Is mechanical pain threshold after transcutaneous electrical nerve stimulation (TENS) increased locally and unilaterally? A randomized placebocontrolled trial in healthy subjects

Background and Purpose. It is not fully understood how transcutaneous electrical nerve stimulation (TENS) intensity affects mechanical pain threshold. Method. Sixty-six healthy volunteers (13 male, 53 female; 132 hands) without prior experience of TENS participated in the study, which comprised a randomized single-blind controlled trial. TENS was administered for 20 minutes through electrodes (25 x 25 mm) placed on the hands and forearms with a fixed frequency of 100 Hz and pulse duration of 150 mu s. TENS intensity was randomized and allocated in a concealed manner so that one arm received TENS with stimulation intensity set at participants' subjective sensory threshold and the other received TENS with stimulation intensity continuously adjusted by physiotherapists to a strong but comfortable non-painful stimulation. Observers were blinded to stimulation intensity levels. Results. Mechanical pain threshold increased significantly, by a mean total of 0.79 kg/cm(2) (95% confidence interval [95% CI]: 0.54-1.04) (p < 0.001) on the strong but comfortable non-painful stimulation side. The mean change in mechanical pain threshold on the sensory threshold side was 0.19 kg/cm(2) and did not reach statistical significance (95% CI-0.15 to 0.51). The mean stimulation intensity level for sensory threshold was 6.7 mA (95% CI: 5.65 to 7.83) which was significantly lower (p < 0.001) than the mean stimulation intensity for the strong stimulation, which was 20.5 mA (95% CI 16.6 to 24.4), respectively. The strong stimulation levels were, on average, 3.05 times higher than sensory threshold, but individual variations were large (range 1.2-6.1). Conclusions. TENS administered at a strong but comfortable non-painful stimulation intensity increases mechanical pain threshold ipsi-laterally in healthy subjects, whereas TENS administered at sensory threshold intensity does not. TENS may be ineffective if electrodes are placed contralaterally or distant to the pain site and if stimulation intensity levels are not titrated to subjective strong levels. Further clinical trials are needed to clarify if these findings may also be generalized to populations of chronic pain sufferers. Copyright (C) 2007 John Wiley & Sons, Ltd.