Introduction
Chronic pain from injury, including injury from whiplash trauma, is associated with
centrally mediated hyperalgesia, also known as central sensitization. [[1]] Several authors have described lowered pain thresholds in uninjured tissues, and
explained the finding as the expression of an abnormal processing of nociceptive information
in the brain and spinal cord. [[2],[3],[4],[5],[6],[7]] Others have postulated that chronicposttraumatic myalgia (muscle pains) mayperpetuateandaccentuate
thepain status of afflicted patients, [[8],[9]] and Ge et al. recently reported experimental evidence of a physiologic link between
myofascial trigger points and central sensitization in patients with shoulder pain
of myofascial origin. [[10]] In contrast, Curatolo et al, in a study of chronic whiplash patients, reported
that anesthetic infiltration of painful or tender points did not alter the signs of
central sensitization. [[11]] An explanation for this discrepancy may lie in the definition of what constitutes
a myofascial pain generator. While both ’tender points’ and myofascial trigger points
are painful to palpation, only myofascial trigger points will fasciculate or “twitch”
when probed with a needle. [[12]] The presence of a twitch response to needle probing has been experimentally demonstrated
as a necessary prerequisite for trigger point deactivation and pain relief with local
anesthetic injection. [[13]] Based on these previously reported findings, it is reasonable to hypothesize that
anesthetization of carefully identified trigger points may alter findings of central
sensitization in patients with chronic neck pain.
In the present study the authors set out to evaluate whether anesthetic infiltration
of myofascial trigger points in patients with chronic and refractory neck pain can
affect pain thresholds in uninjured parts of the body.
Methods
Seventeen participants were recruited from a group of twenty-threepatients who were
referred for surgical evaluation for chronic and refractory neck pain. Inclusion criteria
were as follows:
-
1. Male or female, 19–65 years of age.
-
2. Intrusive daily neck pain for at least 12 months.
-
3. Failure of conservative therapies, e.g. physical therapy, chiropractic manipulation,
or acupuncture.
Exclusion criteria included signs or symptoms of radiculopathy or myelopathy, or radiographic
evidence of significant spine pathology.
Institutional Review Board oversight and approval was provided by the Spinal Injury
Foundation in Westminster, Colorado.
Pre-intervention evaluations
1. Cervical range of motion (CROM) in flexion, extension, right and left lateral flexion,
and right and left rotation was determined with inclinometry. This evaluation was
only performed on the study subjects and not the controls and the subjects were blinded
as to the results.
2. Pressure pain thresholds (PPT) were determined on one side of the body using algometry
as described by Koelbaek-Johansen et al. [[2]] Laterality was determined according to subjects’ indication of the side with the
most intense symptoms. Left was chosen as the default side ifa patient could not differentiate
one side as more symptomatic then the other. The calibrated algometer had a range
of 0.5 – 25 lbs (1.1–13.4 kg) distributed over a 1 cm circular tip. Test sites were
in three different muscles: infraspinatus – 3 cm inferior to the scapular spine and
3 cm lateral to the medial scapular border; wrist extensor – 5 cm distal to the lateral
epicondyle with forearm in full supination; tibialis anterior – 5 cm distal to the
tibial tuberosity, and 2 cm lateral to the anterior tibial margin. Pain thresholds
were determined by slowly and gradually increasing the pressure of the algometer tip
against the marked test site. Identical protocols were used to assess pain thresholds
in the study group and the control group, and all subjects were blinded with to the
values recorded from each test.
3. Presence or absence of photophobia was determined by shining light from an ophthalmoscope
into the ipsilateral eye for 3 seconds. Photophobia was considered present if the
subject confirmed immediate onset or worsening of periocular pain or headaches. This
evaluation was only performed on the study subjects.
4. Subjective assessment of neck painwas madeusing a visual analogue scale (VAS) graded
0–10. This evaluation was only performed on the study subjects.
Intervention
One examiner performed all interventions. Myofascial trigger points (TP) were mapped
through palpation alongthe upper trapezius, and traced on the skin with a permanent
marker. Next, the focal pain generator within each trigger point was identified through
probing witha 25 gauge needleforsharp pain and a twitchresponse, and injected with
1–2 cc 1% lidocaine (10–20 mg). The procedure was repeated at other previously identified
and marked TPs until the subject indicated a significant relief of neck pain. Aminimum
of one, but never more than eight trigger points were injected in any of the subjects,
and no single TP was injected more than once. The total amount of 1% lidocaine injected
to any one subject varied from 2 cc to 10 cc.
Post-intervention evaluation
1. All pre-injection measurements(PPT, CROM, Photophobia, and VAS) were repeated following
the injections of lidocaine. Identical techniques were used, with the exception that
PPTwas only evaluated once. Post-injection PPT was determined within 60 seconds of
the final injection of local anesthetic (study group and controls), and remainingdata
was collected immediately thereafter. On no instance did more than seven minutes elapse
between the time of the final injection andcompletion of the data collection procedure.
Control Group
In addition to the symptomatic subjects a group of 10 volunteers with no history of
chronic neck pain was recruited to serve as a control for the intervention effects.
The controls were each evaluated for PPT twice at the same three sites as the symptomatic
subjects prior to an injection of 6 cc of1% lidocaine in the left thigh, and then
re-evaluated for PPT within 1 minute of the injection. The purpose of the control
group was to assess the effect of systemic lidocaine on PPT versus the more specific
trigger point infiltration in the symptomatic subjects.
Statistical analysis
Paired-sample t-tests were conducted on pre-injection/post-injection pairs for each
of the ten numerical evaluation measures – six cervical range of motion types, three
PPT sites, and the visual analog scale (Analyze-It, Leeds UK).
Results
Of the 23 patients presenting for evaluation 17 fit the inclusion criteria. The 6
patients who were excluded either had chronic pain that was not in the neck or had
neck pain for less than 12 months. The 17 remaining patients consisted of14 (82%)
female and 3 (18%) male subjects ranging in age from 26 to 59, with a mean age of
42.4 (SD 9.7). The duration of symptoms ranged from 1.5 to 18 years, with a mean duration
of 8.7 years (SD 6.0). All except one described a traumatic episode as the precipitating
factor for their neck pain, and most were injured in a traffic crash. ([Table 1]) Six subjects described current or past suicidal ideationbecause ofunremitting pain.
Although all subjects used non-steroidal anti-inflammatory drugson a regularbasis,
none reportedmore than occasional opioid use. All of the patients had been diagnosed
with a chronic pain syndrome that was intended to describe their unremitting symptoms
of neck pain, including myofascial pain syndrome, fibromyalgia syndrome, or simply
“chronic neck pain.” None of the subjects had been diagnosed with radiculopathy or
myelopathy or cervical central stenosis, although there were several with nonspecific
diagnoses of cervical spondylosis.
Table 1
Attibuted cause of chronic neck pain
|
Attributed cause of chronic pain
|
# subjects (%)
|
|
Traffic crash
|
11 (65)
|
|
Low speed rear impact
|
3 (18)
|
|
Moderate speed rear impact
|
3 (18)
|
|
Side impact
|
3 (18)
|
|
Front impact
|
2 (12)
|
|
Skiing injury
|
3 (18)
|
|
Fall
|
1 (6)
|
|
Lifting
|
1 (6)
|
|
Insidious onset
|
1 (6)
|
|
Total
|
17 (100)
|
The control group consisted of 6 male and 4 female volunteers with a mean age of 39.6
(SD 12.1).
As would be expected in a population of chronic neck pain patients, pre-injection
CROMin the study group was generally lower thanstandard reference values. [[14]] Following the TP injection all subjects demonstrated an increase in CROM in all
directions. ([Figure 1]; [Table 2])
Table 2
Pre and post injection cervical range of motion
|
CROM (degrees)
|
Pre-injection (mean, SD)
|
Post-injection (mean, SD)
|
% change
|
p-value
|
|
Flexion
|
33.9, 17.7
|
50.5, 10.5
|
49
|
0.000
|
|
Extension
|
40.6, 20.6
|
58.4, 12.6
|
44
|
0.001
|
|
R Lat Flexion
|
27.2, 11.9
|
40.0, 8.9
|
47
|
0.000
|
|
L Lat Flexion
|
31.0, 12.7
|
39.6, 12.3
|
28
|
0.016
|
|
R Rotation
|
50.2, 19.1
|
63.7, 10.5
|
27
|
0.002
|
|
L Rotation
|
44.3, 15.6
|
64.1, 11.3
|
45
|
0.000
|
Figure 1
Pre and post injection cervical range of motion in degrees. The abbreviations are as follows: Flex – flexion, Ext – extension, rlat flex – right
lateral flexion, llat flex – left lateral flexion, rrot – right rotation, lrot – left
rotation.
In the study group, there was no significant difference in pre-injection pain threshold
values at the shoulder or forearm;readings for the second pre-injection tibialis anterior
PPT test were significantly lower than for the first. ([Table 3]) In order to avoid selection bias in favor of significance, the more conservative
(higher) first pre-injection test was used to establish the pre-injection PPT in tibialis
anterior. The pooled mean of the two pre-tests was used to establish the pre-injection
level of the other two test sites – infraspinatus and wrist extensor.
Table 3
Pre-injection PPT values, and pooled mean used for comparison with post-injection
PPT
|
PPT (lbs)
|
Pre-injection 1 (mean, SD)
|
Pre-injection 2 (mean, SD)
|
p-value (1 vs. 2)
|
Pooled mean (mean, SD)
|
|
Infraspinatus
|
4.1, 2.1
|
3.8, 1.9
|
0.111
|
4.0, 2.0
|
|
Wrist Ext
|
4.3, 2.9
|
4.3, 2.6
|
0.986
|
4.3, 2.7
|
|
Tib Ant
|
8.2, 4.3
|
7.3, 3.9
|
0.003
|
7.7, 4.1
|
Statistically significant increases in pressure pain thresholds were documented within
the symptomatic groupafter the TP injections within 1 minute. ([Tables 3] and [4]) Post-injection pressure pain thresholds were 68, 78, and 64% greater at the infraspinatus,
wrist extensors, and tibialis anterior, respectively, in comparison with the pre-injection
thresholds in the study group. ([Figure 2]).
Table 4
Pre and post injection changes in PPT, all measurements taken within 1 minute of the
final injection
|
PPT (lbs)
|
Pre-injection (mean, SD)
|
Post-injection (mean, SD)
|
% change
|
p-value
|
|
Infraspinatus
|
4.0, 2.0
|
6.7, 2.3
|
68
|
0.000
|
|
Wrist Ext
|
4.3, 2.7
|
7.7, 3.4
|
78
|
0.000
|
|
Tib Ant
|
8.2, 4.3
|
13.4, 6.8
|
64
|
0.002
|
Figure 2
Pre and post pressure pain thresholds as measured in pounds of pressure. The abbreviations are as follows: infra – infraspinatus, Ext – wrist extensors,
Ant tib – Anterior tibialis.
Among the controls there were no significant differences between the two pre-injection
PPT values or between the second pre-injection PPT measurement and the post intramuscular
(IM) thigh injection PPT values at any of the three evaluation sites. ([Table 5])
Table 5
Pressure pain threshold for the controls
|
PPT (lbs)
|
Pre-injection 1 (mean, SD)
|
Pre-injection 2 (mean, SD)
|
p-value (1 vs. 2)
|
Post-injection (mean, SD)
|
p-value
|
|
Infraspinatus
|
11.7, 5.0
|
10.8, 4.8
|
0.548
|
11.1, 4.4
|
0.437
|
|
Wrist Ext
|
8.6. 3.6
|
8.2. 4.1
|
0.348
|
8.3. 4.1
|
0.447
|
|
Tib Ant
|
12.9, 5.4
|
12.7, 5.6
|
0.509
|
12.1, 4.4
|
0.288
|
Of 11 (65%) subjectswho showed evidence of photophobia before TP injections, only2
(12%)described any pain or discomfort from light at the post-injection testing (p
= 0.03).
A 57% reductionin neck pain was documented witha drop inVAS from a 6.1 (SD 1.5) before
the trigger point injections, to 2.6 (SD 1.8) after the trigger point injections (p
= < 0.001). ([Figure 3]). As the anesthetic wore off, however, all pre-injection symptoms returned to baseline
over a matter of hours to several days.
Figure 3
Pre and post injection subjective pain level.
Discussion
The present data demonstrate a remarkably rapid change in central sensitization symptoms
following the anesthetizing of painful trigger points. Since the infraspinatus PPT
site was relatively close to areas that had been infiltrated with anesthetic, the
wrist extensor and tibialis anterior pressure sites may be considered more reliable
indicators of alterations in centralpain modulation within the study group; however,
there were little differences among the three tested sitesand threshold increases
were uniform.
Symptoms of light sensitivity (photophobia) resolved in all but 2 of the 11 subjects
for the duration of the anesthetic. This result suggestsa central mechanism as the
mediator between the myofascial trigger points andlight sensitivity; however, the
phenomenon requires further investigation. The methods used in the present study were
quite elementary; the outcome was simply the perception of eye or head discomfort
when an opthalmoscope set on highest intensity was shined in the eye for 3 seconds.
More precise methods of measuring eye discomfort thresholds vs. light intensity would
have been required in order to draw any detailed quantitative conclusions. In contrast,
the increase in cervical range-of-motion observed in the symptomatic cohort following
the injections suggests that the initial finding of reduced motion was either a result
of peripherally modulated pain inhibition (from the trigger points) or centrally mediated
pain inhibition, or both.
Differences in the techniques adopted to identify and inject trigger points may explain
why the present resultsdiffer from those of Curatolo et al, who found no changes in
central sensitization with tender point (as opposed to trigger point) injections.
Despite the difference in outcome between the present study and what was described
by Curatolo et al., the hypothesis introduced by these authors, that central modulation
of pain is maintained by continued peripheral nociception arising from painful muscles
in the neck, provides a comprehensive explanatory model for our results. While physiological
and anatomical characteristics of myofascial trigger pointsare not completely understood,
it is reasonable to posit that these entities initially arise as a protective response
to injury. Why such focal pain generators persist and remain active after a reasonable
healing period for soft tissue injury remains unclear.
A nuisance effect that we attempted to control for was the potential impact of the
infusion of intramuscular lidocaine on the pain thresholds of the symptomatic subjects.
Prior authors have noted a decrease in experimentally induced hyperalgesia following
intravenous (IV) administration of lidocaine. [[15]] Wu et al. also demonstrated a significant decrease in stump and phantom pain in
an amputee population after 42 minutes of IV lidocaine infusion. [[16]] As opposed to the methods used in the present investigation, these studies involved
IV infusions of lidocaine. Thus, the purpose of the control group in the present study
was to evaluate the effect on pain thresholds of nonspecific IM lidocaine administration.
This is not to say that the present design is ideal; a randomized controlled trial,
in which the controls were identical to the cases (both groups with chronic neck pain)
would have been preferable. For this reason it is reasonable to interpret the results
of the present study with a modicum of caution.
The most important conclusion to be drawn from our results, relate to the immediate
and substantial changes in peripheral pain thresholds observed in the study group,
where some participants had been symptomatic for more than two decades. It has been
postulated that central sensitization is an expression of permanent structural or
biochemical changes (neuroplasticity) in the central nervous system,[[17]] and therefore unlikely to change regardless of intervention Our findings therefore
serve as an argument for central sensitization as a neuromodulatory process perpetuated
by, and dependent upon peripheral sources of nociception referred to as trigger point.
Rather than a neuroplastic condition, central sensitization may be a neuro elastic phenomenon. Our results also argue against psychological components as an etiological
factor rather than consequence of chronic pain after whiplash injury. [[18]] While it has been suggested that litigation or monetary issues may augment chronic
symptoms,[[19]] none of the subjects in the present study were involved in litigation and no reasonable
interpretation of the present data allows for any attribution of the observations
to financial motivation or emotional liability.
The number of subjects in the present study compares with previously published reports
within the same field of investigation, in which the authors have presented data based
on studies of 11–29 subjects. [[1],[3],[5],[6]] Nonetheless, some degree of caution is warranted in extrapolating the results of
this study to the general chronic neck pain population before further randomized and
placebo-controlled trials help bolster the validity of the conclusions presented here.
It is important to note that the success of the injections may largely depend upon
the skill and experience of the operator [[11]].
It is reasonable to suggest, based upon our conclusions and those of prior authors,
that algometry of both symptomatic and asymptomatic body sites may have a practical
clinical application in the contemporary evaluation of treatment success in chronic
neck pain patients.
Permanent solutions for the chronic pain conditions of the subjects in the present
study are few; one suggestion is to surgically excise or ablate symptomatic trigger
points that are associated with a decrease in local and generalized pain following
anesthetization. [[20]] Such an approach, although intriguing, requires further description and study.
Conclusion
Lowered pain thresholds related to chronic neck pain may rapidly reversed by precise
location and anesthetization of trigger points. While the full implications of this
finding are yet to be determined; treatments aimed at permanent ablation of peripheral
pain generators may offer a means of long term relief for this patient population.
The results of the present study refute claims that some or all of the pain experienced
by chronic whiplash patients arises from psychosocial issues, as the only treatment
experienced by the study subjects was directed at the identified trigger points. Although
encouraging, further study is needed to explore the ramifications of these findings.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
MF: Research design, data collection, data analysis, writing and editing of manuscript.
AN: Research design, study execution, writing and editing of manuscript. CC: Research
design, editing of manuscript. All authors read and approved the final manuscript.
