A total of 2103 relevant studies were obtained from the databases after conducting an initial search. Subsequently, we excluded 886 duplicated studies and 1205 studies with irrelevant abstracts and titles. We carefully evaluated the remaining 12 studies by reading the full text to determine whether they were eligible. Six studies were further excluded for the following reasons: studies were reviews and case reports (n = 2) [11, 12], retrospective study (n = 1) , compared ESPB with other types of regional blocks (n = 2) [14, 15], and an ongoing trial (n = 1) . Finally, six studies [8, 17,18,19,20,21] satisfied the inclusion criteria and were used for the meta-analysis. The literature screening process is illustrated in Fig. 1.
Six RCTs with 360 patients (180 patients in the ESPB group and 180 patients in the control group with no intervention or sham intervention) were considered in the present meta-analysis. The publication years of the eligible studies ranged from 2019 to 2021. ESPB was performed between T10 and L3. Four studies [8, 19,20,21] used bupivacaine as a local anesthetic, while two studies used ropivacaine. The concentration of the local anesthetics ranged from 0.025 to 0.5%. PCIA was used to relieve postoperative pain in five trials [8, 17, 18, 20, 21]. Detailed information regarding studies included in the meta-analysis is presented in Table 1.
Risk of Bias
Four studies [8, 17, 19, 21] explicitly reported the method of random sequence generation, and three studies [17, 18, 20] described allocation concealment. None of the studies described the blinding of participants and personnel. All studies mentioned that the assessors were blinded and attrition bias was evaluated. No selective reporting was observed. All studies calculated the sample sizes, and the other bias was classified as "low risk". A summary of the risk of bias is presented in Fig. 2.
All studies reported opioid consumption during the first 24 postoperative hours. Synthesized results revealed that patients who received ESPB had a significantly lower opioid requirements after surgery than patients in the control group (SMD = −3.82, 95% CI [−5.70, −1.94]; P < 0.01, I2 = 97%, Fig. 3).
Postoperative pain scores were assessed at six different time points: 2 h, 4 h, 6 h, 8 h, 12 h, and 24 h after surgery. Forest plots revealed that ESPB reduced postoperative pain scores significantly (2 h: MD = −1.89, 95% CI [−2.64, −1.14]; P < 0.01, I2 = 87%; 4 h: MD = −1.46, 95% CI [−2.45, −0.47]; P < 0.01, I2 = 92%; 6 h: MD = −2.15, 95% CI [−2.60, −1.70]; P < 0.01, I2 = 0%; 8 h: MD = −1.64, 95% CI [−2.26, −1.01]; P < 0.01, I2 = 87%; 12 h: MD = −1.09, 95% CI [−1.71, −0.47]; P < 0.01, I2 = 81%; 24 h: MD = −0.68, 95% CI [−0.88, −0.49]; P < 0.01, I2 = 0%, Fig. 4).
Four studies recorded the numbers of patients who required rescue analgesia. A forest plot revealed that ESPB reduced the number of patients requiring rescue analgesia significantly (RR = 0.36, 95% CI [0.24–0.54], P < 0.01, I2 = 33%, Fig. 5).
Five studies recorded the incidence of postoperative nausea and vomiting. A forest plot revealed that the incidence of postoperative nausea and vomiting were considerably lower in the ESPB group than in the control group (RR 0.25, 95% CI 0.14–0.47, P < 0.01, I2 = 0%, Fig. 6). Two studies recorded the incidence of itching, and a forest plot revealed no significant difference between the ESPB and control groups (RR 0.63, 95% CI 0.06–6.68, P = 0.70, I2 = 76%, Fig. 7). No ESPB operation-related complications such as local anesthetic toxicity, bleeding, neuraxial injury, or infection were reported in the included studies.
We did not evaluate publication bias because the number of eligible studies was small .
All included studies were randomized trials. All the assessors were blinded, there was no selective reporting and the other bias, the "risk of bias" was graded as "not serious"; I2 > 30%, which indicated "inconsistency", was graded as "serious". Three trials [17, 19, 21] reported opioid consumption and pain scores as median (interquartile range) or median (min–max), which indicated "indirectness" was classified as "serious". The overall results of GRADE are presented in Table 2.