1. Applications of Transcutaneous Electrical Nerve
Stimulation in the Management of Patients with Pain
State-of-the-Art Update
MERYL ROTH GERSH and STEVEN L. WOLF
Numerous publications devoted to the topic of transcutaneous electrical nerve
stimulation (TENS) have appeared since the presentation of a special issue of
PHYSICAL THERAPY (December, 1978). This update article addresses contemporary
information on efficacy, mode of application, treatment outcomes, and neurophysiological mechanisms relevant to this modality. Investigators have become
far more specific when presenting this information in the current literature on
treating acute pain conditions with TENS than they were in the literature for the
1978 special issue. Improvement has been made in providing specific details to
enable replication of TENS stimulating characteristics among patients with
chronic pain; yet several clinical researchers still fail to evaluate treatment
outcomes adequately. Perhaps the greatest advances in our understanding of
TENS involve the recent development of mechanisms that might account for how
different types of TENS work. Suggestions for predicting patient responses to
TENS and for avenues of future inquiry are offered.
Key Words: Electric stimulation, Pain, Physical therapy.
A wealth of information is available
on the clinical application of transcutaneous electrical nerve stimulation
(TENS) for pain management. In recent
years, clinicians have studied the effect
of TENS on pain associated with specific pathological conditions and have
sought a relationship between specific
treatment protocols and outcomes. Authors have more closely attended to the
importance of specific electrode placements and stimulation characteristics,
so that studies on particular diagnostic
groups of patients could be compared
and replicated. More sophisticated pain
evaluation tools have been used to assess
a patient's response to TENS therapy.
The purpose of this article is to review
critically literature about TENS, which
has been generated after the publication
of a special issue on TENS in PHYSICAL
THERAPY in 1978, to determine if more
definitive information is available regarding 1) the efficacy of treatment for
specific diagnostic categories, 2) current
methods of application (specific elec-
Mrs. Gersh is a physical therapist at St. Luke's
Memorial Hospital, S 711 Cowley St, Box 288,
Spokane, WA 99210.
Dr. Wolf is Associate Professor, Department of
Rehabilitation Medicine, Emory University School
of Medicine, 1441 Clifton Rd, NE, Atlanta, GA
30322 (USA) and a senior investigator, Emory University Rehabilitation Research and Training Center, Atlanta, GA.
Address all correspondence to Dr. Wolf.
This invited paper was submitted July 16, 1984,
and was accepted September 7, 1984.
314
trode placements and stimulation characteristics) and their effects on treatment
outcomes, and 3) neurophysiological
modes of action. Topics for future clinical study will also be discussed.
TRANSCUTANEOUS
ELECTRICAL NERVE
STIMULATION FOR ACUTE
PAIN
One of the most successful applications of TENS is for postoperative pain
control.1-11 Although treatment protocols vary between different studies, important treatment variables are fairly
consistent among these studies.1 Patients are generally provided with a preoperative exposure to TENS to choose
comfortable stimulation settings. Sterile
electrodes are placed adjacent to the incision in surgery, and TENS treatment
commences in the recovery room, with
the stimulation variables set at a previously established comfort level. Transcutaneous electrical nerve stimulation is
used continuously for the first 48 to 72
hours; the patient regulates the stimulus
intensity to suit his needs. Treatment
outcomes are measured not only by subjective pain report, but also by the type
and amount of pain medication requested by the patient. Incidences of
postoperative ileus and atelectasis, records on compliance with respiratory
therapy regimens, and length of inten-
sive care and hospital stay also provide
objective measures of the patient's response to TENS treatment.
Schomburg and Carter-Baker evaluated the analgesic effect of TENS on 75
postlaparotomy patients.2 In comparing
these patients with a matched control
group by retrospective chart observation, the authors found that patients
using TENS postoperatively required 56
percent fewer doses of pain medication
during the first five postoperative days
than did patients in the control group.
Patients receiving TENS were more mobile and participated in breathing exercises earlier than their control group
counterparts.
Ali et al studied the pulmonary function of 40 patients who had undergone
cholecystectomies.3 Fifteen patients
used TENS continuously for thefirst48
hours postoperatively and then on an
"as needed" basis. Another 15 patients
did not use TENS, and a third group of
10 patients used TENS units with the
batteries reversed so that no current was
delivered to the patient (sham TENS).
Spirometric evaluations of all patients
conducted on the third and fifth postoperative days indicated that patients
who were treated with TENS had significantly higher vital capacities and functional residual capacities than patients
receiving either sham TENS or no
TENS. Patients using TENS had a significantly decreased incidence of postPHYSICAL THERAPY

2. PRACTICE
operative pulmonary dysfunction and
complications. Patients in all groups required supplemental pain medications,
but those patients in the TENS group
required less pain medication than did
those not receiving actual TENS treatment.
Taylor and associates conducted a
similar study with patients who had
undergone abdominal surgery.4 Thirty
patients used actual TENS and 22 patients used sham units for one hour
every four hours for the first three postoperative days. Patients were permitted
to request pain medication after 30 minutes of TENS treatment if the treatment
did not adequately control pain.
Twenty-five patients served as a control
group. Taylor and associates noted that
patients receiving TENS or sham TENS
required less pain medication and ambulated earlier than did those patients
in the control group.4 The results highlighted the placebo potential of TENS
but may also be explained by the noncontinuous mode of TENS application.
Another study examined the analgesic
effect of TENS on patients who had
undergone upper abdominal surgery.6
The patients who used TENS for postoperative pain control required 30 times
less pain medication than did those in
the control group. Improved pulmonary
function, appetite, and ambulation indicated an earlier recovery for those patients who used TENS than for those
patients who did not. Because the report
of this study lacked information on
treatment protocol and technique, replicating or comparing these results with
similar studies is impossible.
Several investigators have studied the
efficacy of TENS for management of
postlaminectomy pain.7-9 In all these
studies, electrodes were placed parallel
to the incision, stimulation was set at
comfortable levels, TENS was used continuously for at least the first 24 to 48
hours, and the treatment was discontinued after that period at each patient's
request. The investigators all reported a
significant decrease in the strength and
amount of pain medication requested
by the patients using TENS in comparison with those patients not using
TENS. Solomon et al reported that
TENS appeared most effective in "drugnaive" patients, those who had not used
narcotics preoperatively for more than
two weeks in the six months before surgery.7 Furthermore, they noted that
poor pain relief was reported by drugVolume 65 / Number 3, March 1985
experienced patients, regardless of
whether TENS or narcotics were used.
This occurrence may suggest a crosstolerance between narcotics and TENS
and activation of a similar neural substrate to explain the analgesic effect of
both TENS and opioid derivative medications.
Richardson and Siquiera carefully recorded the stimulation settings used.8
They observed no correlation between
specific pulse widths, rates, or stimulus
intensities and the degree of pain relief
reported. Other investigators have corroborated this finding.12
Additional benefits of postoperative
pain management with TENS may be
realized by the postcesarean patient.
Nonnarcotic pain control by use of
TENS may facilitate earlier mother-infant bonding. Drug-induced side effects
such as nausea, drowsiness, and respiratory depression are limited. Narcotics
are not passed to the baby by breastfeeding. Pulmonary rehabilitation is facilitated and reduces the occurrence of
pulmonary complications in the
mother.10
Harvie cited rehabilitation benefits
when using TENS to control postoperative pain after knee surgery.11 He studied patients who had undergone total
knee replacements, synovectomies,
meniscectomies, arthrotomies, patchplasties, or fracture reductions. Electrodes placed over the medial and lateral
collateral ligaments provided the most
effective pain control. Narcotic use was
decreased by 75 to 100 percent. Recovery of quadriceps femoris muscle
strength and knee range of motion
(ROM) was facilitated. Four of seven
patients with total knee replacements
achieved 80 to 90 degrees of active knee
flexion by the sixth postoperative day;
the other three patients achieved the
same goal by the eighth postoperative
day. Earlier ambulation and decreased
length of hospital stay were also reported. Clearly, TENS for management
of postoperative knee pain is an important adjunct to a rehabilitation program.
Transcutaneous electrical nerve stimulation can also be applied for control
of acute dental pain.13, 14 Hansson and
Ekblom evaluated 62 patients admitted
to an emergency dental clinic with acute
pain secondary to pulpal inflammation,
apical periodontitis, or postoperative
pain after tooth extraction.13 Patients
were randomly assigned to one of three
groups: those receiving high frequency
TENS (100 Hz; n = 22); those receiving
low frequency TENS (2 Hz; n = 20);
and those receiving a placebo treatment
(batteries removed from the unit; n =
20). Electrodes were placed on the face
over the painful area. Stimulus intensity
was set to three times the sensory threshold for patients in the high frequency
group, and three to five times sensory
threshold for those receiving low frequency TENS. This latter group experienced muscular contractions associated with the higher intensity. Patients
used a visual analog scale to record their
pain intensity before, during, and after
treatment. Seven of 22 patients (31.8%)
in the high frequency group reported
pain relief of greater than 50 percent
after 30 minutes of treatment, compared
with 9 of 20 patients (45%) in the low
frequency group, and 2 of 20 (10%) in
the placebo group. Pain returned within
10 minutes after treatment in 4 of 7
patients in the high frequency group,
and in 2 of 9 patients in the low frequency group. The 2 patients in the
placebo group who reported initial relief
experienced longer lasting relief. Two
other patients in the high frequency
group and 2 in the low frequency group
reported complete pain relief after treatment. Differences in the analgesic effectiveness of TENS demonstrated between
the high and low frequency groups were
not significant. The effectiveness of
TENS for pain control, however, was
significantly greater when either experimental group was compared with the
placebo group. Pain control of longer
duration might have occurred if treatment duration could have been longer
than 30 minutes.
Transcutaneous electrical nerve stimulation is being used, especially outside
of the United States, to control acute
pain associated with labor and delivery.15, 16 Erkola et al evaluated 100 patients who used TENS for pain management during the first stage of labor.15
Electrodes were placed paravertebrally
at T10-11 and S2-4. Stimulus intensity
was set at a tolerable submotor threshold
and regulated by the patient. Thirty-one
percent of the patients reported good
pain relief, and 55 percent reported
moderate relief within one hour of initiating treatment. Details of the pain
rating procedure were not described. Patients using TENS, however, requested
a similar amount of pain medication
during labor in comparison with a control group who did not use TENS.
315

3. Jones reported that 82 percent of the
patients in labor using TENS had substantial relief of back labor pain and 71
percent had significant relief of abdominal labor pain during the first stage of
labor.16 Again, methods used to measure
pain were not described. During the second stage of labor, TENS was frequently
discontinued because it interfered with
the patient's controlled breathing and
pushing efforts. Transcutaneous electrical nerve stimulation also interfered
with continuous fetal monitoring. The
use of TENS did not affect the length of
labor or immediate postnatal health of
the infant.
Further investigation of the role of
TENS in the management of labor pain
is warranted with close attention paid to
application techniques and measurement of treatment outcome. Reduction
of the need for narcotics during labor
could contribute to the improved perinatal and postnatal health of the mother
and the improved respiratory and neurological status of the newborn child.
Methods of application for TENS to
control acute pain are summarized in
Table 1. All but one report provide specific electrode placements for particular
pain locations. Ranges are given most
frequently to describe stimulation settings used, and the frequency and duration of TENS treatment is reported.
The provision of application details in
recent literature allows more accurate
comparison and replication of clinical
research.
Table 2 summarizes the evaluation
tools used to assess TENS treatment
outcomes for acute pain management.
A variety of subjective pain rating scales
and recording of pain medication intake
were used most commonly to assess the
analgesic effect of TENS. In three studies, additional credence was given to
favorable treatment outcomes by use of
objective physical evaluations, such as
pulmonary function studies or joint
range-of-motion measurements. In addition to the patients' reports of pain,
objective evaluation procedures enhance the reliability and validity of these
clinical studies.
Recent literature has been favorable
on the efficacy of TENS for acute pain
control. The location and description of
acute pain is usually precise and allows
for use of a more specific treatment
approach. Homogeneous groups of patients (eg, those with postoperative pain)
and matched control groups are readily
available for evaluation. Treatment outcomes may be objectively measured in
terms of medication intake, respiratory
status, rehabilitation factors, and subjective pain ratings. These advantages are
not as readily available when studying
the management of chronic pain and
may explain the wide variation in response to TENS treatment among
chronic pain patients.
TRANSCUTANEOUS
ELECTRICAL NERVE
STIMULATION FOR CHRONIC
PAIN
Studies examining patients with
widely divergent diagnoses or symptom
complexes are not as prevalent in the
TENS literature today as they were several years ago. These studies can provide
valuable information in selecting which
diagnostic groups of patients respond
most favorably to TENS for pain relief.
Wolf and colleagues evaluated the responses to TENS of 114 patients with
chronic pain.12 Patients reported pain
secondary to peripheral neuropathy, peripheral nerve injury, radiculopathy, or
musculoskeletal trauma. Electrodes
were systematically placed at the painful
TABLE 1
Transcutaneous Electrical Nerve Stimulation Application Methods for Acute Pain
Primary Author
Diagnosis
Electrode Placement
Pulse Width
(µ see)
Pulse Rate
Intensity
(PPS)
0-90 V (comfort)
10-100
Frequency and Duration
of Treatments
Schomburg2
postlaparotomy
parallel to incision
120-340
Ali3
parallel to incision
128-200
10-100
0-135 mA (comfort)
Taylor4
Sodipo6
Solomon7
postcholecystectomy
postlaparotomy
postlaparotomy
postoperative
constant for first 48
hr, then as needed
constant for first 48 hr
80
40
comfort
60 min every 4 hr
Richardson8
postlaminectomy
parallel to incision
parallel to incision
1.0 cm parallel to
incision
5 cm parallel to
incision
Schuster9
postlaminectomy
Riley10
Harvie11
postcesarean
section
postoperative
knee pain
Hansson13
dental pain
2.5 cm parallel to
incision
above and below
incision
over medial and
lateral collateral
ligaments
over painful site
constant for first 48 hr
72.5240.0
8.7-240
0.2-38.5 mA
40-100
25-100
0-90 V
250-400
80-100
20-35 mA
comfort
labor pain
Jones16
labor pain
316
paraspinal T10L1, S2-S4
200
100
84
Erkola15
within first 20 hr postoperatively, for
3-12 days
constant for first
18-24 hr
constant, or 30 min
four times a day
2
2-3 times sensory
threshold or
3-5 times sensory
threshold
20-25 V (comfort)
comfort
30 min
30 min
during first stage of
labor
during first stage of
labor
PHYSICAL THERAPY

4. PRACTICE
TABLE 2
Evaluation Methods for Transcutaneous Electrical Nerve Stimulation Treatment for Acute Pain
Primary Author
Diagnosis
Subjective
Pain Rating
Pain Medication
Taken
Physical
Evaluations
Schomburg2
postlaparotomy
yes
yes
Ali3
no
yes
Taylor4
Sodipo6
Solomon7
Richardson8
postcholecystectomy
postlaparotomy
postlaparotomy
postoperative
postlaminectomy
yes
no
yes
yes
yes
yes
yes
yes
none
Schuster9
postlaminectomy
yes
yes
none
Riley10
postcesarean
section
postoperative
knee pain
yes
yes
none
no
yes
dental pain
labor pain
labor pain
yes
yes
yes
no
yes
yes
Other
knee range of motion, straight leg
raise, early ambulation
none
none
none
Harvie11
Hansson13
Erkola15
Jones 16
site or on related nerve roots or peripheral nerves. Stimulation variables were
set to evoke a strong but comfortable
sensation in the painful region, and exact electrical settings were recorded.
Treatments were conducted on an outpatient basis and were of 30- to 45minute duration. Patients rated their
pain intensity on a 10-cm line before,
during, and immediately after treatment. In addition, some patients completed the pain descriptor word list
found in the McGill Pain Questionnaire.17 Thirteen of 18 patients (72%)
with peripheral neuropathy, 6 of 21 patients (28.5%) with peripheral nerve injury, 8 of 36 patients (22%) with radicular pain, and 15 of 39 patients (38.4%)
with musculoskeletal pain reported
more than 60 percent relief of pain after
TENS treatment.12 In the peripheral
neuropathy group, patients with postherpetic neuralgia responded most
favorably to TENS. Patients with fewer
previous analgesic treatments, no surgical intervention, and limited narcotic
use responded more favorably than
those patients with numerous previous
treatments. We found no significant relationship between specific electrode
placements or stimulation settings and
treatment outcomes, but patients with
radiculopathy or peripheral nerve injury
responded better to higher intensity
stimulation. This observation was also
reported by Melzack in treating patients
with chronic low back pain.18 Follow-
Volume 65 / Number 3, March 1985
spirometry, arterial
blood gases
pulmonary functions
up evaluations on 25 patients who used
TENS at home for one month generally
indicated decreased benefits from treatment as time progressed. These decreased benefits may have been due to
reduced patient compliance when independent TENS application became a
requirement.
Another investigation studied 98 patients with back pain, headache, or a
variety of other pain symptoms.19 Patients used TENS at home, placing electrodes at the site of pain, and setting
stimulation intensity at a comfortable
level. Patients recorded their own subjective pain level before and after treatment. After 12 days of home treatment,
69 percent of the patients with low back
pain, 40 percent of those with headache
pain, and 60 percent of those with pain
from other sources reported more than
50 percent relief of pain. The authors
failed to describe stimulation settings,
pain-rating measures, and duration and
frequency of treatment; they also did
not control for a wide variation in application techniques based on patient
competence and compliance. Thus, this
study provided little valuable information on TENS for chronic pain control.
Santiesteban described the use of low
frequency TENS (2-4 Hz) for treatment
of spinal pain.20 Stimulus pulse width
was set at the maximum for the units
used, and intensity was set at 50 mA to
evoke a muscle contraction within pain
tolerance. Electrodes were placed 2.5 to
resumption of activities
postoperative
complication
resume ambulation
resume ambulation
length of hospital
stay
postoperative
complication
5 cm from the appropriate spinous process in a parallel or crossed configuration.
Distal acupuncture points were also
stimulated. Patients required less analgesic medication when TENS was used
to control pain.
Melzack and colleagues recently compared the analgesic effects of TENS and
massage in a double-blind study of 41
patients with chronic low back pain.21
Transcutaneous electrical nerve stimulation electrodes were placed in the center of the back and on the lateral thigh.
Low frequency stimulation (4-8 Hz)
with a strong but tolerable intensity was
applied. The massage was performed
with a suction cup apparatus. Treatment
was given two times a week for 30 minutes, for a maximum of 10 treatments.
Treatment outcomes were evaluated using both the present-pain intensity (PPI)
scale and the pain-rating index of the
McGill Pain Questionnaire.17 Bilateral
straight leg raising (SLR) and lumbosacral flexion were also measured.
Transcutaneous electrical nerve stimulation produced a significantly greater
improvement than massage in the painrating and the PPI scales and in the
bilateral SLR measures for these patients.21
Transcutaneous electrical nerve stimulation has been used with various degrees of success in the management of
arthritic pain. Taylor et al evaluated the
effect of TENS on osteoarthritic knee
pain.22 Patients used actual TENS or a
317

5. TABLE 3
Transcutaneous Electrical Nerve Stimulation Application Methods for Chronic Pain
Primary Author
Diagnosis
Wolf12
varied
Moore19
Electrode Placement
Pulse Width
(µ sec)
Pulse Rate
(pps)
Intensity
Frequency and Duration
of Treatments
100
50-100
submotor
threshold
30-45 min, 3-5
times a week
varied
site of pain, related
nerve roots, or
peripheral nerve
varied
midrange
10-100
or 1-4
comfort
Santiesteban20
spine pain
paravertebral
maximum
2-4
Melzack21
low back
pain
osteoarthritis
of knee
phantom
limb pain
nonunited
fracture
center of back and
lateral thigh
about knee
4-8
motor
threshold
(50 mA)
to tolerance
30-60 min daily or
as needed
30-60 min
comfort
comfort
stump or contralateral limb
over fracture site
in crossed pattern
100 or 2
peripheral
neuropa-
along nerve trunk
at site of pain
Taylor22
Winnem27
Kahn29
Gersh24
300
minimum
200
110
sensory
threshold
(less than
20 mA)
26-28 mA
30 min, 2 times a
week
30-60 min as
needed
15 min twice a day
30-60 min, 3-4
times a day
continuous, 8-10
hr a day
thy
placebo unit wired to produce various
sounds in a well-monitored home program. After two weeks of home treatment, patients were reevaluated and
sent home to use the other (TENS or
placebo) unit for another two weeks.
Patients were evaluated again and permitted to take home the most beneficial
unit for one more month of home treatment. Responses to treatment were evaluated by subjective pain rating, ambulation distance, and analgesic medication intake. The actual TENS provided
significantly more pain relief than did
the placebo unit in both subjective and
medication analyses. Patients reported
the greatest pain relief while wearing the
active TENS unit. Relief frequently
lasted for several hours after treatment
was completed. Several patients continued to use the TENS at home for several
months. They reported decreasing pain
relief over time, possibly because of increasing joint deterioration.
Transcutaneous electrical nerve stimulation may be an important adjunct in
the rehabilitation of arthritic patients,
particularly when joint replacement is
not possible. In patients with chronic
systemic diseases who may be receiving
a variety of pharmacologic and therapeutic treatments concurrently, the clinician must be alert, however, to adverse reactions to TENS, as reported by
Griffin and McClure.23
318
Patients with a variety of peripheral
neuropathic conditions including peripheral neuropathy,24 postherpetic neuralgia, peripheral nerve injury, reflex
sympathetic dystrophy,25 and Sudeck's
atrophy26 have all responded favorably
to TENS treatment. Transcutaneous
electrical nerve stimulation has also
proven effective in the management of
phantom limb pain27 and the distal
burning paresthesia associated with
Guillain-Barré syndrome.28
Kahn provided radiographic evidence
that TENS facilitated callous formation
and osseous bridging at sites of nonunited fractures in three patients.29
Transcutaneous electrical nerve stimulation was originally applied to control
pain in these patients for nonunited
fractures six months after injury. Electrodes were placed in various configurations to "sandwich" the fracture site.
Pulse width was set for the longest "on"
time, pulse rate was set at the lowest
available frequency, and stimulus intensity was set at the sensory threshold.
Increased callous formation was noticed
on radiographic examination after one
month of treatment in one patient and
after 10 weeks of treatment in the other
two patients.
Millea described another unusual application of TENS.30 A 50-year-old patient with an eight-year history of nonoperative abdominal pain and disten-
tion was relieved of this discomfort after
using TENS for five days. This relief
may be attributed to decreased sympathetic tone and increased gastric motility
associated with TENS application.31
Owens et al observed local vasodilation and skin temperature increases of
1°C when TENS was applied at the ulnar
groove and wrist in seven healthy subjects.31 Such evidence also may explain
the mechanism of pain relief in patients
with causalgia or reflex sympathetic dystrophy. Consistent sympathetic nervous
system responses, however, have not, as
yet, been recorded among a variety of
patients.25
Table 3 summarizes the application
procedures used for chronic pain control
with TENS. Significant effort has been
made by most investigators in recent
years to specify effective electrode placements and stimulating settings. Although specific pulse widths, rates, and
intensities are not always cited, most
reports provide a description of the sensory or motor responses elicited by
TENS during treatment. Treatment duration was usually 30 to 60 minutes, but
the frequency of treatment varied with
each study. Replication of clinical studies is facilitated when these procedures
are described in detail.
Perhaps the weakest aspect of the clinical study of TENS for chronic pain
control is evaluation of treatment outPHYSICAL THERAPY

6. PRACTICE
comes. Table 4 illustrates that most investigators still rely solely on the patient's report of pain to establish the
efficacy of TENS treatment. Often, the
pain-rating scale used by the patient is
not described in detail. The great variety
of pain symptoms, locations, previous
and concomitant treatments, medications, and psychological components associated with chronic pain make objective evaluation much more difficult
than in patients with acute pain. Use of
physical measures, such as joint motion,
strength, muscle girth, and participation
in functional activities, however, would
enhance the objective evaluation of the
efficacy of TENS for chronic pain control.
PREDICTING RESPONSE
TO TRANSCUTANEOUS
ELECTRICAL NERVE
STIMULATION TREATMENT
Successful use of TENS for pain control may be increased as more specific
patient evaluation and selection criteria
are established. Reynolds and associates
examined the predictive value of pain
questionnaires in selecting patients who
would be more likely to respond favorably to TENS treatment.32 Their evaluation indicated that older, retired patients, who had pain of less than one
year duration, who had undergone limited or no surgery, and who used nonnarcotic analgesics were more likely to
experience pain relief with TENS. Site
of injury, sensory deficit, and secondary
gain by financial compensation for injury did not affect response to treatment. The pain questionnaire, however,
seemed to have less predictive value for
TENS than for other treatment regimens.
In another study, Johansson et al suggested that patients with neurogenic
pain responded more favorably to
TENS than did patients with somatogenic or psychogenic pain.33 Patients
with pain in the extremities seemed to
derive more relief with TENS than patients with axial pain. The patient's age,
sex, and pain intensity did not relate to
his response to treatment.
Richardson and colleagues explained
how treatment with TENS could confirm a diagnosis of functional pain compared with organic pain.34 Many patients with suspected functional pain reported increased pain during and after
TENS treatment. Pain was relieved with
a saline injection in the majority of these
patients.
Mannheimer compiled a list of factors
that hinder, enhance, or restore the effectiveness of TENS for pain control.35
Among those factors that enhance
TENS effectiveness are careful, continuous patient evaluation for most effective electrode placement sites and stimulation settings; changing stimulation
modes and characteristics; gradually increasing patient tolerance to stronger
stimulation in the painful area; elec-
trode placement on motor points or superficial aspects of nerves; weaning patients from addictive medications before
treatment; and educating the patient in
the proper use of the modality for home
treatment. Incorporating these selection
and treatment criteria into treatment
protocols and recording which patients
most favorably respond to TENS will
increase the successful use of this modality in the future.
NEUROPHYSIOLOGICAL MODES
OF ACTION
Several years ago, the options available to explain the possible neurophysio l o g y mechanisms through which
TENS could affect pain perception were
limited.36 The prevailing explanation for
most pain attenuating interventions
cited the spinal gate concept developed
by Melzack and Wall in 1965.37 Briefly,
this notion took into account existing
electrophysiological data from animal
experiments that had demonstrated differential effects of collateral axons from
large diameter afferent fibers mediating
touch and pressure and from small diameter afferent fibers conveying nociceptive input upon interneurons within
the substantia gelatinosa (Fig. 1). These
interneurons could be facilitated
through predominantly large diameter
collateral afferent input and inhibited
through primarily collateral axons from
the small diameter system. In addition,
the interneuron was inhibitory onto the
TABLE 4
Evaluation Methods for Transcutaneous Electrical Nerve Stimulation Treatment for Chronic Pain
Primary Author
Diagnosis
Subjective
Pain Rating
Pain Medication
Taken
Physical
Evaluations
Wolf12
varied
McGill Pain
Questionnaire
no
none
Moore19
Santlesteban20
Melzack21
varied
spine pain
low back
pain
yes
no
no
yes
no
Taylor22
osteoarthritis
of knee
yes
yes
Winnem27
phantom
limb pain
nonunited
fracture
peripheral
neuropathy
yes
no
none
none
straight leg raise,
lumbosacral
range of motion
roentgenogram,
ambulation
distance
none
yes
no
roentgenogram
yes
no
none
Other
Kahn29
Gersh24
Volume 65 / Number 3, March 1985
McGill Pain
Questionnaire
resume functional
activities
319

7. Periphery
Spinal Cord
Lamina II & III
Spinal Cord
Lamina V
Fig. 1. Schematic diagram depicting the Melzack-Wall gate theory of pain. Open circles
represent facilitator/ synapses; closed circles indicate inhibitory synapses. Abbreviations SG
= substantia gelatinosa; T = transmission cells.
terminals of both afferent fiber classes.
Consequently, when large diameter
afferent fiber activation was of greater
frequency and intensity than smaller
diameter fiber input, the inhibitory
interneurons would be activated to
presynaptically inhibit transmission
centrally from both the noxious and
nonnoxious inputs. The gate would be
closed. Of course, the opposite effect
would predominate if greater transmission occurred through the smaller diameter system.
This gating theory was subjected to
considerable criticism because it conceptually failed to account for pain relief
among a variety of clinical conditions.
Nonetheless, the test of time has proven
that the framework for the theory has
formed the basis for several more contemporary explanations of pain alleviation through TENS. Specifically what
the Melzack-Wall model brought to the
attention of scientists and clinicians was
the recognition that pain perception
could be modulated somewhere within
the neuraxis if the appropriate stimuli
could be delivered and the appropriate
neural substrate on which such stimuli
might act could be found.
A spinal gate that conceptually follows the original model might incorporate conventional TENS (low intensity,
high frequency stimuli) to effect pain
reduction among patients with a diagnosis of postherpetic neuralgia. This disease process causes selective degeneration among large diameter peripheral
axons. The success with conventional
TENS may reside in the activation of
320
remaining large afferent fibers or those
in close proximity to the painful site but
which enter the neuraxis at the same or
nearby segments as the ongoing noxious
input.38 A similar explanation may be
appropriate to explain how pain following certain kinds of peripheral nerve
injury may respond to conventional
TENS.39
Recently, clinicians have recognized
that conventional TENS may not be the
most effective form of stimulation for
certain types of chronic pain. This
thought was promoted when Ericksson
and co-workers identified a large group
of patients with chronic pain who
showed further improvement in reduced
pain perception when conventional
TENS was supplemented by acupuncture-like TENS (low frequency, high intensity stimulation).40 This latter form
of stimulation showed effects that were
reversible through the administration of
the opioid antagonist, naloxone hydrochloride; this reversal suggests that the
effects of acupuncture-like TENS might
be mediated through an endogenous
opiate system within the neuraxis.41 Previously, Mayer and colleagues had demonstrated that the effectiveness of acupuncture was also reversed by naloxone
hydrochloride.42 These clinical findings
prompted a comprehensive search for
the neural substrates mediating the responsiveness of chronic pain patients to
high intensity cutaneous stimulation.
At the same time, a variety of opiate
receptors and numerous loci of endogenous opiates were being discovered in
many human and subhuman primate
studies.43 A logical marriage from this
exponentially increasing body of knowledge resided in establishing relationships
between neurophysiological and neurohistochemical studies on pain mechanisms and opiate substances, respectively. The mechanismfirstproposed by
Basbaum and Fields in 1978 served to
collate known histochemical and physiological data to explain how high intensity cutaneous electrical stimulation (for
example, acupuncture-like TENS, briefintense TENS, or burst trains of TENS)
might activate endogenous opiates to
alleviate pain.44
This modulatory mechanism is essentially a negative feedback loop that is
schematically illustrated in Figure 2.
Ongoing pain input and the discomfort
often associated with high intensity
TENS activate ascending pathways leading to conscious awareness of pain. Certain axons within the ascending system
are known to form a synapse within
medullary reticular formation nuclei,
and from these nuclei, this input is
transmitted to the periaqueductal gray
region of the midbrain (mesencephalon). This location is exceptionally endowed with high concentrations of endogenous opiates, and when it is activated, either through natural cutaneous
Central Nervous System
Fig. 2. Schematic diagram of negative feedback loop within the neuraxis activated by noxious
input.
PHYSICAL THERAPY

8. PRACTICE
stimulation, iontophoretically applied
morphine, or through direct stimulation, its efferent axons form a synapse
with nuclei (raphe magnus and reticularis magnocellularis) within the medulla oblongata. Output from these nuclear groups descends through the dorsolateral funiculus of the spinal cord to
make enkephalinergic synapses known
to inhibit the spinal transmission of Substance P, a polypeptide implicated as a
neurotransmitter between axons conveying noxious information.45 This last
neural interaction completes the negative feedback loop to modulate ongoing
or subsequent noxious input. For further details, please refer to the Pain:
Mechanism: B. Basic section within the
Bibliography.
Another mechanism that may account for some aspects of pain modulation with TENS involves what LeBars
et al have termed "diffuse noxious inhibitory controls," or DNIC.46 Within
this system, responses elicited through
continuous pain input to convergent
dorsal horn neurons may be suppressed
effectively by noxious or intense cutaneous stimulation, when it is applied
almost anywhere on the body surface.
Responses obtained through activity
within the small diameter afferent fiber
groups are inhibited, but nonnoxious
activation of the same convergent cells
or nonconvergent cells responsive to
only noxious stimuli remain unaffected.
Within animal models, spinalization
eliminates DNIC, thereby suggesting
that descending supraspinal influences
are required to activate this system. Furthermore, the DNIC mechanism is sensitive to naloxone hydrochloride; this
sensitivity indicates an endorphin link.47
Whether this linkage occurs at spinal or
supraspinal levels has yet to be determined. Also, definitive data to test the
validity of the DNIC model in man have
yet to be presented.
Nonetheless, the mechanisms described in this article form plausible explanations for the way in which high
intensity TENS might modulate pain
perception. Other mechanisms have
been proposed, but both the quantity
and quality of research led us to refrain
from addressing these in this article. Undoubtedly, as more data evolve and histochemical and electrophysiological
techniques gain sophistication, additional ways of speculating on or comprehending how TENS modulates pain
perception will be forthcoming.
Volume 65 / Number 3, March 1985
AREAS FOR FUTURE STUDY
To facilitate the continued effective
use of TENS for pain control, several
areas of study must be pursued. Patient
evaluation and selection criteria should
be validated and refined to increase successful treatment with TENS, particularly in patients who have chronic pain.
Specific electrode placements and stimulation characteristics must be evaluated in relation to specific disease entities to establish more effective treatment
protocols. Clinicians should continue to
evaluate the benefits of high versus low
frequency stimulation, auriculotherapy,48 and acupuncture point stimulation. Use of TENS for acute pain control should be expanded within areas
where it is apparently effective (eg, postoperative pain, labor and delivery pain,
and pain from acute injury48). Adverse
responses to treatment such as contact
dermatitis49, 50 should be reported so that
hypoallergenic materials can be developed in the manufacturing of electrodes
and conductive media, and so that patients at high risk for negative responses
to treatment may be screened.23 Ongoing evaluation of long-term use of TENS
by chronic pain patients may yield information on long-term effectiveness
and clarify the neurophysiology on
which treatment is based. The expanding body of knowledge resulting from
applied and basic research on neurochemical and physiological bases for
pain control must address the modus
operandi of TENS, taking into account
the stimulus characteristics applied
within experimental protocols and how
the relationship between stimulation
and response explains the efficacy of this
modality.
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