- Email: [email protected]
Transcutaneous electrical nerve stimulation: Effect on peripheral nerve conduction, mechanical pain threshold, and tactile threshold in humans
1051
Transcutaneous Electrical Nerve Stimulation: Effect on Peripheral Nerve Conduction, Mechanical Pain Threshold, and Tactile Threshold in Humans Deirdre M. Walsh, DPhil, Andrea S. Lowe, DPhil, Kenneth McCormack, G. David Baxter, DPhil, Jim M. Allen, PhD ABSTRACT. Walsh DM, Lowe AS, McCormack K. Willer J-C, Baxter GD, Allen JM. Transcutaneous electrical nerve stimulation: effect on peripheral nerve conduction, mechanical pain threshold, and tactile threshold in humans. Arch Phys Med Rehabil 1998;79:1051-8. Objectives: To investigate the effect of different transcutaneous electrical nerve stimulation (TENS) parameters on nerve conduction in the human superficial radial nerve and on peripheral mechanical pain threshold (MPT) and tactile threshold (TT), and to further the current knowledge of the neurophysiologic effects of TENS. Study Design: Fifty healthy human subjects were randomly allocated in equal numbers to a control group or one of four TENS groups to receive electrical stimulation consisting of four combinations of TENS pulse durations (50psec and 200psec) and frequencies (4Hz and 1IOHz). In the TENS groups, TENS was applied under double-blind conditions for 15 minutes over the superficial radial nerve in the dominant forearm. Over a l-hour period, compound action potentials, MPT readings, and TT readings were recorded bilaterally. Results: Only one combination of TENS parameters (1 lOHz, 200psec) effected consistent changes in all of the variables assessed,ie, TENS produced a significant increase in negative peak latency while simultaneously increasing both MPT and TT. Conclusion: The findings from this study suggest that at least part of TENS-mediated hypoalgesia is a consequence of a direct peripheral effect of TENS, although a “central” effect may not be excluded. 0 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation UBLICATION OF THE gate control theory in 1965l was a P catalyst for the development of small, portable, batteryoperated electrical stimulators for pain control. This theory proposed that selective stimulation of large-diameter afferent fibers could “close the pain gate, ” ie, inhibit incoming nocicepFrom the Rehabilitation Sciences Research Group, School of Health Sciences, University of Ulster at Jordanstown, Northern Ireland (Drs. Walsh, Lowe, Baxter, Allen, Mr. McCormack): and the Ddpartement d’Explorations Fonctionelles Neurologiques, H6pitaJ Pitit-Salp&hiere, Paris, France (Dr. Willer). Submitted for publication December 29, 1997. Accepted in revised form February 25, 1998. Supported by the Wellcome Trust, UK (grant 044316) and the Physiotherapy Research Foundation, United Kingdom. No commercial party having a direct financial interest in the results of the research suuoortinp _. I this article has or will confer a benefit uoon the authors or uoon any oreanization with which the authors are associated. ieprint requests to Dr. Deirdre M. Walsh, Rehabilitation Sciences Research Group, School of Health Sciences, University of Ulster and Jordanstown, Newtownabbey, County Antrim BT37 OQB, Noahem Ireland. 0 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/98/7909-4786$3.00/O
BSc, Jean-Claude
Wilier, MD,
tive information in small diameter afferents. Thus, transcutaneous electrical nerve stimulation (TENS) emerged as a method that could produce pain relief via closure of the pain gate through stimulation of Group II fibers. TENS is currently used to manage a range of both acute and chronic pain conditions, including postoperative, arthritic, labor, and low back pain.2-5In the selection of an optimal treatment regime for a particular patient, the clinician has to choose appropriate settings for a variety of stimulation parameters, eg, pulse duration, pulse frequency, output mode and amplitude. However, there is little information regarding optimal stimulation parameters, and results from the limited number of clinical studies in this area are contentious.6 Despite more than 30 years of clinical applications, there is additional controversy over the possible neurophysiologic and sensory effects of TENS. Peripheral neurophysiologic effects have typically been studied by monitoring changes in compound action potential (CAP) characteristics before and after the application of electrical stimulation. Early uncontrolled work by Campbell and Taub7 reported a peripheral “conduction block” in the human median nerve following the application of high-frequency electrical currents. The authors observed a decrease in amplitude and increase in latency of the Ao wave and a decreasein Aa wave of the median nerve CAP only when stimulation (5OV, lOOHz, OSmsec) was continuous. More recent controlled studies have reported both significantas and nonsignificantlo changes in peripheral nerve conduction following the application of both low- and high-frequency TENS. Furthermore, controversy exists with regard to the specific type of afferent fiber that is stimulated by different TENS parameters.11,12Central spinal neurophysiologic effects associated with TENS have also been studied by several authors: while Chan and Tsang13 have reported a depression of lower limb reflexes after the application of TENS, TENS-mediated facilitation of reflexes has also been reported.14 Studies in this area have therefore produced conflicting evidence of the neurophysiologic effects of TENS. Sensory changes induced by TENS have been studied using both tactile and pain thresholds. Marchand and colleaguesI reported a significant decrease in heat pain perception both during and after high-frequency (100Hz) TENS in a group of healthy volunteers, whereas placebo stimulation did not alter thermal perception. Zoppi and colleagues16 measured thresholds for touch, tingling, and pain in a group of healthy subjects and in a group of chronic myofascial pain patients before and after the application of TENS (lmsec, 50Hz, 24min). This latter study assessedwhether the effect of TENS on sensory thresholds was general, segmental, or unilateral by measuring thresholds in the area of the stimulated nerve and in all four limbs. During TENS application in the healthy subjects, local thresholds increased, whereas outside this area thresholds initially decreased and then increased. Several other authors have Arch
Phys
Med
Rehabil
Voi 79, September
1998
1052
PERIPHERAL
EFFECTS
reported contralateral sensory effects of TENS. Kudo17 reported significant increases in pain threshold bilaterally after the application of TENS (15min, 5Hz, 40mA) to the forearm. Similarly Eriksson and associates18reported a decrease in thermal sensitivity bilaterally after the application of both acupuncture-like (2Hz) and conventional TENS (SOHz) for 15 minutes. Therefore, it appears that while there is ongoing debate on the precise neurophysiologic effects of TENS, there is some agreement on the potential sensory effects of this modality. In previous studies8 we examined the effect of four combinations of TENS pulse durations (50 and 200psec) and frequencies (4 and 11OHz) on conduction in the human superficial radial nerve (SRN) and mechanical pain threshold (MPT) within the associated sensory distribution of the nerve in the dorsum of the hand. We showed that only one combination of TENS parameters (llOHz, 200Psec) effected a significant change in measured conduction latency and MPT. Further, conduction latency changes were shown to correlate highly with changes in MPT. These findings are interesting in relation to the possible mechanisms underlying TENS-mediated hypoalgesia and might suggest that at least part of the hypoalgesic effect of TENS may result from a direct action on the peripheral nerves. In the current study we have extended these observations to include simultaneous measurement of the effect of the same combinations of TENS parameters on negative peak latency (NPL) recorded from the SRN, MPT, and additionally, tactile threshold (TT) within the area of distribution of the stimulated nerve. The recording of NPL values provided a quantifiable indicator of the peripheral neurophysiologic effect of TENS using a noninvasive technique. MPT and TT data were recorded to provide objective data on the concomitant sensory effects of TENS. We have further included measures from an area innervated by the lateral cutaneous nerve of the forearm (LCNF) to exclude any possible nonspecific effects of TENS on MPT or TT in the stimulated forearm. The purpose of this study was therefore to investigate the relation, if any, between the peripheral neurophysiologic and sensory effects of different combinations of TENS parameters by simultaneously measuring nerve conduction, tactile, and pain threshold data. It was hoped that the findings would provide a better understanding of the peripheral/central effects of TENS and the relevance of the selection of specific stimulation parameters to such effects. Study results have been reported in abstract form.lg METHODS General Overview of Procedure After a standardized recruitment procedure, all subjects were required to remain supine on a plinth for the 60 minutes of the experiment. After standard preparation, electrodes were attached to the upper limbs to allow recording of bilateral CAPS from the SRN. In addition, TT and MPT readings were taken from standardized points on both upper limbs using a set of Semmes-Weinstein monofilament9 and a pressure algometer (Electronic Force Gaugeb), respectively. Subjects were randomly allocated to a control group or four TENS groups; in the four TENS groups, electrical stimulation was applied over the SRN in the dominant forearm for three consecutive 5-minute periods, ie, a total of 15 minutes. CAPS were recorded before TENS, after each of the three 5minute TENS applications, and then at the 25, 45, and 60minute points. Both TT and MPT measurements were recorded before TENS, after the last TENS
Arch
Phys
Med
Rehabil
Vol
79, September
1998
OF TENS,
Walsh
application (ie, at the 15-minute point), and then at the 25, 45, and 60-minute points. Control subjects received no TENS. Bilateral skin and ambient temperatures were recorded throughout the experiment. Subjects Fifty healthy subjects (25 men, 25 women; average age 25.5 years) were recruited from staff and students of the university. Ethical approval was obtained. Subjects were screened for relevant contraindications, eg, current pain/medication, history of recent trauma, peripheral neuropathy, and altered skin sensation in the upper limbs. The experimental procedure was explained to each subject, who then signed a consent form. Subjects were randomly allocated to five groups consisting of four TENS groups and a control group (table 1). The parameters of TENS investigated in this study were the same as those used in previous work at this center8 and were chosen because they represent the extremes of ranges of pulse duration and frequency commonly used in clinical practice.6,20 Preparation of Subjects Experiments were conducted at room temperature (20.5”C to 28.O”C) in a sound-attenuated experimental laboratory. Subjects were initially seated at a table and both forearms prepared with alcohol and a colloidal abrasive. Two sets of threshold recording points were then marked with ink bilaterally in a standardized fashion on subjects’ upper limbs; each set of recording points consisted of a proximal TT recording point and a distal MPT recording point. One set of points was marked in the first dorsal web space, an area innervated by the SRN with the proximal recording point located immediately distal to the anatomical snuffbox. The second set was marked in an area innervated by the LCNF with the proximal point located lcm distal to the elbow crease and lcm medial to the lateral border of the forearm (fig 1). Two circular plastic disks mounted in a block of wood were used to standardize marking of the threshold points; each disk was 0.5cm in diameter with a fixed distance of lcm between the disks. Recording of CAPS in the SRN After the threshold points were marked, subjects rested supine on a plinth for the remainder of the experiment. A monopolar stimulator was used to identify the course of the SRN in subjects’ dominant forearms and hands and a bar electrode was attached to the dorsolateral aspect of the forearm for stimulation. Two Ag/AgCl electrodes were used for recording, with the active recording electrode placed over the main portion of the nerve as it crossesthe tendon of extensor pollicis longus, medial to the anatomical snuff box, and a 1Ocmdistance between the cathode and the active recording electrode (fig 1). The reference electrode was placed 3cm distal, over the belly of the first dorsal interosseous muscle. A ground electrode was placed over the dorsum of the hand. The exact position of the stimulating and recording electrodes was determined by the subject’s report. The positioning of the electrodes allowed Table
1: Experimental Group
TENS
Groups
(n = 10 Each
Group)
Parameters 1
TENS 2 TENS 3 TENS 4 Control
11 OHz, 200psec 11 OHz, 50psec 4Hz, 50psec 4Hz. 200psec No stimulation
PERIPHERAL
EFFECTS
OF TENS,
POSTERIOR
1053
Walsh
ANTERIOR
Thermistor = TactileThreshold(proximal) = Mechanical PainThreshold@istal)
Fig 1. Attachment tactile thresholds
of stimulation, (both forearms);
recording, attachment
and earth electrodes of skin thermistor
for CAP recording (both forearms); recording points for mechanical pain and probe (both forearms); position of TENS electrodes (ipsilateral forearm only).
antidromic action potentials to be recorded. This has been shown to be more consistent than orthodromic conduction in producing high-amplitude responses.21In addition, the positioning of electrodes as described minimized the possibility of median nerve sensory potential contamination.22 All electrodes were attached to a Mystro+ recording and stimulation system.c Stimulation consisted of 1OOysecpulses and the stimulus voltage was increased until response amplitude was maximal; in the study supramaximal stimulation was used for all subjects (mean nominal voltage 68.9V). The stimulus consisted of 16 pulses delivered at a frequency of 1Hz with responses averaged and stored; therefore, at each recording time point an average of 16 stimuli were stored for subsequent analysis. Standard settings were used to record the action potentials in the SRN (sensitivity = 50 to lOOuV/div; frequency bandwidth = 5OHz to 2kHz; sweep duration = Smsec). Subjects were allowed to rest for approximately 10 to 15 minutes to acclimatize to room temperature before recording the first CAP. Then CAP recordings were taken at l-minute intervals until three consistent NPLs were obtained; “consistency” was defined as no more than a ? .02msec variation in the NPL. After this standardized regime was completed, the experiment began. The recording of action potentials in the SRN was performed before TENS and immediately after each period of stimulation, ie, at 5, 10, and 15 minutes and then at 25, 45, and 60 minutes. Action potentials were recorded at similar time intervals in the control group. All recordings were stored
on a 3.5” floppy disk on the Mystro+ system at the conclusion of each experiment for subsequent measurement and analysis. Temperature Recording A surface thermistor probe (model EU-U-V3-2,d resolution .OS’C) was positioned on the dorsal aspect of each forearm to record skin temperature; the probe was positioned midway between the TENS electrodes on the dominant forearm and in a similar position on the contralateral forearm. The two surface probes and an ambient probe were connected to a centralized data logger (Squirrel meter, 1200 seriesd).A shift in ambient temperature of ZO.5”C in any one experiment was defined as an exclusion criterion for the current study; no such ambient temperature shifts were recorded and, thus, all data were suitable for analysis. Threshold Recording All threshold data were recorded by the same experimenter (experimenter 1) in a standardized fashion. Subjects were given a demonstration of the procedure involved in recording both tactile and mechanical pain thresholds on the nondominant side before the experiment started. Bilateral tactile and mechanical pain thresholds were recorded immediately before the first CAP recording, after the CAP recording at 15 minutes (ie, after TENS), and again after the CAP was recorded at 25,45, and 60 minutes.
Arch
Phys
Med
Rehabil
Vol 79, September
1998
1054
PERIPHERAL
EFFECTS
TT Recording TTs were recorded from the proximal recording points in the first dorsal web space (SRN point) and lateral forearm (LCNF point) using the Semmes-Weinstein monofilaments.a The set consisted of 20 nylon monofilaments that were graded according to diameter. Semmes-Weinstein monofilaments are widely used for measurement of sensory function and their reliability has been previously reported. 23-26Starting with the smallest diameter, each monofilament was applied to the center of the recording point in a standardized manner. The monofilament was applied until it bent (- 1Ssec), held in contact with the skin for 1Ssec, and then removed. Each monofilament was applied twice on each occasion; subjects were asked to keep their eyes closed and to report when they felt a sensation. Once this was reported the number of the filament was recorded as the basis of calculation of TT. MPT Recording The Electronic Force Gauge pressure algometerb was used to measure MPT at the distal recording points in the first dorsal web space (SRN point) and lateral forearm (LCNF point). The reliability and reproducibility of results obtained using the pressure algometer to measure pain threshold have been reported elsewhere.27-29The algometer was held by the experimenter and applied perpendicularly to the skin. Once initial contact was made, the circular probe head (0.9cm diameter) was applied at a slow steady rate until pain threshold was reached; the point of pain threshold was determined by the subject’s verbal report. Two MPT readings were taken from both distal recording points at each time interval outlined below. TENS Procedure A 1202 TENS unite was used for stimulation. In common with the majority of commercially available TENS units, this unit produces an asymmetrical biphasic waveform. In all groups (ie, control and TENS), two carbon electrodes (3Scm X 5cm) with hydrogel pads were attached to the skin (lcm apart, cathode proximal) directly over the course of the superficial radial nerve in the dominant forearm. Transpore tape was also used to secure the electrodes firmly and thus maximize skin-to-electrode contact. Parameter combinations for each TENS group were calibrated for 4 separate TENS units using an oscilloscope (Model 1602f). Once calibration was complete, the parameters were set and marked on each unit as TENS 1, TENS 2, etc; therefore, each unit was used to deliver the same combination of parameters in a standardized fashion. In groups TENS 1 through TENS 4, TENS was applied for three consecutive 5-minute periods by a second experimenter (experimenter 2). Experimenter 1 was solely responsible for recording data and was unaware of the stimulation delivered to subjects; therefore, this study was double-blinded. When the TENS unit was switched on, subjects were told to report the onset of any “tingling” or “buzzing” sensation beneath the TENS electrodes. The intensity of the current was increased until subjects reported a “strong but comfortable” sensation. The time at which subjects reported the latter was taken as zero. After the first, third, and fourth minute of each 5-minute stimulation period, subjects were asked if the reported sensation had decreased. If so, the intensity was increased to maintain a “strong but comfortable” sensation; thus, clinically representative intensity levels were used in the study. Subjects were given a demonstration of TENS on the nondominant forearm before the
Arch
Phys
Med
Rehabil
Vol
79, September
1998
OF TENS,
Walsh
experiment began. In the control group, the TENS electrodes were attached but no treatment was given; subjects were told they were not going to receive TENS during the experiment. Data Analysis For statistical analysis of NPL and MPT results, difference scores (ie, the variation from time zero) were used as a means of standardization. Analysis of variance (ANOVA) and post hoc tests were used, as appropriate, to assessdifferences over time and between groups for these two variables. A Kruskal-Wallis test was additionally used to analyze the TT data. RESULTS Results are presented in figures 2, 3, and 4. To improve clarity, results for TENS 1 and TENS 4 groups (both have 200,usecpulse duration) are presented separately from TENS 2 and TENS 3 groups (both have 50usec pulse duration). NPL NPL values were taken at the point of maximum negative peak amplitude for all subjects (resolution of NPL 0.20
c
I
I
3,lol------do 5
10
15
20
25
Time
0.20
30
35
40
45
50
55
60
(min)
1
TENS
i
0.15
":o.lo
B0
I
1 5
1 10
1
1
1
1
'
1
'
'
1
1
15
20
25
30
35
40
45
50
55
60
Time Fig 2. Summary (means -C SEM); and TENS 4 (0). TENS 3 (A).
(min)
of ipsilateral NPL difference scores for all groups n = 10 in each group. (A) Control (0). TENS 1 ( *p 5 .Ol; **p 5.05. (B) Control (0). TENS 2 (A), and
PERIPHERAL
EFFECTS
OF TENS,
1055
Walsh
at the 5-minute point; at 10, 15, 25, 45, and 60 minutes the increase in NPL in the TENS 1 group was significant compared with the control group; at 15,25,45, and 60 minutes there was a significant increase in NPL in the TENS 1 group compared with the TENS 4 group. There was no significant treatment effect between control, TENS 2, and TENS 3 groups. Repeated measures ANOVA showed no significant treatment effects for contralateral NPL data.
A -0
5
10
15 20 25 30
Time .I-c d
10
35 40 45
50
55
60
(min)
TENS
Ambient Temperature and Skin Temperature All five experimental groups were analyzed together for temperature data. Repeated measures ANOVA showed no significant interactive effect or differences between groups for ambient temperature; however, there was a significant increase over time (p = .OOOl). By the 60minute point the average increase in ambient temperature across all groups was .14 k .02”C (mean +- SEM). There was also a significant difference over time for both ipsilateral and contralateral skin temperature (p = .OOll and p = .OOOl, respectively), but no significant interactive effect or differences between groups.
8 I2 2 23 t-r
B -0
5
10 15 20
25 30 35 40 45
Time
50 55
18 16
TENS *
14 12 I--
i
60
(min)
Fig 3. Summary of ipsilateral SRN MPT difference scores for all groups (means f SEM); n = 10 in each group. (A) Control (0). TENS ), and TENS 4 (0). (B) Control (O), TENS 2 (A), and TENS 3 (f!J.
readings = .Olmsec). Results for ipsilateral NPL are presented in figure 2. Table 2 summarizes NPL data recorded from ipsilateral and contralateral sides for all experimental groups. The greatest increase in NPL occurred in the TENS 1 group (fig 2). NPL values continued to increase after the period of TENS application to reach a maximum at the 60-minute point (NPL difference score mean _f SEM = .l 1 k .04msecfor TENS 1 compared with - .03 t .02msec in the control group). In the TENS 3 group, NPL also continued to increase after TENS application to reach a maximum value at the 60-minute point (NPL difference score mean F SEM = .09 2 .05msec for TENS 3). Figure 5 illustrates two CAPSrecorded from a subject in the TENS 1 group. Analysis of NPL data using repeated measures ANOVA showed a significant treatment effect (p = .0036) between control, TENS 1, and TENS 4 groups; one factor ANOVA further showed significant differences at 5, 10, 15, 2.5, 45, and 60 minutes (p = .02, p = ,001, p = .OOl, p = .006, p = .03 andp = .02, respectively). Corrected post hoc Fisher tests showed a significant increase in NPL in both TENS groups compared with the control group
0
5
10
15
20 25 30
A
35 40 45
50
55 60
Time (min)
TENS
I--
K"
5
10 15 20 25 30 35 40 45 50
B
55 60
Time (min)
Fig 4. Summary of ipsilateral SRN TT difference scores for all groups (means f SEM); n = 10 each group. (A) Control (O), TENS 1 ( TENS 4 (0). *p 5 .Ol. (B) Control (O), TENS 2 (A), and TENS 3 (a).
Arch
Phys
Med
Rehabil
Vol 79, September
1998
1056
PERIPHERAL
Table
2: Summary
of lpsilateral
Control
Time
EFFECTS
OF TENS,
and Contralateral
TENS 1
Walsh
NPL Data TENS 2
(msec,
mean
f
SEMI
TENS 3
TENS 4
lpsilateral OMin 5Min
2.08
+ .02
2.26
+ .06
2.20
i
.04
2.31
t .09
2.05
t
2.29
t
IOMin 15Min
2.04 2.07
+ .03 + .03
2.33 t .06 2.35 2 .07
2.22 2.24
+- .03 + .02
2.30 2.31
t .08 2 .09
2.20
2 .04
25Min 45Min
2.05 t .03 2.08 k .03
2.34 t .07 2.36 + .08
2.28 2.26
2 .02 -+ .03
2.33 2.34
2 .09 + .09
2.19 2.18
-+ .05 + .04
GOMin Contralateral
2.04
2 .03
2.37
2.23 2.24
k .03 + .03
2.38 2.39
+ .I0 t .I 1
2.18 2.18
+ .05 + .05
OMin 5Min
2.11
+ .Ol
2.30
2 .06
2.16
+ .05
2.33
2 .08
2.16
+ .05
2.10 + .02 2.12 + .02
2.31
2 .06
2.15 2.16
i- .05 I! .05
2.35 2.36
-t .08 2 .09
2.17 2.17
2 .06 i .05
.02
.06
+ .08
2.10
-+ .02
2.33 2.33
t- .06 + .07
2.18
2 .05
2.37
i .09
2.18
I! .05
25Min 45Min
2.09 2.12
? .02 2 .02
2.31 2.36
+ .07 + .08
2.18 2.18
t .04 + .04
2.38 2.42
i- .09 t .09
2.16 2.17
+ .05 i- .05
6OMin
2.11
t- .03
2.37
2 .08
2.19
I
2.45
2 .09
2.18
t
TT The force (mN) of each filament was calculated from data supplied by the manufacturers. The most pronounced change in TT was observed at the ipsilateral SRN point; figure 4 shows changes in TT difference scores over time for this point. The TENS 1 group showed a marked increase in threshold immediately after TENS application (mean increase = 10.25mN), which then decreased to the same level as the other groups. Analysis of control, TENS 1, and TENS 4 data using KruskalWallis test showed significant differences at the 15minute point for ipsilateral SRN data (p = .Ol), with the TENS 1 group showing the greatest increase in TT. There was also a significant
I
lms
Fig 5. Two SRN CAPS recorded from a subject in the TENS 1 group; the upper CAP was recorded at 0 minutes (before TENS) and the lower at 15 minutes (ie, after TENS application). The two vertical cursors are positioned at the NPL values at 0 and 15 minutes, respectively. There was a change in NPL from 2.15msec at Omin to 2.29msec at 15min.
Phys
+ .05 i .05
IOMin 15Min
MPT The lower of the two MPT readings taken from each of the recording points was used for the purposes of analysis. Figure 3 shows ipsilateral SRN changes in MPT, after TENS application (ie, at the 15-minute point) there was a large increase in MPT in the TENS 1 group, in contrast to the smaller changes in MPT in the other three TENS groups. Analysis of MPT data for all 4 recording points, ie, ipsilateral SRN/LCNF and contralateral SRN/LCNF points, revealed no significant differences between groups; however, there were significant differences over time.
Arch
2.19 2.20
Med
Rehabil
Vol79,
September
1998
4
.04
.06
difference at the 45minute point for contralateral SRN T’I scores (p = .Ol), with the TENS 1 group again showing the greatest increase in TT when compared with the control and TENS 4 groups. No other significant differences were observed for TT data at any of the other recording points. In summary, statistical analysis of all data showed consistent effects of the TENS 1 group on NPL, MPT, and TT data on the ipsilateral side. DISCUSSION The purpose of this study was to determine the effect of different combinations of TENS pulse durations (50 and 200psec) and frequencies (4 and 11OHz) on conduction in the SRN and on MPT and TT within its area of distribution. Nonspecific effects (ie, outside the area of distribution of the stimulated nerve) of TENS on MPT and TT in the stimulated forearm were assessedusing MPT and TT measurements from an area innervated by the LCNF, and possible systemic or “central” effects were assessedusing concomitant measurements from the contralateral limb. Results clearly show that only one combination of TENS parameters (1 lOHz, 200psec) effected obvious and statistically significant changes in two of the variables assessed, ie, a measurable increase in NPL, with concomitant increases in TT within the area of distribution of the stimulated nerve. Although TENS 2 did effect an increase in measured NPL, this did not persist after the stimulation period. No combination of TENS parameters significantly affected MPT. These data extend our previous findings in showing that TENS, when applied at 1lOHz and 200ysec, effects the most significant hypoalgesia on experimentally induced mechanical pain in humans8 and would suggest that there may indeed be some functional relation between the action of TENS on NPL and its hypoalgesic efficacy. Concomitant measures on the contralateral limb showed an isolated significant effect of the TENS 1 group on contralateral SRN TT scores; however, overall results provided no direct evidence for either “systemic” or “central” effects of this particular combination of TENS parameters and, although not excluding the possibility of a “central” component to TENS-mediated hypoalgesia, further strengthen the argument for a “locally” mediated action. Although not presented in detail in the results section, statistical analysis of peak-to-peak amplitude and peak-to-peak duration data from the CAP records did not show any signifi-
PERIPHERAL
EFFECTS
groups. This would suggest that the various components of the CAP were equally affected by TENS because the results indicate that despite the shift in latency, the general shape of the waveform was not
cant changes for any of the experimental
significantly changed. Although the changes in conduction velocity of the SRN were very small, the data presented in this study are consistent with earlier findings9; there was a decrease in mean conduction velocity of 2.05m/sec in the TENS 1 group
by the 60-minute point, compared with an increase in the control group of O.gm/sec. Further studies using microneurographic techniques would be required to determine if the observed changes in conduction velocity were related to specific types of nerve fibers. Although we have consistently found an increase in NPL in response to TENS applied at 1lOHz and 200psec, previous
studies on the peripheral neurophysiologic effects of TENS have provided contradictory findings7*10,30;however lack of control data and variations in the stimulation parameters employed makes comparisons between the current study and previous work difficult. In contrast, most previous studies7J7 have reported measurable effects of TENS on pain, thermal, and touch sensitivity, although not all reports confine these to within the area of distribution of the stimulated nerve, and contralateral effects have been reported.‘5~16~1s In contrast, the current study has reported mainly ipsilateral effects following TENS application. Notwithstanding the contradictions evident in the literature, the findings from the current and previous studies12,3’strongly
suggest some peripheral component to TENS-mediated hypoalgesia. It is unlikely that this would be related simply to a reduction in nerve conduction velocity (as measured in this and other studies); the changes observed between the sensory thresholds and measured increase in NPL suggests there may be some relation between the two. Interestingly our findings are supported by Francini and associates,32who reported evidence of a relation between changes in sensory responses and reflex thresholds induced by TENS in a group of healthy volunteers. The authors measured the tendon and Hoffman reflexes and cutaneous pain threshold before and after the application of TENS (50Hz, lmsec). The results showed that the magnitudes of the TENS-induced changes in the pain thresholds and in the Hoffman and tendon reflexes were correlated, thereby suggesting some relation between these two variables. CONCLUSION The results of this study have demonstrated concurrent peripheral neurophysiologic and sensory effects after application of TENS over a peripheral nerve; the results suggest that these two effects may be related. In addition, the data have extended previous work at this center in showing parameterspecific peripheral effects of TENS. The greatest changes in both sensory and neurophysiologic effects of TENS were observed in the TENS 1 (1 lOHz, 200psec) group. Acknowledgment: The expert technical assistance of Mr. Robin Ritchie is gratefully acknowledged. References 1. Melzack R, Wall PD. Pain mechanisms: a new theory. Science 1965;150:971-9. 2. Marchand S, Charest J, Li J, Chenard J-R, Lavignolle B, Laurencelle L. Is TENS purely a placebo effect? A controlled study on chronic low back pain. Pain 1993;54:99-106. 3. Forster EL, Kramer JF, Lucy SD, Scudds RA, Novick RJ. Effect of TENS on pain, medications, and pulmonary function following coronary artery bypass graft surgery. Chest 1994;106:1343-8.
OF TENS,
1057
Walsh
4. Carroll D, Tram&r M, McQuay H, Nye B, Moore A. Transcutaneous electrical nerve stimulation in labour pain: a systematic review. Br J Obstet Gyn 1997;104:169-75. 5. Grimmer K. A controlled double blind study comparing the effects of strong burst mode TENS and high rate TENS on painful osteoarthritic knees. Aust J Phys Ther 1992;38:49-56. 6. Walsh DM. TENS: clinical applications and related theory. Edinburgh: Churchill Livingstone; 1997. 7. Campbell JN, Taub A. Local analgesia from percutaneous electrical nerve stimulation. Arch Neurol 1973;28:347-50. 8. Walsh DM, Foster NE, Baxter GD, Allen JM. Transcutaneous electrical nerve stimulation (TENS): relevance of stimulation parameters to neurophysiological and hypoalgesic effects. Am J Phys Med Rehabil1995;74: 199-206. 9. Walsh DM, Greer K, Baxter GD. Relevance of transcutaneous electrical nerve stimulation parameters to neurophysiological effects [abstract]. In: Proceedings of the 12th International Congress of the World Confederation for Physical Therapy: 1995 June 25-30; Washington. Washington: American Physical Therapy Association; 1995. p. 576. 10. Cox PD, Kramer JF, Hartsell H. Effect on different TENS stimulus parameters on ulnar motor nerve conduction velocity. Am J Phys Med Rehabil 1993;72:294-300. 11. Janko M, Trontelj JV Transcutaneous electrical nerve stimulation: a microneurographic and perceptual study. Pain 1980;9:219-30. 12. Levin MF, Hui-Chan C. Conventional and acupuncture-like transcutaneous electrical nerve stimulation excites similar afferent fibers. Arch Phys Med Rehabil 1993;74:54-60. 13. Chan CYW, Tsang H. Inhibition of the human flexion reflex by low intensity high frequency transcutaneous electrical nerve stimulation TENS has a gradual onset and offset. Pain 1987;28:239-53. 14. Hui-Chan C, Levin ME Stretch reflex latencies in spastic hemiparetie subjects are prolonged after transcutaneous electrical nerve stimulation. Can J Neurol Sci 1993;20:97-106. 15. Marchand S, Bushnell MC, Duncan GH. Modulation of heat pain perception by high frequency transcutaneous electrical nerve stimulation (TENS). Clin J Pain 1991;l: 122-9. 16. Zoppi M, Francini F, Maresca M, Procacci P. Changes of cutaneous sensory thresholds induced by non-painful transcutaneous electrical nerve stimulation in normal subjects and in subjects with chronic pain. J Neurol Neurosurg Psychiatry 1981;44:708-17. 17. Kudo S. Effects of TENS on the stimulated and non-stimulated sides: changes of pain threshold and skin temperature. J Phys Ther Sci 1994;6:109-13. 18 Eriksson MBE, Rosen I, Sjolund B. Thermal sensitivity in healthy subjects is decreased by a central mechanism after TENS. Pain 1985;22:235-42. 19. Lowe AS, McCormack K, Walsh DM, Baxter GD, Allen JM. The effects of hanscutaneous electrical nerve stimulation (TENS) parameters upon bilateral nerve conduction, mechanical pain and tactile thresholds in humans [abstract]. J Physiol 1996;495: 13 1. 20. Mannheimer JS, Lampe GN. Clinical transcutaneous electrical nerve stimulation. Philadelphia (PA): FA Davis; 1984. 21. Downie AW, Scott TR. An improved technique for radial nerve conduction studies. J Neurol Neurosurg Psychiatry 1967;30:332-6. 22. Mackenzie K, DeLisa JA. Distal sensory latency measurement of the superficial radial nerve in normal adult subjects. Arch Phys Med Rehabil 1981;62:31-4. 23. Bell-Krotoski J, Tomancik E. The repeatability of testing with semmes-Weinstein monofilaments. J Hand Surg 1987;12A:155-61. 24. MacDermid JC, Kramer JF, Roth JH. Decision making in detecting abnormal semmes-Weinstein monofilament thresholds in carpal tunnel syndrome. J Hand Ther 1994;7:158-62. 25. Bell-Krotoski JA. Fess EE, Figarola JH: Hiltz D. Threshold detection and Semmes-Weinstein monofilaments. J Hand Ther 1995;8:155-62. 26. Owen BM, Stratford CJ. Assessment of the methods for testing sensation in leprosy patients in a rural setting. Lepr Rev 1995;66: 55-62.
Arch
Phys
Med
Rehabil
Vol 79, September
1998
1058
PERIPHERAL
EFFECTS
27. Shapira SC, Magora F, Chrubasik S, Feigin E, Vatine JJ, Weinstein D. Assessment of pain threshold and pain tolerance in women in labour and in the early post-partum period by pressure algometry. Eur J Anaesthesiol 1995;12:495-9. 28. Reeves JL, Jaeger B, Graffradford SB. Reliability of the pressure algometer as a measure of myofascial trigger point sensitivity. Pain 1986;24:313-21. 29. Ohrbach R, Gale EN. Pressure pain thresholds in normal musclesreliability, measurement effects, and topographic differences. Pain 1989;37:257-63. 30. Ignelzi RJ, Nyquist JK. Excitability changes in peripheral nerve fibres after repetitive electrical stimulation. J Neurosurg 1979;51: 824-33. 31. Francini F, Maresca M, Procacci P, Zoppi M. The effects of non-painful transcutaneous electrical nerve stimulation on cutaneous pain threshold and muscular reflexes in normal men and in subjects with chronic pain. Pain 198 1; 11:49-63.
Arch
Phys
Med
Rehabil
Vol 79, September
1998
OF TENS,
Walsh
32. Walmsley RP, Monga TN, Prouix M. Effect of transcutaneous nerve stimulation on sensory nerve conduction velocity: a pilot project. Physiother Prac 1986;2:117-20. Suppliers a. Stoelting Company, Oakwood Centre, 620, Wheat Lane, Wood Dale, IL60191. b. Salter Weigh-Tronix, Ltd., George Street, West Bromwich, West Midlands B70 6AD, United Kingdom. c. Medelec Ltd., Manor Way, Old Woking, Surrey GU22 9JU, United Kingdom. d. Grant Instruments (Cambridge) Ltd., Barrington, Cambridge CB2 5QZ, United Kingdom. e. IT0 Company, Ltd., Tokyo, Japan. f. Gould Electronics Ltd., Roebuck Road, Hainault, Ilford, United Kingdom.
Transcutaneous Electrical Nerve Stimulation: Effect on Peripheral Nerve Conduction, Mechanical Pain Threshold, and Tactile Threshold in Humans Deirdre M. Walsh, DPhil, Andrea S. Lowe, DPhil, Kenneth McCormack, G. David Baxter, DPhil, Jim M. Allen, PhD ABSTRACT. Walsh DM, Lowe AS, McCormack K. Willer J-C, Baxter GD, Allen JM. Transcutaneous electrical nerve stimulation: effect on peripheral nerve conduction, mechanical pain threshold, and tactile threshold in humans. Arch Phys Med Rehabil 1998;79:1051-8. Objectives: To investigate the effect of different transcutaneous electrical nerve stimulation (TENS) parameters on nerve conduction in the human superficial radial nerve and on peripheral mechanical pain threshold (MPT) and tactile threshold (TT), and to further the current knowledge of the neurophysiologic effects of TENS. Study Design: Fifty healthy human subjects were randomly allocated in equal numbers to a control group or one of four TENS groups to receive electrical stimulation consisting of four combinations of TENS pulse durations (50psec and 200psec) and frequencies (4Hz and 1IOHz). In the TENS groups, TENS was applied under double-blind conditions for 15 minutes over the superficial radial nerve in the dominant forearm. Over a l-hour period, compound action potentials, MPT readings, and TT readings were recorded bilaterally. Results: Only one combination of TENS parameters (1 lOHz, 200psec) effected consistent changes in all of the variables assessed,ie, TENS produced a significant increase in negative peak latency while simultaneously increasing both MPT and TT. Conclusion: The findings from this study suggest that at least part of TENS-mediated hypoalgesia is a consequence of a direct peripheral effect of TENS, although a “central” effect may not be excluded. 0 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation UBLICATION OF THE gate control theory in 1965l was a P catalyst for the development of small, portable, batteryoperated electrical stimulators for pain control. This theory proposed that selective stimulation of large-diameter afferent fibers could “close the pain gate, ” ie, inhibit incoming nocicepFrom the Rehabilitation Sciences Research Group, School of Health Sciences, University of Ulster at Jordanstown, Northern Ireland (Drs. Walsh, Lowe, Baxter, Allen, Mr. McCormack): and the Ddpartement d’Explorations Fonctionelles Neurologiques, H6pitaJ Pitit-Salp&hiere, Paris, France (Dr. Willer). Submitted for publication December 29, 1997. Accepted in revised form February 25, 1998. Supported by the Wellcome Trust, UK (grant 044316) and the Physiotherapy Research Foundation, United Kingdom. No commercial party having a direct financial interest in the results of the research suuoortinp _. I this article has or will confer a benefit uoon the authors or uoon any oreanization with which the authors are associated. ieprint requests to Dr. Deirdre M. Walsh, Rehabilitation Sciences Research Group, School of Health Sciences, University of Ulster and Jordanstown, Newtownabbey, County Antrim BT37 OQB, Noahem Ireland. 0 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/98/7909-4786$3.00/O
BSc, Jean-Claude
Wilier, MD,
tive information in small diameter afferents. Thus, transcutaneous electrical nerve stimulation (TENS) emerged as a method that could produce pain relief via closure of the pain gate through stimulation of Group II fibers. TENS is currently used to manage a range of both acute and chronic pain conditions, including postoperative, arthritic, labor, and low back pain.2-5In the selection of an optimal treatment regime for a particular patient, the clinician has to choose appropriate settings for a variety of stimulation parameters, eg, pulse duration, pulse frequency, output mode and amplitude. However, there is little information regarding optimal stimulation parameters, and results from the limited number of clinical studies in this area are contentious.6 Despite more than 30 years of clinical applications, there is additional controversy over the possible neurophysiologic and sensory effects of TENS. Peripheral neurophysiologic effects have typically been studied by monitoring changes in compound action potential (CAP) characteristics before and after the application of electrical stimulation. Early uncontrolled work by Campbell and Taub7 reported a peripheral “conduction block” in the human median nerve following the application of high-frequency electrical currents. The authors observed a decrease in amplitude and increase in latency of the Ao wave and a decreasein Aa wave of the median nerve CAP only when stimulation (5OV, lOOHz, OSmsec) was continuous. More recent controlled studies have reported both significantas and nonsignificantlo changes in peripheral nerve conduction following the application of both low- and high-frequency TENS. Furthermore, controversy exists with regard to the specific type of afferent fiber that is stimulated by different TENS parameters.11,12Central spinal neurophysiologic effects associated with TENS have also been studied by several authors: while Chan and Tsang13 have reported a depression of lower limb reflexes after the application of TENS, TENS-mediated facilitation of reflexes has also been reported.14 Studies in this area have therefore produced conflicting evidence of the neurophysiologic effects of TENS. Sensory changes induced by TENS have been studied using both tactile and pain thresholds. Marchand and colleaguesI reported a significant decrease in heat pain perception both during and after high-frequency (100Hz) TENS in a group of healthy volunteers, whereas placebo stimulation did not alter thermal perception. Zoppi and colleagues16 measured thresholds for touch, tingling, and pain in a group of healthy subjects and in a group of chronic myofascial pain patients before and after the application of TENS (lmsec, 50Hz, 24min). This latter study assessedwhether the effect of TENS on sensory thresholds was general, segmental, or unilateral by measuring thresholds in the area of the stimulated nerve and in all four limbs. During TENS application in the healthy subjects, local thresholds increased, whereas outside this area thresholds initially decreased and then increased. Several other authors have Arch
Phys
Med
Rehabil
Voi 79, September
1998
1052
PERIPHERAL
EFFECTS
reported contralateral sensory effects of TENS. Kudo17 reported significant increases in pain threshold bilaterally after the application of TENS (15min, 5Hz, 40mA) to the forearm. Similarly Eriksson and associates18reported a decrease in thermal sensitivity bilaterally after the application of both acupuncture-like (2Hz) and conventional TENS (SOHz) for 15 minutes. Therefore, it appears that while there is ongoing debate on the precise neurophysiologic effects of TENS, there is some agreement on the potential sensory effects of this modality. In previous studies8 we examined the effect of four combinations of TENS pulse durations (50 and 200psec) and frequencies (4 and 11OHz) on conduction in the human superficial radial nerve (SRN) and mechanical pain threshold (MPT) within the associated sensory distribution of the nerve in the dorsum of the hand. We showed that only one combination of TENS parameters (llOHz, 200Psec) effected a significant change in measured conduction latency and MPT. Further, conduction latency changes were shown to correlate highly with changes in MPT. These findings are interesting in relation to the possible mechanisms underlying TENS-mediated hypoalgesia and might suggest that at least part of the hypoalgesic effect of TENS may result from a direct action on the peripheral nerves. In the current study we have extended these observations to include simultaneous measurement of the effect of the same combinations of TENS parameters on negative peak latency (NPL) recorded from the SRN, MPT, and additionally, tactile threshold (TT) within the area of distribution of the stimulated nerve. The recording of NPL values provided a quantifiable indicator of the peripheral neurophysiologic effect of TENS using a noninvasive technique. MPT and TT data were recorded to provide objective data on the concomitant sensory effects of TENS. We have further included measures from an area innervated by the lateral cutaneous nerve of the forearm (LCNF) to exclude any possible nonspecific effects of TENS on MPT or TT in the stimulated forearm. The purpose of this study was therefore to investigate the relation, if any, between the peripheral neurophysiologic and sensory effects of different combinations of TENS parameters by simultaneously measuring nerve conduction, tactile, and pain threshold data. It was hoped that the findings would provide a better understanding of the peripheral/central effects of TENS and the relevance of the selection of specific stimulation parameters to such effects. Study results have been reported in abstract form.lg METHODS General Overview of Procedure After a standardized recruitment procedure, all subjects were required to remain supine on a plinth for the 60 minutes of the experiment. After standard preparation, electrodes were attached to the upper limbs to allow recording of bilateral CAPS from the SRN. In addition, TT and MPT readings were taken from standardized points on both upper limbs using a set of Semmes-Weinstein monofilament9 and a pressure algometer (Electronic Force Gaugeb), respectively. Subjects were randomly allocated to a control group or four TENS groups; in the four TENS groups, electrical stimulation was applied over the SRN in the dominant forearm for three consecutive 5-minute periods, ie, a total of 15 minutes. CAPS were recorded before TENS, after each of the three 5minute TENS applications, and then at the 25, 45, and 60minute points. Both TT and MPT measurements were recorded before TENS, after the last TENS
Arch
Phys
Med
Rehabil
Vol
79, September
1998
OF TENS,
Walsh
application (ie, at the 15-minute point), and then at the 25, 45, and 60-minute points. Control subjects received no TENS. Bilateral skin and ambient temperatures were recorded throughout the experiment. Subjects Fifty healthy subjects (25 men, 25 women; average age 25.5 years) were recruited from staff and students of the university. Ethical approval was obtained. Subjects were screened for relevant contraindications, eg, current pain/medication, history of recent trauma, peripheral neuropathy, and altered skin sensation in the upper limbs. The experimental procedure was explained to each subject, who then signed a consent form. Subjects were randomly allocated to five groups consisting of four TENS groups and a control group (table 1). The parameters of TENS investigated in this study were the same as those used in previous work at this center8 and were chosen because they represent the extremes of ranges of pulse duration and frequency commonly used in clinical practice.6,20 Preparation of Subjects Experiments were conducted at room temperature (20.5”C to 28.O”C) in a sound-attenuated experimental laboratory. Subjects were initially seated at a table and both forearms prepared with alcohol and a colloidal abrasive. Two sets of threshold recording points were then marked with ink bilaterally in a standardized fashion on subjects’ upper limbs; each set of recording points consisted of a proximal TT recording point and a distal MPT recording point. One set of points was marked in the first dorsal web space, an area innervated by the SRN with the proximal recording point located immediately distal to the anatomical snuffbox. The second set was marked in an area innervated by the LCNF with the proximal point located lcm distal to the elbow crease and lcm medial to the lateral border of the forearm (fig 1). Two circular plastic disks mounted in a block of wood were used to standardize marking of the threshold points; each disk was 0.5cm in diameter with a fixed distance of lcm between the disks. Recording of CAPS in the SRN After the threshold points were marked, subjects rested supine on a plinth for the remainder of the experiment. A monopolar stimulator was used to identify the course of the SRN in subjects’ dominant forearms and hands and a bar electrode was attached to the dorsolateral aspect of the forearm for stimulation. Two Ag/AgCl electrodes were used for recording, with the active recording electrode placed over the main portion of the nerve as it crossesthe tendon of extensor pollicis longus, medial to the anatomical snuff box, and a 1Ocmdistance between the cathode and the active recording electrode (fig 1). The reference electrode was placed 3cm distal, over the belly of the first dorsal interosseous muscle. A ground electrode was placed over the dorsum of the hand. The exact position of the stimulating and recording electrodes was determined by the subject’s report. The positioning of the electrodes allowed Table
1: Experimental Group
TENS
Groups
(n = 10 Each
Group)
Parameters 1
TENS 2 TENS 3 TENS 4 Control
11 OHz, 200psec 11 OHz, 50psec 4Hz, 50psec 4Hz. 200psec No stimulation
PERIPHERAL
EFFECTS
OF TENS,
POSTERIOR
1053
Walsh
ANTERIOR
Thermistor = TactileThreshold(proximal) = Mechanical PainThreshold@istal)
Fig 1. Attachment tactile thresholds
of stimulation, (both forearms);
recording, attachment
and earth electrodes of skin thermistor
for CAP recording (both forearms); recording points for mechanical pain and probe (both forearms); position of TENS electrodes (ipsilateral forearm only).
antidromic action potentials to be recorded. This has been shown to be more consistent than orthodromic conduction in producing high-amplitude responses.21In addition, the positioning of electrodes as described minimized the possibility of median nerve sensory potential contamination.22 All electrodes were attached to a Mystro+ recording and stimulation system.c Stimulation consisted of 1OOysecpulses and the stimulus voltage was increased until response amplitude was maximal; in the study supramaximal stimulation was used for all subjects (mean nominal voltage 68.9V). The stimulus consisted of 16 pulses delivered at a frequency of 1Hz with responses averaged and stored; therefore, at each recording time point an average of 16 stimuli were stored for subsequent analysis. Standard settings were used to record the action potentials in the SRN (sensitivity = 50 to lOOuV/div; frequency bandwidth = 5OHz to 2kHz; sweep duration = Smsec). Subjects were allowed to rest for approximately 10 to 15 minutes to acclimatize to room temperature before recording the first CAP. Then CAP recordings were taken at l-minute intervals until three consistent NPLs were obtained; “consistency” was defined as no more than a ? .02msec variation in the NPL. After this standardized regime was completed, the experiment began. The recording of action potentials in the SRN was performed before TENS and immediately after each period of stimulation, ie, at 5, 10, and 15 minutes and then at 25, 45, and 60 minutes. Action potentials were recorded at similar time intervals in the control group. All recordings were stored
on a 3.5” floppy disk on the Mystro+ system at the conclusion of each experiment for subsequent measurement and analysis. Temperature Recording A surface thermistor probe (model EU-U-V3-2,d resolution .OS’C) was positioned on the dorsal aspect of each forearm to record skin temperature; the probe was positioned midway between the TENS electrodes on the dominant forearm and in a similar position on the contralateral forearm. The two surface probes and an ambient probe were connected to a centralized data logger (Squirrel meter, 1200 seriesd).A shift in ambient temperature of ZO.5”C in any one experiment was defined as an exclusion criterion for the current study; no such ambient temperature shifts were recorded and, thus, all data were suitable for analysis. Threshold Recording All threshold data were recorded by the same experimenter (experimenter 1) in a standardized fashion. Subjects were given a demonstration of the procedure involved in recording both tactile and mechanical pain thresholds on the nondominant side before the experiment started. Bilateral tactile and mechanical pain thresholds were recorded immediately before the first CAP recording, after the CAP recording at 15 minutes (ie, after TENS), and again after the CAP was recorded at 25,45, and 60 minutes.
Arch
Phys
Med
Rehabil
Vol 79, September
1998
1054
PERIPHERAL
EFFECTS
TT Recording TTs were recorded from the proximal recording points in the first dorsal web space (SRN point) and lateral forearm (LCNF point) using the Semmes-Weinstein monofilaments.a The set consisted of 20 nylon monofilaments that were graded according to diameter. Semmes-Weinstein monofilaments are widely used for measurement of sensory function and their reliability has been previously reported. 23-26Starting with the smallest diameter, each monofilament was applied to the center of the recording point in a standardized manner. The monofilament was applied until it bent (- 1Ssec), held in contact with the skin for 1Ssec, and then removed. Each monofilament was applied twice on each occasion; subjects were asked to keep their eyes closed and to report when they felt a sensation. Once this was reported the number of the filament was recorded as the basis of calculation of TT. MPT Recording The Electronic Force Gauge pressure algometerb was used to measure MPT at the distal recording points in the first dorsal web space (SRN point) and lateral forearm (LCNF point). The reliability and reproducibility of results obtained using the pressure algometer to measure pain threshold have been reported elsewhere.27-29The algometer was held by the experimenter and applied perpendicularly to the skin. Once initial contact was made, the circular probe head (0.9cm diameter) was applied at a slow steady rate until pain threshold was reached; the point of pain threshold was determined by the subject’s verbal report. Two MPT readings were taken from both distal recording points at each time interval outlined below. TENS Procedure A 1202 TENS unite was used for stimulation. In common with the majority of commercially available TENS units, this unit produces an asymmetrical biphasic waveform. In all groups (ie, control and TENS), two carbon electrodes (3Scm X 5cm) with hydrogel pads were attached to the skin (lcm apart, cathode proximal) directly over the course of the superficial radial nerve in the dominant forearm. Transpore tape was also used to secure the electrodes firmly and thus maximize skin-to-electrode contact. Parameter combinations for each TENS group were calibrated for 4 separate TENS units using an oscilloscope (Model 1602f). Once calibration was complete, the parameters were set and marked on each unit as TENS 1, TENS 2, etc; therefore, each unit was used to deliver the same combination of parameters in a standardized fashion. In groups TENS 1 through TENS 4, TENS was applied for three consecutive 5-minute periods by a second experimenter (experimenter 2). Experimenter 1 was solely responsible for recording data and was unaware of the stimulation delivered to subjects; therefore, this study was double-blinded. When the TENS unit was switched on, subjects were told to report the onset of any “tingling” or “buzzing” sensation beneath the TENS electrodes. The intensity of the current was increased until subjects reported a “strong but comfortable” sensation. The time at which subjects reported the latter was taken as zero. After the first, third, and fourth minute of each 5-minute stimulation period, subjects were asked if the reported sensation had decreased. If so, the intensity was increased to maintain a “strong but comfortable” sensation; thus, clinically representative intensity levels were used in the study. Subjects were given a demonstration of TENS on the nondominant forearm before the
Arch
Phys
Med
Rehabil
Vol
79, September
1998
OF TENS,
Walsh
experiment began. In the control group, the TENS electrodes were attached but no treatment was given; subjects were told they were not going to receive TENS during the experiment. Data Analysis For statistical analysis of NPL and MPT results, difference scores (ie, the variation from time zero) were used as a means of standardization. Analysis of variance (ANOVA) and post hoc tests were used, as appropriate, to assessdifferences over time and between groups for these two variables. A Kruskal-Wallis test was additionally used to analyze the TT data. RESULTS Results are presented in figures 2, 3, and 4. To improve clarity, results for TENS 1 and TENS 4 groups (both have 200,usecpulse duration) are presented separately from TENS 2 and TENS 3 groups (both have 50usec pulse duration). NPL NPL values were taken at the point of maximum negative peak amplitude for all subjects (resolution of NPL 0.20
c
I
I
3,lol------do 5
10
15
20
25
Time
0.20
30
35
40
45
50
55
60
(min)
1
TENS
i
0.15
":o.lo
B0
I
1 5
1 10
1
1
1
1
'
1
'
'
1
1
15
20
25
30
35
40
45
50
55
60
Time Fig 2. Summary (means -C SEM); and TENS 4 (0). TENS 3 (A).
(min)
of ipsilateral NPL difference scores for all groups n = 10 in each group. (A) Control (0). TENS 1 ( *p 5 .Ol; **p 5.05. (B) Control (0). TENS 2 (A), and
PERIPHERAL
EFFECTS
OF TENS,
1055
Walsh
at the 5-minute point; at 10, 15, 25, 45, and 60 minutes the increase in NPL in the TENS 1 group was significant compared with the control group; at 15,25,45, and 60 minutes there was a significant increase in NPL in the TENS 1 group compared with the TENS 4 group. There was no significant treatment effect between control, TENS 2, and TENS 3 groups. Repeated measures ANOVA showed no significant treatment effects for contralateral NPL data.
A -0
5
10
15 20 25 30
Time .I-c d
10
35 40 45
50
55
60
(min)
TENS
Ambient Temperature and Skin Temperature All five experimental groups were analyzed together for temperature data. Repeated measures ANOVA showed no significant interactive effect or differences between groups for ambient temperature; however, there was a significant increase over time (p = .OOOl). By the 60minute point the average increase in ambient temperature across all groups was .14 k .02”C (mean +- SEM). There was also a significant difference over time for both ipsilateral and contralateral skin temperature (p = .OOll and p = .OOOl, respectively), but no significant interactive effect or differences between groups.
8 I2 2 23 t-r
B -0
5
10 15 20
25 30 35 40 45
Time
50 55
18 16
TENS *
14 12 I--
i
60
(min)
Fig 3. Summary of ipsilateral SRN MPT difference scores for all groups (means f SEM); n = 10 in each group. (A) Control (0). TENS ), and TENS 4 (0). (B) Control (O), TENS 2 (A), and TENS 3 (f!J.
readings = .Olmsec). Results for ipsilateral NPL are presented in figure 2. Table 2 summarizes NPL data recorded from ipsilateral and contralateral sides for all experimental groups. The greatest increase in NPL occurred in the TENS 1 group (fig 2). NPL values continued to increase after the period of TENS application to reach a maximum at the 60-minute point (NPL difference score mean _f SEM = .l 1 k .04msecfor TENS 1 compared with - .03 t .02msec in the control group). In the TENS 3 group, NPL also continued to increase after TENS application to reach a maximum value at the 60-minute point (NPL difference score mean F SEM = .09 2 .05msec for TENS 3). Figure 5 illustrates two CAPSrecorded from a subject in the TENS 1 group. Analysis of NPL data using repeated measures ANOVA showed a significant treatment effect (p = .0036) between control, TENS 1, and TENS 4 groups; one factor ANOVA further showed significant differences at 5, 10, 15, 2.5, 45, and 60 minutes (p = .02, p = ,001, p = .OOl, p = .006, p = .03 andp = .02, respectively). Corrected post hoc Fisher tests showed a significant increase in NPL in both TENS groups compared with the control group
0
5
10
15
20 25 30
A
35 40 45
50
55 60
Time (min)
TENS
I--
K"
5
10 15 20 25 30 35 40 45 50
B
55 60
Time (min)
Fig 4. Summary of ipsilateral SRN TT difference scores for all groups (means f SEM); n = 10 each group. (A) Control (O), TENS 1 ( TENS 4 (0). *p 5 .Ol. (B) Control (O), TENS 2 (A), and TENS 3 (a).
Arch
Phys
Med
Rehabil
Vol 79, September
1998
1056
PERIPHERAL
Table
2: Summary
of lpsilateral
Control
Time
EFFECTS
OF TENS,
and Contralateral
TENS 1
Walsh
NPL Data TENS 2
(msec,
mean
f
SEMI
TENS 3
TENS 4
lpsilateral OMin 5Min
2.08
+ .02
2.26
+ .06
2.20
i
.04
2.31
t .09
2.05
t
2.29
t
IOMin 15Min
2.04 2.07
+ .03 + .03
2.33 t .06 2.35 2 .07
2.22 2.24
+- .03 + .02
2.30 2.31
t .08 2 .09
2.20
2 .04
25Min 45Min
2.05 t .03 2.08 k .03
2.34 t .07 2.36 + .08
2.28 2.26
2 .02 -+ .03
2.33 2.34
2 .09 + .09
2.19 2.18
-+ .05 + .04
GOMin Contralateral
2.04
2 .03
2.37
2.23 2.24
k .03 + .03
2.38 2.39
+ .I0 t .I 1
2.18 2.18
+ .05 + .05
OMin 5Min
2.11
+ .Ol
2.30
2 .06
2.16
+ .05
2.33
2 .08
2.16
+ .05
2.10 + .02 2.12 + .02
2.31
2 .06
2.15 2.16
i- .05 I! .05
2.35 2.36
-t .08 2 .09
2.17 2.17
2 .06 i .05
.02
.06
+ .08
2.10
-+ .02
2.33 2.33
t- .06 + .07
2.18
2 .05
2.37
i .09
2.18
I! .05
25Min 45Min
2.09 2.12
? .02 2 .02
2.31 2.36
+ .07 + .08
2.18 2.18
t .04 + .04
2.38 2.42
i- .09 t .09
2.16 2.17
+ .05 i- .05
6OMin
2.11
t- .03
2.37
2 .08
2.19
I
2.45
2 .09
2.18
t
TT The force (mN) of each filament was calculated from data supplied by the manufacturers. The most pronounced change in TT was observed at the ipsilateral SRN point; figure 4 shows changes in TT difference scores over time for this point. The TENS 1 group showed a marked increase in threshold immediately after TENS application (mean increase = 10.25mN), which then decreased to the same level as the other groups. Analysis of control, TENS 1, and TENS 4 data using KruskalWallis test showed significant differences at the 15minute point for ipsilateral SRN data (p = .Ol), with the TENS 1 group showing the greatest increase in TT. There was also a significant
I
lms
Fig 5. Two SRN CAPS recorded from a subject in the TENS 1 group; the upper CAP was recorded at 0 minutes (before TENS) and the lower at 15 minutes (ie, after TENS application). The two vertical cursors are positioned at the NPL values at 0 and 15 minutes, respectively. There was a change in NPL from 2.15msec at Omin to 2.29msec at 15min.
Phys
+ .05 i .05
IOMin 15Min
MPT The lower of the two MPT readings taken from each of the recording points was used for the purposes of analysis. Figure 3 shows ipsilateral SRN changes in MPT, after TENS application (ie, at the 15-minute point) there was a large increase in MPT in the TENS 1 group, in contrast to the smaller changes in MPT in the other three TENS groups. Analysis of MPT data for all 4 recording points, ie, ipsilateral SRN/LCNF and contralateral SRN/LCNF points, revealed no significant differences between groups; however, there were significant differences over time.
Arch
2.19 2.20
Med
Rehabil
Vol79,
September
1998
4
.04
.06
difference at the 45minute point for contralateral SRN T’I scores (p = .Ol), with the TENS 1 group again showing the greatest increase in TT when compared with the control and TENS 4 groups. No other significant differences were observed for TT data at any of the other recording points. In summary, statistical analysis of all data showed consistent effects of the TENS 1 group on NPL, MPT, and TT data on the ipsilateral side. DISCUSSION The purpose of this study was to determine the effect of different combinations of TENS pulse durations (50 and 200psec) and frequencies (4 and 11OHz) on conduction in the SRN and on MPT and TT within its area of distribution. Nonspecific effects (ie, outside the area of distribution of the stimulated nerve) of TENS on MPT and TT in the stimulated forearm were assessedusing MPT and TT measurements from an area innervated by the LCNF, and possible systemic or “central” effects were assessedusing concomitant measurements from the contralateral limb. Results clearly show that only one combination of TENS parameters (1 lOHz, 200psec) effected obvious and statistically significant changes in two of the variables assessed, ie, a measurable increase in NPL, with concomitant increases in TT within the area of distribution of the stimulated nerve. Although TENS 2 did effect an increase in measured NPL, this did not persist after the stimulation period. No combination of TENS parameters significantly affected MPT. These data extend our previous findings in showing that TENS, when applied at 1lOHz and 200ysec, effects the most significant hypoalgesia on experimentally induced mechanical pain in humans8 and would suggest that there may indeed be some functional relation between the action of TENS on NPL and its hypoalgesic efficacy. Concomitant measures on the contralateral limb showed an isolated significant effect of the TENS 1 group on contralateral SRN TT scores; however, overall results provided no direct evidence for either “systemic” or “central” effects of this particular combination of TENS parameters and, although not excluding the possibility of a “central” component to TENS-mediated hypoalgesia, further strengthen the argument for a “locally” mediated action. Although not presented in detail in the results section, statistical analysis of peak-to-peak amplitude and peak-to-peak duration data from the CAP records did not show any signifi-
PERIPHERAL
EFFECTS
groups. This would suggest that the various components of the CAP were equally affected by TENS because the results indicate that despite the shift in latency, the general shape of the waveform was not
cant changes for any of the experimental
significantly changed. Although the changes in conduction velocity of the SRN were very small, the data presented in this study are consistent with earlier findings9; there was a decrease in mean conduction velocity of 2.05m/sec in the TENS 1 group
by the 60-minute point, compared with an increase in the control group of O.gm/sec. Further studies using microneurographic techniques would be required to determine if the observed changes in conduction velocity were related to specific types of nerve fibers. Although we have consistently found an increase in NPL in response to TENS applied at 1lOHz and 200psec, previous
studies on the peripheral neurophysiologic effects of TENS have provided contradictory findings7*10,30;however lack of control data and variations in the stimulation parameters employed makes comparisons between the current study and previous work difficult. In contrast, most previous studies7J7 have reported measurable effects of TENS on pain, thermal, and touch sensitivity, although not all reports confine these to within the area of distribution of the stimulated nerve, and contralateral effects have been reported.‘5~16~1s In contrast, the current study has reported mainly ipsilateral effects following TENS application. Notwithstanding the contradictions evident in the literature, the findings from the current and previous studies12,3’strongly
suggest some peripheral component to TENS-mediated hypoalgesia. It is unlikely that this would be related simply to a reduction in nerve conduction velocity (as measured in this and other studies); the changes observed between the sensory thresholds and measured increase in NPL suggests there may be some relation between the two. Interestingly our findings are supported by Francini and associates,32who reported evidence of a relation between changes in sensory responses and reflex thresholds induced by TENS in a group of healthy volunteers. The authors measured the tendon and Hoffman reflexes and cutaneous pain threshold before and after the application of TENS (50Hz, lmsec). The results showed that the magnitudes of the TENS-induced changes in the pain thresholds and in the Hoffman and tendon reflexes were correlated, thereby suggesting some relation between these two variables. CONCLUSION The results of this study have demonstrated concurrent peripheral neurophysiologic and sensory effects after application of TENS over a peripheral nerve; the results suggest that these two effects may be related. In addition, the data have extended previous work at this center in showing parameterspecific peripheral effects of TENS. The greatest changes in both sensory and neurophysiologic effects of TENS were observed in the TENS 1 (1 lOHz, 200psec) group. Acknowledgment: The expert technical assistance of Mr. Robin Ritchie is gratefully acknowledged. References 1. Melzack R, Wall PD. Pain mechanisms: a new theory. Science 1965;150:971-9. 2. Marchand S, Charest J, Li J, Chenard J-R, Lavignolle B, Laurencelle L. Is TENS purely a placebo effect? A controlled study on chronic low back pain. Pain 1993;54:99-106. 3. Forster EL, Kramer JF, Lucy SD, Scudds RA, Novick RJ. Effect of TENS on pain, medications, and pulmonary function following coronary artery bypass graft surgery. Chest 1994;106:1343-8.
OF TENS,
1057
Walsh
4. Carroll D, Tram&r M, McQuay H, Nye B, Moore A. Transcutaneous electrical nerve stimulation in labour pain: a systematic review. Br J Obstet Gyn 1997;104:169-75. 5. Grimmer K. A controlled double blind study comparing the effects of strong burst mode TENS and high rate TENS on painful osteoarthritic knees. Aust J Phys Ther 1992;38:49-56. 6. Walsh DM. TENS: clinical applications and related theory. Edinburgh: Churchill Livingstone; 1997. 7. Campbell JN, Taub A. Local analgesia from percutaneous electrical nerve stimulation. Arch Neurol 1973;28:347-50. 8. Walsh DM, Foster NE, Baxter GD, Allen JM. Transcutaneous electrical nerve stimulation (TENS): relevance of stimulation parameters to neurophysiological and hypoalgesic effects. Am J Phys Med Rehabil1995;74: 199-206. 9. Walsh DM, Greer K, Baxter GD. Relevance of transcutaneous electrical nerve stimulation parameters to neurophysiological effects [abstract]. In: Proceedings of the 12th International Congress of the World Confederation for Physical Therapy: 1995 June 25-30; Washington. Washington: American Physical Therapy Association; 1995. p. 576. 10. Cox PD, Kramer JF, Hartsell H. Effect on different TENS stimulus parameters on ulnar motor nerve conduction velocity. Am J Phys Med Rehabil 1993;72:294-300. 11. Janko M, Trontelj JV Transcutaneous electrical nerve stimulation: a microneurographic and perceptual study. Pain 1980;9:219-30. 12. Levin MF, Hui-Chan C. Conventional and acupuncture-like transcutaneous electrical nerve stimulation excites similar afferent fibers. Arch Phys Med Rehabil 1993;74:54-60. 13. Chan CYW, Tsang H. Inhibition of the human flexion reflex by low intensity high frequency transcutaneous electrical nerve stimulation TENS has a gradual onset and offset. Pain 1987;28:239-53. 14. Hui-Chan C, Levin ME Stretch reflex latencies in spastic hemiparetie subjects are prolonged after transcutaneous electrical nerve stimulation. Can J Neurol Sci 1993;20:97-106. 15. Marchand S, Bushnell MC, Duncan GH. Modulation of heat pain perception by high frequency transcutaneous electrical nerve stimulation (TENS). Clin J Pain 1991;l: 122-9. 16. Zoppi M, Francini F, Maresca M, Procacci P. Changes of cutaneous sensory thresholds induced by non-painful transcutaneous electrical nerve stimulation in normal subjects and in subjects with chronic pain. J Neurol Neurosurg Psychiatry 1981;44:708-17. 17. Kudo S. Effects of TENS on the stimulated and non-stimulated sides: changes of pain threshold and skin temperature. J Phys Ther Sci 1994;6:109-13. 18 Eriksson MBE, Rosen I, Sjolund B. Thermal sensitivity in healthy subjects is decreased by a central mechanism after TENS. Pain 1985;22:235-42. 19. Lowe AS, McCormack K, Walsh DM, Baxter GD, Allen JM. The effects of hanscutaneous electrical nerve stimulation (TENS) parameters upon bilateral nerve conduction, mechanical pain and tactile thresholds in humans [abstract]. J Physiol 1996;495: 13 1. 20. Mannheimer JS, Lampe GN. Clinical transcutaneous electrical nerve stimulation. Philadelphia (PA): FA Davis; 1984. 21. Downie AW, Scott TR. An improved technique for radial nerve conduction studies. J Neurol Neurosurg Psychiatry 1967;30:332-6. 22. Mackenzie K, DeLisa JA. Distal sensory latency measurement of the superficial radial nerve in normal adult subjects. Arch Phys Med Rehabil 1981;62:31-4. 23. Bell-Krotoski J, Tomancik E. The repeatability of testing with semmes-Weinstein monofilaments. J Hand Surg 1987;12A:155-61. 24. MacDermid JC, Kramer JF, Roth JH. Decision making in detecting abnormal semmes-Weinstein monofilament thresholds in carpal tunnel syndrome. J Hand Ther 1994;7:158-62. 25. Bell-Krotoski JA. Fess EE, Figarola JH: Hiltz D. Threshold detection and Semmes-Weinstein monofilaments. J Hand Ther 1995;8:155-62. 26. Owen BM, Stratford CJ. Assessment of the methods for testing sensation in leprosy patients in a rural setting. Lepr Rev 1995;66: 55-62.
Arch
Phys
Med
Rehabil
Vol 79, September
1998
1058
PERIPHERAL
EFFECTS
27. Shapira SC, Magora F, Chrubasik S, Feigin E, Vatine JJ, Weinstein D. Assessment of pain threshold and pain tolerance in women in labour and in the early post-partum period by pressure algometry. Eur J Anaesthesiol 1995;12:495-9. 28. Reeves JL, Jaeger B, Graffradford SB. Reliability of the pressure algometer as a measure of myofascial trigger point sensitivity. Pain 1986;24:313-21. 29. Ohrbach R, Gale EN. Pressure pain thresholds in normal musclesreliability, measurement effects, and topographic differences. Pain 1989;37:257-63. 30. Ignelzi RJ, Nyquist JK. Excitability changes in peripheral nerve fibres after repetitive electrical stimulation. J Neurosurg 1979;51: 824-33. 31. Francini F, Maresca M, Procacci P, Zoppi M. The effects of non-painful transcutaneous electrical nerve stimulation on cutaneous pain threshold and muscular reflexes in normal men and in subjects with chronic pain. Pain 198 1; 11:49-63.
Arch
Phys
Med
Rehabil
Vol 79, September
1998
OF TENS,
Walsh
32. Walmsley RP, Monga TN, Prouix M. Effect of transcutaneous nerve stimulation on sensory nerve conduction velocity: a pilot project. Physiother Prac 1986;2:117-20. Suppliers a. Stoelting Company, Oakwood Centre, 620, Wheat Lane, Wood Dale, IL60191. b. Salter Weigh-Tronix, Ltd., George Street, West Bromwich, West Midlands B70 6AD, United Kingdom. c. Medelec Ltd., Manor Way, Old Woking, Surrey GU22 9JU, United Kingdom. d. Grant Instruments (Cambridge) Ltd., Barrington, Cambridge CB2 5QZ, United Kingdom. e. IT0 Company, Ltd., Tokyo, Japan. f. Gould Electronics Ltd., Roebuck Road, Hainault, Ilford, United Kingdom.