Adjusting Pulse Amplitude During Transcutaneous Electrical Nerve Stimulation Does Not Provide Greater Hypoalgesia

Abstract

Objectives:

Transcutaneous electrical nerve stimulation (TENS) is an electrotherapeutic modality commonly used in rehabilitation to relieve pain. Adjusting pulse amplitude (intensity) during TENS treatment has been suggested to overcome nerve habituation. However, it is still unclear if this procedure leads to greater hypoalgesia. The aim of this study was to determine if the hypoalgesic effect of TENS is greater when pulse amplitude is adjusted throughout the TENS treatment session in chronic low-back pain patients.

Design:

Randomized double-blind crossover study.

Setting:

Recruitment and assessment were conducted at the Clinique universitaire de réadaptation de l'Estrie (CURE) of the Faculty of Medicine and Health Sciences of the Université de Sherbrooke.

Participants:

Twenty-one volunteers with chronic low-back pain were enrolled and completed this investigation.

Interventions:

Each patient received two high-frequency TENS treatments on two separate sessions: (1) with adjustment of pulse amplitude and (2) without pulse amplitude adjustment.

Main outcome measures:

Pain intensity and unpleasantness were assessed before, during, and after TENS application with a 10 cm visual analog scale.

Results:

Both TENS conditions (with and without adjustment of intensity) decreased pain intensity and unpleasantness when compared with baseline. No difference was observed between the two stimulation conditions for both pain intensity and unpleasantness.

Conclusion:

The current results suggest that adjustment of pulse amplitude during TENS application does not provide greater hypoalgesia in individuals with chronic low-back pain. Future studies are needed to confirm these findings in other pain populations.

Introduction

Transcutaneous electrical nerve stimulation (TENS) is a simple therapeutic tool commonly used in rehabilitation to decrease pain.¹ Although TENS has now been studied for almost half a century,^2,3 its clinical efficacy still remains a matter of debate.^4,5 One potential reason for the inconsistent results could arise from the stimulation parameters used during TENS application (e.g., pulse amplitude, pulse duration, frequency). The pulse amplitude (intensity) of TENS stimulation), in particular, appears to be an important parameter, with studies showing superior hypoalgesia with a greater pulse amplitude.^4,6,7

An important aspect of TENS delivery related to stimulation intensity concerns the necessity to adjust pulse amplitude (i.e., stimulation intensity) during TENS application. Many electrotherapeutic textbooks suggest that it is important to adjust pulse amplitude during TENS application to compensate for nerve habituation and maximize TENS hypoalgesia.^8,9

Surprisingly, very few studies have investigated the effects of pulse amplitude adjustment on TENS-induced hypoalgesia.⁶ Pantaleao et al. noted that the hypoalgesic effect of TENS, measured in healthy participants using mechanical pain thresholds, was greater when pulse amplitude was adjusted during treatment compared with when TENS intensity was kept constant.⁶ However, given the population studied and pain paradigm used, the results of Pantaleao et al. can hardly be transferred in clinical settings.

In an attempt to address this issue, Elserty et al. recently conducted a similar study, in a population of chronic low-back pain. Contrarily to Pantaleao et al., Elserty et al. showed that adjusting pulse amplitude during TENS did not produce superior pain relief when compared with a fixed TENS amplitude protocol.¹⁰ Yet, Elserty et al. measured the effect of TENS several weeks after its application. This can be problematic, given that the greatest effect of TENS on pain is observed during and immediately after its application.⁴

The present study addresses these limitations by looking at the short-term TENS effect of pulse amplitude adjustment in a population of chronic low-back pain individuals. More specifically, the objective of this study was to compare, in individuals suffering from chronic low-back pain, the short-term hypoalgesic effect of two TENS treatments: (1) with adjustment of pulse amplitude and (2) without adjustment of pulse amplitude. Based on the results of Pantaleao et al.,⁶ we hypothesized that pain reductions after TENS would be higher when pulse amplitude (intensity) is adjusted compared with when pulse amplitude is kept constant during the entire treatment session.

Methods

Participants

Twenty-one chronic low-back pain patients participated in this study. Participants were recruited through advertisements posted in local hospitals and physical therapy clinics. All participants were 18 years and older, suffered from low-back pain for more than 6 months, and had pain at rest. Participants had to refrain from using analgesics and consuming caffeine 6 h before testing, and from consuming nicotine 2 h before testing. For security reasons, participants were excluded if they were implanted with a pacemaker, had been diagnosed with cancer, or were pregnant.¹ Individuals with prior experience with TENS treatments and patients taking opioids were also excluded.

Experimental design

A randomized, double-blind, crossover trial was conducted. Experimentation and assessments were conducted at the Clinique universitaire de réadaptation de l'Estrie (CURE) of the Faculty of Medicine and Health Sciences of the Université de Sherbrooke between March 2013 and April 2014. Each participant underwent two TENS sessions: one with adjustment of pulse amplitude and one without adjustment of pulse amplitude. Approximately, 1 week interval separated the two TENS sessions.

Randomization

The order of presentation for both conditions was determined using a random number table designed by the statistician of the Research Center on Aging, using Minitab Statistical Software (version 15.0; State College, PA). The principal investigator assigned the participants to the two groups. The participants and the person assessing the outcomes were blinded to the interventions during the entire study. Eleven participants received the adjusted stimulation first, and 10 participants received the unadjusted stimulation first. No changes were made to the design or to the methods (including outcomes) after trial commencement. All the 21 participants completed the study.

TENS stimulation protocol

At each visit, the participant was comfortably installed (in prone position) on a mobilization table. The painful area was determined and carefully marked with a pen to optimize electrode placement.

TENS stimulations were delivered with 48 cm² carbon electrodes placed on the painful area and connected to a Sonopuls 692 apparatus (Enraf-Nonius, Rotterdam, Netherlands). The electrodes were placed to obtain maximal paresthesia over the painful area of each participant. Electrode positioning was reassessed with the TENS stimulator turned ON to ensure that the induced paresthesia covered the painful region entirely. In cases where stimulations did not properly cover the painful area, the stimulator was turned OFF and the electrodes were repositioned.⁵ For both sessions, the stimulation was applied for 25 min with a frequency of 100 Hz and pulse duration of 60 μsec using rectangular biphasic symmetrical waveforms (conventional TENS).^4,11 No harms or unintended effects were reported by the participants.

Pulse amplitude (intensity) was initially adjusted to generate the strongest, nonpainful, tingling sensation at the beginning of both sessions. For the adjusted TENS condition, the pulse amplitude was regularly adjusted (i.e., at the 5th, 10th, 17th, and 23rd min) to obtain the same level of paresthesia throughout the treatment session. During the unadjusted condition, the therapist pretended to adjust parameters on the TENS apparatus at the same time intervals during stimulation (i.e., 5th, 10th, 17th, and 23rd min; to keep patients and the person assessing the outcomes blinded). However, all TENS parameters (including pulse amplitude) were kept constant throughout the unadjusted session.

Pain evaluations

Pain intensity and unpleasantness were evaluated using two separate visual analog scales (VAS; pain intensity: 0 = no pain, 10 = unbearable pain; pain unpleasantness: 0 = not unpleasant, 10 = extremely unpleasant). The distinction between these aspects of pain was clearly explained to the participants before testing. Participants were asked to evaluate the intensity and unpleasantness of their low-back pain on four separate occasions: before TENS application (T0), during TENS application (i.e., after 20 min of stimulation; T1), immediately after stimulation (T2), and 15 min after stimulation (T3). All pain measures were taken while the patient maintained the prone position.

At the end of each visit, the Patient's Global Impression of Change (PGIC) scale, a single-item rating of improvement on a 7-point scale (range: −3 = very much worse to 3 = very much improved), was used to document the overall impressions regarding the efficacy of the TENS application.¹²

Sample size calculation

The study was designed to detect a difference of 2 points on the 0–10 VAS (clinically important difference).¹³ To detect a 2-point difference between both TENS conditions, with 80% statistical power and a 5% significance level, we determined that 21 adults had to be enrolled in the study (estimated standard deviation of 3.1, based on preliminary results).

Statistical analysis

Because of the relatively small number of participants included in the study, and since the data were not normally distributed, nonparametric tests were used. All tests were performed using SPSS (version 17; IBM Corp, Armonk, NY). Friedman tests were used to compare the pain scores before, during, and after TENS application (TIME variable). This allowed us to determine if each TENS session (adjusted or unadjusted) influenced pain perception. Wilcoxon signed-rank tests were used to compare the pain scores across the two TENS treatments for the same time measure (CONDITION variable). This allowed us to directly compare the efficacy of the adjusted and unadjusted TENS sessions. The Wilcoxon signed-rank test was also used to compare the PGIC score reported between the two TENS conditions. For all analyses, differences were considered to be significant if p < 0.05 (two-tailed) was obtained. Bonferroni corrections were not applied to minimize the risk of committing a Type-II error.¹⁴

Previous studies have observed that the order of presentation can influence the results of clinical trials.^15,16 Therefore, we performed between-subject analyses to determine if the pattern of hypoalgesic response observed following adjusted or unadjusted TENS application was influenced by the order of presentation. To do this, delta pain scores, representing pain reduction experienced during TENS application (delta pain score = pain at baseline − pain during or after TENS), were calculated and compared between participants who received adjusted TENS during their first session and those who received unadjusted TENS during their first session.

Results

Characteristics of the participants

The characteristics of the participants, including McGill Pain Questionnaire,¹⁷ Brief Pain Inventory,¹⁸ and Pain Catastrophizing Scale¹⁹ scores are presented in Table 1. Diagnoses for low-back pain included herniated disc (n = 7), spondylosis (n = 1), osteoarthritis of the lumbar spine (n = 5), scoliosis (n = 1), ankylosing spondylitis (n = 1) postfracture pain (n = 1), and idiopathic nonspecific mechanical low-back pain (n = 5).

Table 1.

Characteristics of Participants (n = 21)

Characteristics	Mean ± SD
Age (years)	47 ± 15
Gender	10 men
Duration of pain symptoms (months)	167 ± 183
McGill Pain Questionnaire (0–72)	30 ± 16
Impact of pain on physical function (Brief Pain Inventory) (0–70)	31 ± 18
Pain Catastrophizing Scale (0–52)	24 ± 12

SD, standard deviation.

Pulse amplitude during TENS stimulations

Pulse amplitude values throughout both TENS sessions are presented in Table 2. The TENS sensory thresholds were similar between the two conditions. Pulse amplitudes steadily increased during TENS application for the adjusted condition. As determined, there was no change in pulse amplitude throughout the treatment session for the unadjusted condition. The difference in pulse amplitude between the two conditions reached statistical significance only at the end of the TENS session (at the 23rd min; p = 0.02).

Table 2.

Pulse Intensity (Amplitude) and Density Throughout Transcutaneous Electrical Nerve Stimulation Sessions (Mean ± SD)

	Intensity (mA)		Density (mA/cm²)
	Adjusted	Unadjusted	Adjusted	Unadjusted	p^a
Sensory threshold	16.0 ± 5.6	16.0 ± 6.2	0.33 ± 0.12	0.33 ± 0.13	0.6
0 Min	48.3 ± 24.7	53.8 ± 28.0	1.00 ± 0.52	1.12 ± 0.58	0.1
5 Min	52.8 ± 25.9	53.8 ± 28.0	1.10 ± 0.54	1.12 ± 0.58	0.9
10 Min	56.1 ± 26.2	53.8 ± 28.0	1.17 ± 0.55	1.12 ± 0.58	0.4
17 Min	59.8 ± 27.3	53.8 ± 28.0	1.25 ± 0.57	1.12 ± 0.58	0.1
23 Min	62.9 ± 28.1	53.8 ± 28.0	1.31 ± 0.59	1.12 ± 0.58	0.02^*
p ^b	<0.001^*	1.0	<0.001^*	1.0

Wilcoxon signed-rank test between the adjusted and unadjusted TENS conditions.

Wilcoxon signed-rank test between 0, 5, 10, 17, and 23 min.

Statistically significant.

mA, milliampere; SD, standard deviation; TENS, transcutaneous electrical nerve stimulation.

Order of presentation

Analyses revealed that the order of presentation did not influence the pattern of results (i.e., similar hypoalgesia following TENS for participants who received adjusted TENS during their first visit and participants who received unadjusted TENS during their first visit; all p-values >0.25).

Pain intensity

Mean pain intensity scores obtained before, during, and after TENS applications for each condition are shown in Figure 1. As can be seen from Figure 1, there was a decrease in pain intensity during and after TENS for both the adjusted and unadjusted conditions. These observations were confirmed by Friedman tests, which revealed a significant change in pain intensity over time for the two TENS conditions (all p-values <0.01). Post hoc Wilcoxon signed-rank tests showed that there was a significant reduction in pain intensity during (T1) and after (T2 and T3) TENS application when compared with baseline (T0) for both the adjusted and unadjusted TENS conditions (all p-values <0.01). Wilcoxon signed-rank tests also revealed no significant differences between the two TENS conditions for all time measures (all p-values >0.05), suggesting that pulse amplitude adjustment had no effect on pain intensity.

FIG. 1.

Mean pain intensity scores (±SEM) before (T0), during (T1), immediately after (T2) and 15 min after TENS (T3) for adjusted and unadjusted pulse amplitude TENS conditions. SEM, standard error of the mean; TENS, transcutaneous electrical nerve stimulation; VAS, visual analog scale.

Pain unpleasantness

Analyses performed for pain unpleasantness revealed a similar pattern of results (Fig. 2). More specifically, Friedman tests revealed that there was a significant change in pain unpleasantness across the different time measures for both TENS conditions (all p-values <0.001). Post hoc Wilcoxon tests showed that there was a significant reduction in pain unpleasantness during (T1) and after TENS (T2 and T3) when compared with baseline (T0) for both the adjusted and unadjusted TENS conditions (all p-values <0.05). Wilcoxon signed-rank tests also revealed no significant differences between the two TENS conditions for all time measures (all p-values >0.05) suggesting that pulse amplitude adjustment had no effect on pain unpleasantness.

FIG. 2.

Mean pain unpleasantness scores (±SEM) before (T0), during (T1), immediately after (T2), and 15 min after TENS (T3) for adjusted and unadjusted pulse amplitude TENS conditions. SEM, standard error of the mean; TENS, transcutaneous electrical nerve stimulation; VAS, visual analog scale.

Global impression of change

The PGIC scores for the adjusted and unadjusted conditions are presented in Table 3. The majority of patients reported improvements following TENS for both conditions. The Wilcoxon signed-rank test confirmed that there was no difference between the adjusted and unadjusted conditions (p = 0.78), indicating, again, that the two TENS conditions had similar effects.

Table 3.

Global Impression of Change After Transcutaneous Electrical Nerve Stimulation Sessions

PGIC score	Adjusted pulse amplitude (n)	Unadjusted pulse amplitude (n)
3 = Very much improved	3	3
2 = Much improved	12	12
1 = Minimally improved	5	4
0 = No change	0	1
−1 = Minimally worse	1	1
−2 = Much worse	0	0
−3 = Very much worse	0	0

PGIC, Patient's Global Impression of Change.

Discussion

In the present study, we evaluated the effect of pulse amplitude adjustment on the hypoalgesic effect of high-frequency TENS in patients suffering from chronic low-back pain. Contrary to our initial hypothesis, we observed no difference for pain intensity, pain unpleasantness, and PGIC scores between the adjusted and unadjusted TENS conditions, suggesting that adjusting pulse amplitude has no additional benefit for individuals suffering from chronic low-back pain.

During high-frequency TENS application, the tingling sensation induced by the constant electrical impulses often decreases over time, a phenomenon that has been attributed to changes in the excitability of the nerve membrane and that is commonly known as nerve habituation.^6,8,20 In the now famous gate control theory of pain, Melzack and Wall postulated that, during prolonged nonnociceptive stimulation, A-Beta fibers (responsible for the closing of the pain gate and for the associated hypoalgesic effect) could adapt, favoring a “reopening” of the pain gate.²¹ The premise of Melzack and Wall regarding the potential reopening of the pain gate probably contributed to the long-term belief regarding the importance of adjusting pulse amplitude during TENS treatments. Indeed, many recent electrotherapy manuals suggest that clinicians and patients should frequently readjust the intensity of TENS stimulation to counter nerve habituation and maximize the effect of TENS on pain.^6,9

Pantaleao et al. showed that the hypoalgesic effect of TENS (evaluated using mechanical pain thresholds in healthy individuals) was higher when pulse amplitude was frequently readjusted, compared with when pulse amplitude was kept constant.⁶ According to Pantaleao et al., adjusting pulse amplitude could prevent nerve habituation and activate a greater number of afferent fibers responsible for the closing of the pain gate.⁶ Current density adjustments made in the present study (+0.31 mA/cm²) were more important than the ones reported by Pantaleao et al. (+0.12 mA/cm²).⁶ Similarly, greater differences in current density between adjusted and unadjusted conditions at the end of the TENS treatments were noted in our study (0.19 mA/cm²) compared with Pantaleao et al. (0.09 mA/cm²).⁶ The absence of difference between the two TENS conditions noted in our study is, therefore, probably not attributable to insufficient increases in pulse amplitude/current density.

For their part, Defrin et al. observed that adjusting the pulse amplitude of interferential current (IFC) during treatment did not affect IFC pain relief (short- and long-term effects) in patients with chronic knee osteoarthritis.²⁰ Despite the important differences between TENS and IFC (e.g., presence of a carrier frequency, sinusoidal waveform), both electrotherapeutic modalities rely on the same physiologic mechanism to alter pain perception (i.e., stimulation of large afferent fibers activating gate-control inhibitory interneurons in the substantia gelatinosa).²² More recently, Elserty et al. reported that adjusting pulse amplitude of TENS did not produce greater hypoalgesic effect, compared with fixed TENS amplitude, in a population of chronic low-back pain patients.¹⁰ As mentioned previously, an important drawback of this investigation is that pain evaluations were obtained several weeks after TENS treatments. As pointed out by Sluka et al., this could be problematic given that the peak hypoalgesic effect of TENS is observed during and immediately after stimulations.⁴

In this study, pain measures were taken during and immediately after TENS application. Still, no difference could be observed between the two conditions (i.e., with and without adjustment of pulse amplitude). Taken together these results suggest that the vanishing sensation perceived during electrotherapeutic treatments does not affect the associated hypoalgesic effect. The discrepancies between the results of Pantaleao et al. and the results obtained in other studies can probably be explained by the different pain paradigms used (i.e., clinical vs experimental pain paradigm). Detection of pain thresholds is believed to rely on the activity of A-delta fibers, while ongoing clinical pain at rest (chronic low-back pain, osteoarthritis pain) probably mostly depends on C-fiber activation.²³

In the present study, stimulation intensity in the two TENS conditions was initially adjusted to obtain the strongest, nonpainful paresthesia possible. Several studies have highlighted the importance of adequate stimulation intensity to obtain successful hypoalgesia.^7,24
–26 In a review published by Claydon et al. on the effect of TENS on experimental pain, the authors concluded that high-intensity (i.e., high pulse amplitude) stimulations had a more favorable hypoalgesic profile compared with low-intensity stimulation parameters.⁷ The results of the present study do not invalidate these conclusions, but suggest that when pulse amplitude is fixed at the highest level at the beginning of a TENS treatment, any future increase in pulse amplitude is of little benefit. Hence, for individuals suffering from chronic low-back pain, we believe that high-frequency TENS treatments should be initiated at the strongest, although nonpainful, intensity, with no need to adjust intensity afterward to account for nerve habituation.

In this study, short-term effects of TENS were evaluated in a clinical pain population. The use of a clinical pain paradigm maximizes external validity, thus facilitating the generalization of our results for practitioners using TENS in clinical settings. However, clinical pain paradigms can present some limitations to the internal validity of a study. Several elements were used to minimize the threats to internal validity. For instance, the use of a crossover design allowed us to test two TENS protocols in the same sample of participants, thus eliminating the chances that the condition of the participant (e.g., pathologic origin of the low-back pain, baseline back pain intensity) affected the pattern of results. Similarly, only participants who reported stable, nonfluctuating low-back pain at rest were included. Another strength of this study is the evaluation of the TENS effect on both pain intensity and pain unpleasantness. Surprisingly, the specific effect of TENS on these two important components of pain has often been neglected in past studies.^23,27 Evaluating the effect of TENS on the sensory-discriminative (pain intensity) and affective (pain unpleasantness) components of pain is crucial, since treatments can differently affect each component of pain.²³

Even though the aim of the present study was not to test the clinical efficacy of TENS by comparing real and sham TENS, the absence of a control (placebo) remains a limitation. The heterogeneity of the participants included in our study (regarding diagnosis and supposed mechanisms of maintained chronic pain) must also be underlined. However, it should be noted that most cases of chronic low-back pain disorders have no specific diagnosis and, even when a specific radiologic diagnosis is reached, the underlying pain mechanism cannot always be assumed.²⁸

Another important limit of this study concerns the relatively small sample size. This study was designed to detect a difference of 2 points on a 0–10 VAS. According to Farrar et al., changes equal to or greater than two points must be obtained for the result to be considered clinically significant.¹³ A larger sample size could have potentially revealed a statistically significant difference between the two TENS protocols. This difference, however, would be of little clinical significance. At the moment, the tendency observed actually suggests that the unadjusted condition would be superior to the adjusted condition, which is contrary to the initial hypothesis.

Finally, it should be noted that the difference in pulse amplitude between the two groups was modest, with differences reaching statistical significance only at the end of the stimulation session. This situation can be attributed to a steady, but slow, increase in pulse amplitude throughout the adjusted pulse amplitude condition. Different results could potentially be obtained with longer TENS sessions or more important changes in pulse amplitude.

Conclusions

In the present study, we demonstrated that the hypoalgesic effect of high-frequency TENS is not affected by the adjustment of pulse amplitude during treatment in individuals with chronic low-back pain. Adjustment of pulse amplitude produced no superior effect for pain intensity, pain unpleasantness, and global impression of change. These results suggest that clinicians working with patients suffering from chronic low-back pain can achieve optimal results, even without adjusting the pulse amplitude during the treatment session. Future research is needed to confirm these results in other pain populations and to better understand the physiologic mechanisms underlying nerve habituation.

Footnotes

Acknowledgments

The authors wish to thank Vicky Teasdale Dubé, Stéphanie Morasse and Marie-Claude Girard for their help with data collection as well as Philippe Chalaye for his thoughtful comments on the article. This work was supported by a grant from the Fonds de recherche du Québec-Santé (FRQ-S).

Author Disclosure Statement

No competing financial interests exist.

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