High-Frequency Transcutaneous Electrical Nerve Stimulation Alters Thermal But Not Mechanical Allodynia Following Chronic Constriction Injury of the Rat Sciatic Nerve.

High-Frequency Transcutaneous Electrical Nerve Stimulation Alters Thermal But Not Mechanical Allodynia Following Chronic Constriction Injury of the Rat Sciatic Nerve

Somers DL, Clemente FR (Department of Physical Therapy, John G Rangos Sr School of Health Sciences, Duquesne University, Pittsburgh, Pa), Arch Phys Med Rehabil. 1998;79:1370-1376.

This study compared the effects on pain threshold of daily intervention provided over 14 days when initiated at 3 different times: immediately, 20 to 30 hours after nerve insult, or 3 days after nerve insult. The subjects were 6 groups of 7 to 11 rats per group. Four groups had the right sciatic nerve ligated; 3 groups were provided transcutaneous electrical nerve stimulation (TENS) initiated at 3 different times after surgery, and the fourth group received the same treatment as the immediate intervention group but did not receive TENS. Two groups underwent sham surgery; one group received a TENS intervention comparable to the immediate intervention group, and the other group was not treated.

TENS electrodes were placed above the right dorsal rami of the lumbar spinal nerves 1 through 6. All treatments lasted 60 minutes except for the first treatment of the immediate intervention group, which lasted 90 minutes. The frequency was set at 100 Hz, and the stimulation intensity was 80% of the intensity that resulted in a visible contraction. The response to mechanically induced pain was measured by pressing a calibrated Semmes-Weinstein monofilament against the plantar aspect of the hind paws. The smallest filament that produced a withdrawal movement indicated the mechanical pain threshold. Withdrawal latency from a radiant heat source served as the measure of thermal pain threshold. Responses were measured at 2, 7, 12, and 14 days after sciatic nerve ligation or sham surgery.

The control group of rats with ligated sciatic nerves demonstrated mechanical and thermal allodynia. The effect of TENS on thermal allodynia was greatest in the groups treated immediately or within a day's time, but the effect varied greatly within each group, from no effect to preventing allodynia and raising the withdrawal latency above that of the contralateral paw. No effect on thermal allodynia was noted when TENS was initiated 3 days after insult. Conversely, no effect of TENS on mechanical allodynia was noted, with the exception of assessments taken on days 12 and 14 of the group whose treatment was initiated 3 days post-trauma.

Because the best response occurred with those rats that received treatment soon after insult, the authors hypothesized that TENS may interfere with mechanisms within the nervous system that result in thermal hypersensitivity. The authors believed that the lack of improvement in mechanical allodynia using TENS may be related to the timing of the assessments, which were made at 12 to 18 hours after the last treatment. The effects may have been transient. The authors also believed it was possible that low-frequency TENS may result in analgesia through the release of opiates and may activate regions not affected by high frequency.

Barbara Hanley, PT
Helen Hayes Hospital
West Haverstraw, NY