Efficacy of transversus abdominis plane block on the chronic pain after colorectal surgery: a retrospective cohort study

Background: Chronic postsurgical pain (CPSP) is common and would reduce the quality of life of patients. Transversus abdominal plane (TAP) block has been widely used in lower abdominal surgery and many researchers demonstrated that it could improve acute postsurgical pain. We aim to determine whether TAP block could improve chronic postoperative pain at 3 months and 6 months after colorectal surgery. Methods: A total of 307 patients received selective colorectal surgery under general anesthesia between January, 2015 and January, 2019 in a single university hospital were included: 128 patients received TAP block combined with patient-controlled intravenous analgesia (PCIA) for postsurgical analgesia (group TP) and 179 only administrated with PCIA (group P). Main outcome was the incidence of pain at 3 months and 6 months after colorectal surgery, and NRS score of pain at 24 hours, 48 hours, 3 months and 6 months after colorectal surgery. The data was analyzed by two-way repeated measures anova and the chi-square test. Results: The NRS score decreased significantly at 24 hours after surgery (rest NRS 1.07±1.34 vs 1.65±1.67, P =0.003; and movement NRS 3.00±1.45 vs 3.65±1.89; p =0.003) and at 3 months after surgery (movement NRS 0.59±1.23 vs 0.92±1.65, p =0.045 )in group TP than that in group P. There were no significant difference of NRS score at 48 hours and 6 months after surgery (all p >0.05). Among 307 patients, 62/307(20.2%) developed chronic post-surgical pain (CPSP) at 3 months after surgery, and 42/307(13.7%) experienced CPSP at 6 months after surgery. The prevalence of CPSP in group TP showed no significant difference compared with group P at 3 months (19.5% vs 20.7%, p =0.806) and 6 months (13.2% vs 13.9%, p =0.863) after surgery. Conclusions: TAP block could decrease the movement pain score at 3 months after surgery, but could not reduce the incidence of CPSP at 3 months and 6 months after selective colorectal surgery. may have positive effect on chronic postsurgical


Introduction
Chronic postsurgical pain (CPSP) is defined by the International Association for the Study of Pain (IASP) as pain that develops after a surgical procedure and persists for at least 3 months after 3 surgery, where all other causes of pain (e.g. infection, recurring malignancy) or pre-existing pain problems are excluded. [1,2] Few studies reported the prevalence of CPSP after colorectal surgery [3,4]. A prospective study showed that 19% of patients underwent colorectal surgery reported CPSP at 6 months, 63% of which taking medication for their pain, 53% of which were using opioids to manage their pain [3]. In another retrospective study, 17% of patients reported CPSP after laparoscopic colorectal surgery, 37% of which suffering from severe pain (pain score ≥ 7) [4]. CPSP has a negative impact on the quality of life, such as mood, sleep, walking ability, and normal work, so it is important to find out methods to reduce CPSP.
It was found that acute postoperative pain was an important risk factor of CPSP, [4][5][6] and many research demonstrated that CPSP could be partly improved by optimizing perioperative pain management, including intravenous analgesia, epidural analgesia, paravertebral blocks and regional analgesia. [7][8][9] Transversus abdominis plane (TAP) block is an effective regional analgesia technique to reduce perioperative pain intensity [10], especially combined with ultrasound-guided technique.
TAP block is the injection of local anesthetics to neurofascial plane between internal oblique and transversus abdominis muscles aiming at blocking the neural afferents from the anterolateral abdominal wall (T7-L1). [11] TAP block has been widely used in many abdominal operation, with a satisfactory duration of analgesic action for up to 24 hours after surgery. [10,[12][13][14] Many studies confirmed that TAP block can improve acute postoperative pain [15,16], but whether TAP block can reduce chronic pain after colorectal surgery has been reported scarcely.
The main aim of our study was to evaluate whether TAP block could improve CPSP after colorectal surgery. We compared the outcomes for patients who had TAP block and PCIA with those patients without TAP block.

Methods
Our research was approved by the Ethics Committee of Third Xiangya Hospital, Central South University (No:2019-S473). We reviewed the medical records of patients undertaking colorectal surgery at the third Xiangya hospital of Central South University from January, 2015 to January 2019.
Eligibility criteria included patients who were over 18 years and under 85 years old, American Society of Anesthesiologists (ASA) status Ⅰ-III, receiving selective colorectal surgery under general anesthesia and patient-controlled intravenous analgesia (PCIA) for postsurgical analgesia. Exclusion criteria were patients unable to communicate, wound infection after surgery, receiving a second operation.
For those patients who received TAP block, bilateral TAP block was performed by ultrasound (SonoSite EdgeⅡ, American) before induction of general anesthesia. 40ml of 0.5% ropivacaine (AstraZneca AB, Sweden, 20 ml each side) was injected into the transversus abdominis plane under real-time ultrasound guidance.
Postoperatively, electronic analgesic pump with wireless analgesic system was used for 48 hours of PCIA (Renxian Medtech, Jiangsu). The PCIA pump was filled with sufentanil 150 μg, azasetron 10 mg, diluted to 150 ml with 0.9% normal saline. It was programmed to give 1.5-2 ml/h background infusion with a 1.5-2 μg bolus of sufentanil solution, with a 5min lockout time. Patients were routinely informed that they could control the pain by pressing a self-controlled button to only slight pain. Pain was assessed twice a day during rest and movement with an 11-point Numeric Rating Scale (NRS) (0 = no pain; 10 = pain ''as bad as you can imagine'') by the same nurse of acute pain service group within 48h after operation. The NRS score recorded at the afternoon (2-4 pm) was extracted for analysis in this study.
After discharged from hospital, postoperative follow-up team contacted patients via telephone at three months and six months after operation to complete pain evaluation (NRS score and painful place).

The data extraction
Patients' data including patients' ASA status, sex, age at the time of operation, body mass index were extracted from medical records. Patients' information about surgical and anesthetic management such as anesthesia duration, surgery duration, perioperative opioid dose and pain scores was extracted from Anesthesia records. All patients were allocated to two groups: group TP were those patients who received TAP block and PCIA; and group P were those patients who only received PCIA.

Statistical analysis
In order to facilitate comparison, all perioperative analgesics were equivalent to morphine. Sufentanil 0.01mg, fentanyl 0.1mg, remifentanil 0.1mg equal to morphine 10mg. Opioid usage were analyzed with the unpaired t-test between two groups. Two-way repeated measures anova was used for NRS scores of 24 hours, 48 hours, 3 months and 6 months after surgery. The chi-square test was used for Patients' demographics, ASA status, duration of surgery and anesthesia, and operative method were similar in the two groups (Table 1).
All opioids were converted to morphine to facilitate data statistics. Compared with group P, there was a significant reduction in intraoperative morphine use and total morphine use in group TP 6 (180.6±57.3 mg vs 205.3±67.6 mg, P=0.001; 271.0±76.1 vs 290.0±84.1, P=0.044, respectively).
But the postoperative requirement for morphine showed no significant difference at 24h and 48h between group TP and group P (P=0.255, P=0.320, respectively) ( Table 2).
The interaction effect between TAP block and time was statistically significant (movement NRS, P=0.042; rest NRS, P=0.048), which means that both TAP block and time had effects on postoperative pain. Thus we identified the effect of each factor on postoperative pain.
Compared with group P, the NRS score decreased significantly in group TP at 24 hours after surgery while the same was observed in movement NRS score at 3 months after surgery (0.59±1.23 vs 0.92±1.65, P=0.045). There were no significant difference of NRS score at rest and movement at 48 hours and 6 months after surgery (all P>0.05) (Fig 2).
In time comparison, both in group TP and group P, the effect of time on pain was statistically significant (all P 0.001) and the degree of pain decreased with time (Fig 2).
The prevalence of CPSP at 3 months and 6 months after surgery were summarized in Figure 3. CPSP occurred in 62 patients (20.2%) at 3 months after surgery and 42 patients (13.7%) at 6 months after surgery in our patients. The prevalence of CPSP in group TP showed no significant difference compared with group P (19.5% vs 20.7%,P=0.806) at 3 months after surgery. At 6 months after surgery, the results also showed no significant difference between the two groups (13.2% vs 13.9%, P=0.863).

Discussion
Our research assumptions were partially supported. TAP block not only significantly decreased the intraoperative opioids and perioperative total opioid dosage, but also lessen pain intensity at 24 hours and 3 months after colorectal surgery; however, such an effect did not occur at other time point.
In group TP, patients received bilateral TAP block before surgery, and there was a significant reduction in intraoperative morphine use compared with group P. At postoperative first 24 hour, group TP had significantly lower pain scores at rest and movement compared with group P, but the postoperative requirement for morphine showed no significant difference between the two groups.

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There was no difference of pain score and morphine use at 48 hours after surgery. These results were similar to some studies, which showed that TAP block with levobupivacaine could reduce postoperative visual analogue pain scores in the first 24 hours after major abdominal surgery, [17] and TAP block provided superior analgesia with lower pain scores at rest and movement in the first 24 hours after colorectal surgery, when compared to PCIA alone. [18] All suggested that TAP block could improve acute postoperative pain and the effect lasted at least 24 hours. [10] In our study, the overall incidence of chronic pain was 20.2% at postoperative three months and13.7% at postoperative 6 months. A prospective research documented that the prevalence of CPSP was 19% at 6 months after colorectal surgery. [3] A retrospective analysis confirmed that the prevalence of CPSP in patients with colorectal cancer was 13% at 38 months after laparoscopic colorectal surgery [4], which was a little higher than ours. These maybe have something to do with the difference of race and analgesia method.
However, there was no literature on how TAP block affects the incidence of CPSP in colorectal surgery.
In our study, at 3 months after surgery, the prevalence of CPSP was 19.5% in group TP and 20.7% in group P; and at 6 months after surgery, the prevalence of CPSP was 13.2% in group TP and 13.9% in group P. There was no significant difference about the prevalence of CPSP between the two groups after elective colorectal surgery. Similarly, a prospective cohort follow-up study indicated TAP block did not influence the prevalence of CPSP at 6 months and 12 months after breast reconstruction. [19] A randomized controlled trail also demonstrated that TAP block did not decrease the incidence of CPSP at 6 months after cesarean delivery. [20] They told us that TAP block could not decrease the incidence of CPSP.
Although TAP block did not improve the incidence of chronic pain after colorectal surgery, pain scores at 3 months was lower in group TP than that in group P, which means TAP block has positive effect on CPSP. A randomized controlled trial showed that TAP block could not reduce the incidence of chronic pain after inguinal hernia repair, but reduce the pain scores in rest and movement at 6 months, which was similar to us. [14] Our study has some limitations. Firstly, as the prevalence of CPSP after colorectal surgery is low, we 8 need to increase the sample size to verify whether TAP block has any effect on CPSP at 6 months or more. Secondly, the design of the retrospective study made it difficult for us to strictly exclude the use of perioperative non-opioid analgesics, which may also influence postoperative pain scores.
In conclusion, TAP block could relieve pain movement score at 3 months after surgery, but could not reduce the incidence of CPSP at 3 months and 6 months after selective colorectal surgery. Further studies need to expand sample size and carry out prospective research. Declarations block does not improve early or late pain outcomes after Cesarean delivery: a randomized controlled trial. Can J Anaesth 2014, 61 (7):631-640.  Figure 1 Flow chart showing patients enrollment, grouping and analysis.

Figure 2
The NRS score of patients at rest and movement at different moment. Data was presented as mean±SD. Compared with group P, the NRS score decreased significantly at 24 hours (rest and movement) and 3 months (movement) after surgery in group TP. There were no significant difference of NRS score at 48 hours and 6 months after surgery (all P>0.05).
NRS=numerical rating scale.
15 Figure 3 The prevalence of CPSP at 3 months and 6 months after surgery. Data was presented as percentage. The prevalence of CPSP in group TP showed no significant difference compared with group P at 3 months and 6 months after surgery (all P>0.05). CPSP=chronic postsurgical pain.