether segmentally or heterotopically, produced powerful and long-lasting inhibitory after-effects, especially with the heterotopic paradigm. These effects were associated with long-lasting local changes to the skin of the neurogenic inflammation type, which were well tolerated by all subjects. Application of Sham PEC did not result in significant modification of either the RIII reflex or the skin.

CONCLUSIONS: These data are discussed in terms of possible spinal and supraspinal mechanisms involving inhibitory descending controls and underline the potential clinical use of PECs1 in the treatment of pain.