The effect of acupuncture duration on analgesia and peripheral sensory thresholds

Abstract

Background: Acupuncture provides a means of peripheral stimulation for pain relief. However, the detailed neuronal mechanisms by which acupuncture relieves pain are still poorly understood and information regarding optimal treatment settings is still inadequate. Previous studies with a short burst of unilateral electroacupuncture (EA) in the Tendinomuscular Meridians (TMM) treatment model for pain demonstrated a transient dermatomally correlated bilateral analgesic effect with corresponding peripheral modality-specific sensory threshold alterations. However, the impact of EA duration on the analgesic effect in this particular treatment model is unknown. To obtain mechanistically and clinically important information regarding EA analgesia, this current prospective cross-over study assesses the effects of EA duration on analgesia and thermal sensory thresholds in the TMM treatment model.

Methods: Baseline peripheral sensory thresholds were measured at pre-marked testing sites along the medial aspects (liver and spleen meridians) of bilateral lower extremities. A 5-second hot pain stimulation was delivered to the testing sites and the corresponding pain Visual Analog Scale (VAS) scores were recorded. Three different EA (5Hz) stimulation durations (5, 15 and 30 minutes) were randomly tested at least one week apart. At the last 10 seconds of each EA session, 5 seconds of subject specific HP stimulation was delivered to the testing sites. The corresponding pain and EA VAS scores of de qi sensation (tingling) during and after the EA were recorded. The measurements were repeated immediately, 30 and 60 minutes after the EA stimulation. A four-factor repeat measures ANOVA was used to assess the effect of stimulation duration, time, location (thigh vs. calf) and side (ipsilateral vs. contralateral) of EA on sensory thresholds and HP VAS scores.

Results: A significant (P < 0.01) main effect of time and location with warm, cold and hot pain thresholds at the four testing sites without any significant difference in duration effect was observed. Similar time and location effects were observed with HP VAS with the longer durations (15 and 30 minutes) of stimulation showed a slower onset, but a more sustainable bilateral analgesic benefit than the short stimulation duration (5 minutes). The 15-minute stimulation resulted in an earlier onset of analgesic effect than the 30-minute stimulation paradigm.

Conclusion: Longer durations of EA stimulation provide a more sustainable analgesic benefit to hot noxious stimulation than a shorter duration of stimulation. The increase of cold threshold with sustained warm threshold temperature elevation as observed in the longer durations of EA suggests that as the duration of EA lengthened, there is a gradual shifting from an initial predominantly spinally mediated analgesic effect to a supraspinally mediated modulatory mechanism of thermal pain. The 15-minute stimulation appeared to be the optimal setting for treating acute pain in the lower extremities.

Figure 1

Dermatomal Relationship for EA and Testing Sites.

Figure 2

Cold threshold. The four-factor repeated measures ANOVA revealed a significant main effect of location ¶(F [1, 15] = 15.4, p < 0.01), main effect of time* (F [3, 45] = 3.1, p < 0.05), and main effect of side# (F [1, 15] = 5.7, p < 0.05), with the ipsilateral calf threshold temperatures being mostly lower than the ipsilateral thigh threshold temperatures.

Figure 3

Warm threshold. The overall four-factor repeated measures ANOVA revealed a significant main effect of location ¶(F [1, 15] = 82.9, p < 0.0001), significant main effect of time* (F [3, 45] = 35.7, p < 0.0001), and a significant interaction of location and time (F [3, 45] = 8.0, p < 0.001). The main effect of location was accounted for by the significantly higher thresholds in calf versus thigh at all time points.

Figure 4

Hot pain threshold. The four-factor ANOVA revealed a significant main effect of location ¶(F [1, 15] = 16.8, P < 0.001) and main effect of time * (F [3, 45] = 8.5, P < 0.0001).

Figure 5

Hot pain VAS. There is a significant main effect of duration €(F [2, 30] = 6.1, p < 0.01), a main effect of side (F [1, 15] = 6.3, p < 0.05), and a main effect of time * (F [3, 45] = 3.5, p < 0.05). In addition, a significant interaction of duration and location ¶(F [2, 30] = 6.6, p < 0.005) and of duration and time (F [6, 90] = 5.6, p < 0.0001) were noted.

Figure 6

De Qi sensation and VAS score.