Objectives: To examine the effect of varying frequency, intensity, and pulse duration of transcutaneous electrical nerve stimulation (TENS) on primary hyperalgesia (increased response to a noxious stimuli) to heat and mechanical stimuli induced by carrageenan paw inflammation in rats.
Design: Inflammation was induced by injection of 3% carrageenan into the hindpaw. Two frequencies (high, 100 Hz; low, 4 Hz), 2 intensities (high, motor; low, sensory), and 2 pulse durations (100 microsec, 250 microsec) were applied for 20 minutes to the inflamed paw. The paw withdrawal latency (PWL) to radiant heat, threshold to mechanical stimuli, and spontaneous pain-related behaviors were measured before and 4 hours after induction of inflammation, after TENS, and at 8, 12, and 24 hours after inflammation. A 3-factor (frequency, intensity, pulse duration) repeated-measures (time) design was used to analyze the changes in PWL. Mechanical threshold and spontaneous pain-related behaviors were compared for frequency, intensity, and pulse duration with a Kruskal-Wallis analysis of variance.
Results: For changes in PWL to heat, there was an effect for time (p = .0001) and frequency (p =.0001), but not for intensity (p = .45) or pulse duration (p = .21). For changes in mechanical threshold, there was also an effect for frequency (p = .007), but not for intensity (p = .055) or pulse duration (p = .058), after treatment with TENS. High-frequency TENS significantly reduced the primary hyperalgesia to heat and mechanical stimuli when compared with controls receiving no TENS or treatment with low-frequency TENS. High-frequency motor TENS also reduced spontaneous pain-related behaviors for 1 day after treatment.
Conclusion: High-frequency TENS reduces primary hyperalgesia to heat and mechanical stimuli for up to 1 day after treatment. In contrast, low-frequency TENS is ineffective in reducing primary hyperalgesia. Varying intensity or pulse duration had no effect on the degree of antihyperalgesia produced by high-frequency TENS.