Randomized clinical trial of trigger point infiltration with lidocaine to diagnose anterior cutaneous nerve entrapment syndrome

Randomized clinical trial
Randomized clinical trial of trigger point infiltration
with lidocaine to diagnose anterior cutaneous nerve
entrapment syndrome
O. B. A. Boelens1, M. R. Scheltinga1, S. Houterman2 and R. M. Roumen1
1 Department of Surgery and 2M´axima Medical Centre Academy, M´axima Medical Centre, Veldhoven, The NetherlandsCorrespondence to: Mr O. B. A. Boelens, Department of Surgery, M´axima Medical Centre, de Run 4600, PO Box 7777,5500 MB Veldhoven, The Netherlands (e-mail: [email protected]) Background: Anterior cutaneous nerve entrapment syndrome (ACNES) is hardly considered in the
differential diagnosis of chronic abdominal pain. Some even doubt the existence of such a syndrome and
attribute reported successful treatment results to a placebo effect. The objective was to clarify the role
of local anaesthetic injection in diagnosing ACNES. The hypothesis was that pain attenuation following
lidocaine injection would be greater than that after saline injection.
Methods: Patients aged over 18 years with suspected ACNES were randomized to receive an injection
of 10 ml 1 per cent lidocaine or saline into the point of maximal abdominal wall pain just beneath the
anterior fascia of the rectus abdominis muscle. Pain was recorded using a visual analogue scale (VAS;
1–100 mm) and a verbal rating scale (VRS; 0, no pain; 4, severe pain) during physical examination just
before and 15–20 min after injection. A reduction of at least 50 per cent on the VAS and/or 2 points on
the VRS was considered a successful response.
Results: Between August 2008 and December 2010, 48 patients were randomized equally (7 men and 41
women, median age 47 years). Four patients in the saline group reported a successful response compared
with 13 in the lidocaine group (P = 0·007).
Conclusion: Entrapped branches of intercostal nerves may contribute to the clinical picture in some
patients with chronic abdominal pain. Pain reduction following local infiltration in these patients was
based on an anaesthetic mechanism and not on a placebo or a mechanical (volume) effect. Registration
number: NTR2016 (Nederlands Trial Register; http://www.trialregister.nl)
Paper accepted 29 August 2012
Published online in Wiley Online Library (www.bjs.co.uk). DOI: 10.1002/bjs.8958
Introduction
A cohort study recently reported on the results of a tai- lored regimen on diagnosis and treatment of patients with Up to 30 per cent of patients with chronic abdominal suspected ACNES (139 patients)10. The diagnosis is sug- pain suffer from pain localized in the abdominal wall1–4.
gested by the combination of a patient’s history (chronic Although these patients are seldom able to discriminate pain), physical examination (pain localization) and the visceral (organ-related) from parietal (abdominal wall) absence of objective abnormalities (laboratory, ultrasonog- pain themselves, simple testing allows distinction between raphy and/or computed tomography). If these findings are the two varieties2,5,6. A positive test result is frequently consistent with ACNES, the diagnosis may be confirmed associated with anterior cutaneous nerve entrapment syn- by local subfascial anaesthetic injection, so-called trigger drome (ACNES), although some doubt the existence of point infiltration (TPI). Some state that a salutary effect of this somewhat illusive pain entity7. ACNES is a pain syn- a single injection is placebo-based, whereas others hypoth- drome thought to be the result of entrapment of cutaneous esize a dry-needling or acupuncture-like mechanism11–14.
branches of an intercostal nerve at the level of the rectus Controlled data in the diagnostic setting of ACNES are  2012 British Journal of Surgery Society Ltd O. B. A. Boelens, M. R. Scheltinga, S. Houterman and R. M. Roumen
The primary aim of the present study was to compare of the rectus abdominis muscle; tenderness increased by the effect of a single TPI using either lidocaine or saline abdominal muscle tensing using Carnett’s test; normal on pain perception in the diagnostic setting in patients laboratory findings (C-reactive protein concentration with suspected ACNES. It was hypothesized that pain below 6 mg/l, serum leucocyte count 4–10 × 109/l, attenuation following lidocaine injection would be greater normal urine sedimentation); and no abnormal abdominal imaging, if performed previously. Carnett’s test involvesan investigator localizing and stabilizing the point ofmaximum pain using an index finger. The patient is then asked to lift the upper torso or both legs while the palpating This single-centre randomized double-blind placebo- index finger remains on the painful spot. When pain controlled trial was conducted at a large teaching hospital intensity is increased by this manoeuvre, the origin of the (M´axima Medical Centre (MMC), Veldhoven) in the pain is likely to be located in the abdominal wall2,5,6,18,19.
Netherlands. The institute has gained a reputation Exclusion criteria were: previous injection at the site for specializing in diagnosing and treating chronic of maximum pain, surgical scar-related pain syndromes, pain presenting with abdominal wall or groin nerve recent intra-abdominal pathology, lidocaine allergy, co- entrapment syndromes, so patients are increasingly being morbidity and impaired communication.
referred from other Dutch hospitals10,15–17. The medical If the patient fulfilled these criteria, the possible ethics committee of MMC approved the study design, diagnosis of ACNES was communicated and informed protocol and informed consent procedures. The study was consent obtained for the local administration of a registered in the Nederlands Trial Register (NTR2016), first diagnostic injection. After explaining the injection and is reported according to Consolidated Standards of procedure, the participant was randomized to receive a Reporting Trials (CONSORT) guidelines.
subfascial injection of either 10 ml 1 per cent lidocaine or All patients aged over 18 years suffering from loco- 10 ml saline at the point of maximum pain. The injection regional abdominal pain for at least 1 month were eligible was performed in outpatients by a freehand technique for this study if all of the following criteria were met: unilateral single tender spot (trigger point); constant site of The primary endpoint was the proportion of patients abdominal tenderness with a small (less than 2 cm2) area of achieving at least a 50 per cent improvement in pain maximal intensity (fingertip) within the lateral boundaries perception measured on a visual analogue scale (VAS; Did not meet inclusion criteria n = 30Declined to participate n = 6Other reason n = 42 Fig. 1 CONSORT diagram for the trial. TPI, trigger point injection
 2012 British Journal of Surgery Society Ltd Diagnosis of anterior cutaneous nerve entrapment syndrome
where 0 mm represented absence of pain and 100 mm indicated excruciating pain) and/or an improvement of atleast 2 points on a verbal rating scale (VRS; 0, no pain; 4, Patients were recruited from August 2008 to December severe pain), during physical examination 15–20 min after 2010. A total of 126 patients were referred and evaluated the TPI compared with directly before20. The secondary for alleged abdominal wall-related pain (Fig. 1). Based on endpoint was the investigator’s or patient’s ability to predict physical examination or laboratory or imaging findings, the type of injection administered based on the observed the origin of the abdominal pain was considered unlikely effect (both subjective and physical findings) 15–20 min to be abdominal wall-related in 13 patients. A total of after TPI. Patient and primary investigator (both blinded) 78 patients were excluded, including 25 who had already were asked to register their opinion on the injected agent, received TPI. Six patients did not consent to participation.
A total of 48 patients were randomized and received the Participants were assigned randomly to one of the allocated intervention, 24 in each group.
treatment groups following a computer-generated list There were no significant differences between groups of random numbers in blocks of eight. The allocation regarding baseline demographics, pain characteristics sequence was concealed from the researcher enrolling, (VAS, VRS) and disability scores (Table 1). All patients injecting and assessing participants by use of sequentially received TPI according to allocation. Data were complete numbered, opaque and sealed envelopes that were prepared for all participants, with no dropouts or loss to follow-up.
by a secretary who had no involvement in the trial. Afterenrolment, an outpatient department nurse opened the Table 1 Baseline patient demographics, pain characteristics and
next consecutively numbered envelope, and a syringe was prepared according to the allocation and checked by adoctor not involved in the trial. The name and date of birth of the participant were written on the envelope. As both fluids were colourless and odourless, the investigator and participant remained blinded. The allocation was revealed only after follow-up at 2 weeks, when outcomes were assessed by the primary investigator and communicated Statistical analysis
Based on previous experience with a cohort of patients with ACNES, the study was powered for the primary endpoint to detect a difference in the proportion of successful responses (at least 50 per cent improvement in pain perception) of 75 per cent in the lidocaine group versus an expected response of 30 per cent in the saline (placebo) group, with a two-sided 5 per cent significance level and a power of 80 per cent10. To achieve this, a sample size of 22 patients per group was required. To allow for possible dropouts (10 per cent), enrolment of 48 patients Continuous data are presented as median (range).
Differences in baseline characteristics between placebo and intervention groups were tested using χ2 test for categorical variables, and Student’s t test (normal distribution) or Mann–Whitney U test (skewed distribution) for continuous variables. The difference in success rates between groups was calculated using Yates’ corrected χ2 test. P < 0·050 was considered statistically significant.
Data analysis was performed using SPSS version 16.0 for *Values are median (range). VAS, visual analogue scale; VRS, verbal Windows (IBM, Armonk, New York, USA).
 2012 British Journal of Surgery Society Ltd O. B. A. Boelens, M. R. Scheltinga, S. Houterman and R. M. Roumen
Concerning the primary endpoint, the proportion of may be due to interaction between these agents and patients demonstrating a successful response (at least the make-up of sodium channel isomers found on the 50 per cent VAS difference and/or 2 or more VRS nerve axons27. Moreover, pain levels can be influenced categories) was significantly higher in the group receiving by using anaesthetic agents to block nerve transmission.
lidocaine (13 of 24 versus 4 of 24 in saline group; P = 0·007).
Data from the present study therefore strongly support Evaluation of the secondary endpoint showed that 26 the contention that ACNES is a nerve-related abdominal of the 48 patients correctly predicted the type of agent wall problem. The clinical observation that more than administered (lidocaine 8 of 24, saline 18 of 24). In contrast, two-thirds of the patients with ACNES had an area the principal investigator was correct about the nature of of several centimetres around the trigger point with the injected agent in 36 patients (lidocaine 14 of 24, saline sensory disturbances (hypaesthesia, hyperalgia or allodynia; Table 1) is highly suggestive of a peripheral nerve lesion.
No adverse events occurred apart from an occasional On the other hand, as local trigger points are also the small haematoma that resolved spontaneously. Seven hallmark of myofascial pain syndromes, these entities patients reported increased pain during the first few days should also be considered in the differential diagnosis28.
following examination and TPI (lidocaine 3, saline 4).
Future research in patients with suspected ACNESneeds to focus on substantiating these sensory skin Discussion
alterations objectively by performing quantitative sensorytesting29,30.
This controlled trial demonstrated that individuals Although all 48 patients had signs and symptoms with abdominal pain suggestive of ACNES experienced suspicious of ACNES, only just over half of the lidocaine significantly more pain reduction after lidocaine infiltration group experienced significant pain reduction. Several than those who received saline. Interestingly, a blinded factors may explain this limited response. First, it is thought experienced investigator was able to predict the type of that nerve entrapment in ACNES is usually situated at the injected agent correctly in the majority (three-quarters) of level of the ventral fascia of the rectus abdominis muscle.
patients with suspected ACNES. In contrast, the patient’s Lidocaine was therefore injected immediately after the estimation was equivalent to tossing a coin. These findings needle was felt to cross the superficial fascia. Some of suggest that an experienced physician is often able to the non-responders may have had a form of ACNES predict the presence of a pain entity such as ACNES on characterized by nerve entrapment (or herniation) at dorsal the basis of a diagnostic injection. This is in line with the or lateral portions of the muscle8. Second, a per-protocol fact that patients with ACNES often report pseudo-visceral wait of 15–20 min after injection may have been too short in some individuals. In some patients who received A long-lasting effect after just a single anaesthetic lidocaine, a pain relief response of less than 50 per cent injection is frequently reported in various abdominal wall was reported initially after injection, although substantial pain syndromes. This observation led critics to conclude pain reduction or even absence of pain in the first few that a placebo phenomenon may be involved7. Controlled hours or days after the injection was reported at the 2-week investigations were advised but considered unethical5.
Others suggested that the pain cycle is interrupted by evaluation. Third, lidocaine may not have reached the exact a dry-needling effect, as in acupuncture11–14. It has point of entrapment as a freehand injection technique was also been hypothesized that the injected volume results used. Ultrasound-guided injection may be more effective in hydrodissection, leading to release of an entrapped as small fascial openings allowing nerve passage may be nerve23. This latter mechanism may be involved as the volume effect of a 10-ml bolus of any type of fluid ACNES is still frequently overlooked and patients are within a confined subfascial space may possibly reduce subjected to prolonged investigation2,4,31–33. The median a small fat pad that is herniated and compresses the delay to diagnosis in the present trial was 13 months (up nerve during its passage through the rectus abdominis to 120 months). A long delay inevitably leads to central muscle, as is hypothesized in ACNES8,24–26. The finding sensitization in some patients. An accelerated diagnostic of attenuated pain levels in four patients following saline pathway in patients with suspected abdominal wall pain, injection may be explained by either of these suggested searching for a trigger point in the rectus abdominis muscle with a simple diagnostic injection of local anaesthetic, may Others have postulated that long-lasting beneficial cut costs by reducing the number of unnecessary visceral effects of anaesthetic agents in chronic pain syndromes  2012 British Journal of Surgery Society Ltd Diagnosis of anterior cutaneous nerve entrapment syndrome
Disclosure
15 Roumen RM, Scheltinga MR. [Abdominal intercostal neuralgia: a forgotten cause of abdominal pain.] Ned Tijdschr The authors declare no conflict of interest.
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 2012 British Journal of Surgery Society Ltd

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