Pain, 15 (1983) 157-165

Elsevier Biomedical Press

157

Transcutaneous Electrical Nerve Stimulation (TENS) as Compared to Placebo TENS for the

Relief of Acute Oro-facial Pain

Per Hansson and Anders Ekblom

Department of Physiology II, Karolinska Institutet, Stockholm (Sweden)

(Received 24 March 1982, accepted 14 June 1982)

Summary

The present paper describes the effect of high frequency, low frequency and piacebo TENS on acute oro-facial pain in 62 patients, attending to an emergency clinic for dental surgery; they had all suffered pain for l-4 days. The patients were randomly assigned to one of three groups receiving either high frequency (100 Hz), low frequency (2 Hz) or placebo TENS. In the two groups receiving TENS (42 patients) 16 patients reported a reduction in pain intensity exceeding 50%; out of these 16 patients, 4 patients reported complete relief of pain. In the placebo group (20 patients) 2 patients reported a pain reduction of more than 50%; out of these 2 patients, none reported a complete pain relief. Mechanical vibratory stimulation augmented the pain reduction obtained by TENS in 5 out of 10 patients.

Introduction

Since the presentation of the ‘gate control theory’ [8] numerous reports have been published concerning various methods for the treatment of pain based upon activa- tion of afferent nerve fibres by electrical stimulation. One of the most widely used methods, transcutaneous electrical nerve stimulation (TENS), has in several investi- gations proved to be effective in chronic pain conditions of different origin [cf. 4,6,7,10]. There are, however, relatively few reports concerning the effect of TENS in acute pain [cf. 1,3,11,16] and to our knowledge there are no comparative studies concerning the effect of high frequency, low frequency and placebo TENS in acute oro-facial pain. In the present paper we report on the pain reducing effect of high frequency and low frequency TENS as compared to placebo TENS in a group of 62 patients suffering from acute oro-facial pain. Vibratory stimulation was also used in 10 patients.

0304-3959/83/0~-O~O/$O~.~ 0 1983 Elsevier Biomedical Press

TENS was found to be more effective in reducing acute oro-facial pain than was

placebo TENS. Pain reduction obtained by using TENS was further increased if

followed by mechanical vibratory stimulation.

Methods

The effect of TENS and placebo TENS was studied in 62 patients aged 19-54 years (26 males and 36 females) admitted to an emergency clinic for dental surgery

for treatment of oro-facial pain. The most common causes for the pain were pulpal inflammation, apical periodontitis (including facial abscesses) and postoperative

pain following removal of a tooth. Patients with these diagnoses were represented in equal numbers in the test groups and in the placebo group. All patients suffered from acute pain; most of the patients had experienced pain for l--4 days. None of them had taken analgesics within 6 h before submitted for treatment. All the

patients were examined, told the diagnosis and asked if they would take part in the experiments. Those willing to participate were informed about their role in the

experiment. Furthermore the subjects were told that they might or might not experience pain relief as well as aggravation of pain during stimulation. Every effort

was made to avoid suggestion. The patients were told that they could stop the stimulation at any time and all the patients were informed that they would get a

conventional dental treatment after termination of the stimulation. The patients were assigned randomly to one of the three groups, high frequency,

low frequency or placebo TENS. They were asked to rate their pain intensity before they received any stimulation, by using a 5-graded verbal scale - light, light to

K M-S

al 5

Fig. 1. Number of patients with different degrees of pain (L = light pain, M = moderate pain, S = severe pain) which were treated with high frequency (100 Hz), low frequency (2 Hz) or placebo TENS. Patients

who experienced pain reduction exceeding 50% are represented by black areas.

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moderate, moderate, moderate to severe or severe pain. The ratings of the different patients are shown in Fig. 1. The subjects also rated their pain intensity before the

stimulation period using a visual analogue scale consisting of a horizontal line on a card. The words ‘no pain’ and ‘worst pain ever’ were placed at the left respectively

right extreme end of the line. The patients were instructed to mark the line at a point

representing their pain [5]. During the period of stimulation the patients continuously rated their pain by

using a device with a graphic rating scale, connected to an ink-writer out of sight of

the patient 191. After the stimulation period they were again asked to rate their pain intensity on a visual analogue scale and the value observed was compared with that

scored by graphic rating. The results obtained with the two methods were consistent. Due to consideration for the patients it was generally not possible to study

post-conditioning effects for more than 30 min. The patients who had used analgesics to reduce their pain were asked to compare

the effect of TENS with that of medication.

High frequency TENS (100 Hz) Twenty-two patients received high frequency TENS for a period of 30 min. The

stimulator (CEFAR SIII, Lund, Sweden) produced monopolar square wave pulses, duration of 0.2 msec, frequency 100 Hz. A pair of conducting rubber electrodes, each measuring 2 cm x 3 cm was positioned on the skin overlaying the painful area. The stimulus intensity was adjusted to 2-3 times the perception threshold, i.e. the

current needed to produce a tingling non-painful sensation from the stimulated area. The electrical output from the stimulator was adjusted during TENS in order to

maintain a constant tingling sensation.

Low frequency TENS (2 Hz)

Twenty patients received low frequency TENS, according to the method described by Eriksson and Sjolund [2], at a frequency of 2 Hz, i.e., a 71 Hz pulse train with a total duration of 84 msec delivered at 2fsec (monopolar square wave pulses). The intensity used was adjusted to 3-5 times the perception threshold which produced non-painful muscular contractions in the stimulated area. The overall procedure was the same as that used with 100 Hz TENS.

Placebo TENS Twenty patients received in all ways, except two, the same treatment as the two

groups receiving TENS. One difference was that the TENS stimulators used were

not equipped with batteries; and the other difference was that these patients were told that some people may not experience the stimulation. For practical reasons a

double-blind technique could not be used.

Mechanical vibratory stimulation

In 10 patients, in whom TENS produced 30% or less reduction of pain, mechani- cal vibratory stimulation at a frequency of 100 Hz was also tested. This was done l-5 min after the discontinuation of the TENS session. The patients were stimulated

for 10 min over the painful area (for a technical description see Ottoson et al. 191).

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Results

As seen in Fig, 1 patients having different pain intensities prior to stimulation were about evenly distributed on the basis of the 5-graded verbal scale (see Methods). Furthermore it is evident that the relative number of patients receiving high frequency TENS, low frequency TENS or placebo TENS were nearly the same

in each group with the same subjective pain intensity. Black columns in Fig. 1 indicate the number of patients who experienced pain reduction of more than 50%.

High frequency TENS

In 22 patients tested with 100 Hz TENS, 7 reported pain reduction exceeding 50% including 2 having a total pain relief (Fig. 2). In less than 5 min after discontinua-

tion of electrical stimulation 4 of the 7 patients reported a gradual return of pain which left them with a post-conditioning pain relief of less than 50%. In the

remaining 3 patients the post-conditioning relief exceeded 50% for a period longer than 10 min. Nine patients experienced a pain relief of less than 50%. Of the

remaining 6 patients 1 patient was unaffected and 5 patients reported increased pain during stimulation. In 4 of these last 5 patients the increased pain persisted for at

least 10 min after the stimulation period.

100 Hz 2 Hz (n=22) (n= 20)

r

II a b

Placebo (f-+=20)

c d a b c d a

-

-

I d

Fig. 2. Effects of high frequency (100 Hz), low frequency (2 Hz) and placebo TENS on subjective pam

intensity. (a) pain increase; (b) no changes in pain intensity; (c) pain reduction less than 50%: (d) pain

reduction exceeding 50%. Number of patients who experienced total pain relief represented by hatched

areas.

Low frequency TENS In 20 patients tested with 2 Hz TENS, 9 reported pain reduction exceeding 50%

including 2 having total pain relief (Fig. 2). Six patients reported a pain reduction of less than 50%. Three patients had no relief and 2 reported an increase in pain. In less than 5 min after discontinuation of electrical stimulation 2 of the 9 patients, who

during stimulation experienced a relief exceeding 50%, reported a gradual return of pain which left them with a post-conditioning pain relief of less than 50%, as

compared to 4 out of 7 in the group receiving high frequency TENS. In the remaining 7 patients the period of pain reduction after stimulation was longer than 10 min. One of the 2 patients who experienced pain increase during stimulation

reported increased pain for some minutes following cessation of stimulation.

Placebo TENS In the 20 patients receiving placebo TENS, 8 reported some degree of pain

reduction including 2 with pain reduction exceeding 50% (Fig. 2). Eight patients were unaffected by placebo TENS and 4 reported an increase in pain during

stimulation. In the 2 patients who experienced a pain reduction of more than 50% during placebo TENS, this effect persisted for at least 10 min after stimulation. There was also a sustained effect, with a duration exceeding 10 min, in the 4 patients

reporting increased pain during placebo stimulation.

Induction time for analgesia

To see if there was any difference in the induction time for partial and total pain relief between the methods used, we noted the time till the first report of subjective

pain reduction and of maximal pain reduction. In the two test groups, 100 and 2 Hz TENS, about 80% of the patients reported a reduction of pain within less than 5 min after onset of stimulation. The rest of the patients in the two test groups, experienc- ing some pain reduction, reported increased relief during the following 25 min

(Fig. 3). In the two test groups the time needed to obtain maximal pain reduction was less

than 15 min in about 35% of the patients while the rest of the patients experienced maximal effect after 15-30 min of stimulation. There was no significant difference in induction time for first detectable or maximal pain reduction between the patients receiving 100 or 2 Hz TENS.

Of the 8 patients experiencing pain relief in the placebo group 1 patient reported a first reduction of pain less than 5 rnin after onset of stinlulation while 7 patients reported that pain began to diminish after 5- 15 min. The time required for maximal pain reduction was in 5 of the 8 patients more than 15 min.

Vibratory stimulation

In 10 patients who after 30 min of TENS experienced less than 30% pain reduction, TENS was followed by vibratory stimulation at 100 Hz for 10 min. This resulted in a total pain reduction exceeding 50% in 5 of the 10 patients.

Comparison with the effect of analgesics

Twenty-three patients, who had used analgesics (mostly acetylsalicylic acid and paracetamol in doses of OS- 1 g for 1-4 times a day during the days before visiting

162

0’ I 1 . 5 10 15 20 25 30

TIME (MINI

&

2 Hz

10 ..

5 ..

04 1 .

5 10 15 20 25 30

T Placebo

5 10 15 20 25 30 5 10 15 20 25 30

F = 10 F

:

i 100 Hz

hrL t

10 I

5 10 15 20 25 30

TIME (MIN)

2 Hz

I -

0 * 5 10 15 20 25 30

I Pfacebo

Fig. 3. Induction time for first report of subjective pain reduction (left side diagrams) and maximal

obtained subjective pain reduction (right side diagrams) in patients who experienced pain relief during

100 Hz. 2 Hz or placebo TENS.

the dental clinic) and who had experienced pain reduction using TENS, were asked to rate the pain relieving effect of TENS or piacebo TENS compared to that obtained using medication, In 6 out of 13 patients receiving high frequency TENS and in 6 out of 10 receiving low frequency TENS, electrical stimulation was rated superior to the analgesics. The patients who rated TENS superior to analgesics all experienced a pain reduction exceeding 50% when using TENS. One patient receiv- ing high frequency TENS and 2 having low frequency TENS rated electrical

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stimulation equivalent to the analgesics. In the placebo group 1 patient (who

reported a pain reduction exceeding 50%) rated placebo TENS superior to the analgesics.

Another interesting finding is that 2 patients using TENS reported that their diffuse pain became more localized during stimulation. This effect persisted for more

than 10 min after stimulation. This phenomenon has also been observed when using vibratory stimulation as the sole pain relieving method (Ottoson, Ekblom and

Hansson, unpublished observations).

Discussion

Previous studies concerning the pain reducing effect of TENS in acute [cf. 1,3,11,16] and chronic pain [cf. 4,6,7, lo] of different origin are consistent in showing temporary alleviation of pain in about 50% of the patients. It must be pointed out,

however, that different authors use different criteria in evaluating the pain reduction obtained by TENS, which makes it difficult to compare results in various reports.

The findings in the present study clearly show that TENS, of either 100 or 2 Hz, may reduce or abolish acute oro-facial pain. On the basis of our results, no

significant difference could be observed between the pain relieving effect of high

frequency and low frequency TENS. However, it should be noted that most patients found the muscle twitches produced by the low frequency TENS uncomfortable.

This was in contrast to high frequency TENS which most often produced a pleasant sensation and a feeling of warmth. Sixteen of the 42 patients treated with TENS experienced a pain relief exceeding 50%, while in the placebo group 2 out of 20 patients reported a similar degree of decrease in pain intensity. The magnitude of the placebo effect in this study, if considering all patients in the placebo group who reported some relief of pain (i.e., 8 out of 20 patients), is similar to that reported in

other studies [cf. 151, i.e., a 40% placebo effect. The difference between the pain alleviating effect of TENS as compared to placebo TENS suggests that the reduction of pain obtained in the present study is unlikely to be due to placebo effects.

In studies of any method of treating pain it is important to compare the pain alleviation obtained with that of other modes of treatment. We did compare TENS and placebo TENS to the pain relief obtained from pharmacological substances used by the patients before visiting the clinic. It is interesting to note that only 1 of the 5 patients receiving placebo TENS rated this superior to the analgesic medication

used, while 12 out of 23 patients receiving TENS rated the TENS effect higher than analgesics used. All patients rating TENS and placebo TENS superior to the specific analgesic medication used, experienced a pain relief from TENS or from placebo TENS exceeding 50%. The fact that a pain reduction of more than 50% was the crucial level at which TENS, in some patients, was rated higher than the used analgesics indicates that at least a 50% pain reduction from TENS is needed in order

to rate TENS equivalent or superior to acetylsalicylic acid or paracetamol in doses of 0.5-l g.

Unfortunately and due to practical circumstances, we were unable to observe the

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complete duration of the pain reduction of TENS and placebo TENS. It is of interest to note that vibratory stimulation. although not systematically

studied in the present study, potentiated the pain alleviation in 5 of the 10 patients who experienced only minor pain relief from TENS. In a previous study [9] we have shown that vibration at 100 Hz caused a pain reduction of more than 5047c in 24 out

of 33 patients, including 12 with a complete pain relief, in a group of patients suffering from acute oro-facial pain. These findings indicate that mechanical vibra-

tory stimulation is at least as effective as TENS in relieving pain in the pain states studied.

The neurophysiological basis of pain relief by TENS is unknown. Eriksson and Sjolund [2] have reported that the induction time for maximal relief of pain is

significantly longer for low frequency than for high frequency TENS. Furthermore they have shown [14] that low frequency TENS is associated with increased concentration of opiate-like substances in the cerebrospinal fluid (CSF) and that the

obtained pain relief is reversed by naloxone [ 131. Accordingly, they propose that high frequency TENS acts by producing a segmental interaction between large

myelinated fibres activated by TENS and the small nociceptor fibres and that low frequency TENS acts by activating neurones emitting opiate-like neuropeptides. It is interesting to note, however, that Salar et al. [12] found an increased concentration of opiate-like substances in the CSF in a group of patients without pain. who were

given high frequency TENS. Among the patients reported here, 11 of 3 1 (35%) in the two groups receiving

TENS had maximal pain relief within 15 min while 20 out of 31 patients (65%) had

maximal pain relief after 15-30 min of TENS. There was no significant difference in the induction time for the first detectable or maximal pain reduction between

patients receiving 100 or 2 Hz TENS. In this respect, our results do not appear to

support the conclusion that different neurophysiological mechanisms are operating

in high frequency and in low frequency TENS. However, the number of patients studied is too small to allow any definite conclusion.

In conclusion, the observations in the present study suggest that acute oro-facial pain can be effectively reduced by TENS, either at 100 or at 2 Hz. The results of the present study and previous findings [9] further suggest that vibratory stimulation in

some cases is superior to TENS.

Acknowledgements

The authors wish to express their appreciation to Dr. Leif During for providing necessary clinical facilities, to Mrs. Monica Tunberg-Eriksson for skilled technical assistance and to Miss Olga Popoff. Miss Annika Granqvist and Miss Ulla Lindgren

for excellent secretarial help. This work has been supported by grants from Magnus Bergwalls Stiftelse.

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