In recent years, regional interfascial blocks have expanded the scope of regional anesthesia providing postoperative analgesia in a variety of surgical interventions. In comparison with the peripheral nerve block wherein a local anesthetic agent is injected around a singular nerve or group of nerves, the injectate for regional interfascial blocks is placed between fascial layers thereby anesthetizing all nerves transversing along the tissue plane as well as adjacent areas [24]. Amongst the several popular blocks used routinely in clinical practice is the ESPB, developed quite recently in 2016 by Forero et al [4]. While it was initially described by the authors for the management of thoracic neuropathic pain, its usage has expanded greatly with anesthetists reporting its use for breast [7], thoracic [8], and spinal surgeries [25]. Since the anesthetic agent in ESPB is deposited beneath the erector spinae muscle near the tip of the transverse process of the vertebrae [26], and the muscle itself transverses the entire span of the spinal cord, it is postulated that the level of injection can have different analgesic effects and can be used for a variety of surgical procedures. In the current review, we investigated the efficacy of ESPB for pain control after liver surgeries by pooling data from nine RCTs.

Opioids constitute the primary drugs that are used in the management of postoperative pain in most surgical procedures. Therefore, any reduction in opioid consumption is directly indicative of the analgesic potential of the regional block. Assessing the 24-hour total opioid consumption, we noted that there was a tendency for reduced morphine consumption with ESPB as compared to control with an overall reduction of 35 mg. However, the CI was wide-ranging from − 77.01 to 6.52, turning the results non-significant. It can be noted from the forest plot that of the four studies comparing ESPB vs. control, the studies of Kim et al [15] and Mostafa et al [20] found limited or no difference in total opioid consumption while the remaining studies noted a significantly large reduction of opioid consumption. This could be because the procedures were minimally invasive (percutaneous and laparoscopic) in the former studies resulting in limited pain which was easily managed by the standardized analgesic protocol and ESPB had little additive effect. The review also did not find any significant difference in opioid consumption between ESPB and spinal analgesia suggesting that there may be equivalence between the two regional analgesic modalities. However, of the three studies in this comparison, two used intrathecal morphine while one used epidural block. The former studies noted better outcomes with intrathecal morphine while Zubair et al [16] found ESPB to be better than epidural analgesia. The primary disadvantage of spinal analgesia is its associated complications like headaches, respiratory depression, hypotension, backache, etc [27]. However, these could not be compared in a meta-analysis owing to a limited sample size of the RCTs resulting in too few complications. Given the differences among studies and limited data, further trials comparing ESPB with spinal analgesic techniques are needed to demonstrate equivalence between the two.

In the second part of the meta-analysis, we noted only a minimal reduction of pain scores with ESPB vs. control, and that too only at 12 and 24 h. The reduction of pain on a ten-point scale was only 0.41 and 0.11 respectively. The results might have been statistically significant but would not qualify for the minimum clinically important difference which is considered worthwhile by the patient [28]. On the other hand, the meta-analysis also noted no significant difference in pain scores between ESPB and spinal analgesia at all time points. Given the lack of difference in pain scores, it is necessary to distinguish between studies which used ESPB as a single shot or as continuous blocks, as the latter would produce a longer effect. However, the two studies using continuous blocks reported conflicting evidence. Kang et al [21] compared programmed intermittent bolus injections of ESPB with intrathecal morphine and found no difference in 48 h opioid consumption between the two techniques. On the other hand, Zubair et al [16] noted that continuous ESPB provided superior pain control as compared to thoracic epidural analgesia. Due to the scarcity of data, further trials are needed to confirm if continuous ESPB results in better outcomes as compared to single shot blocks.

The only adverse event which could be quantitatively examined in the meta-analysis was PONV. Given the tendency of reduced opioid consumption with ESPB vs. control, one may expect a significantly reduced incidence of PONV with ESPB. However, there was no difference in the risk of PONV between ESPB vs. control. Limited data from two trials found no difference in the risk of PONV between ESPB and spinal analgesia as well. Furthermore, none of the trials reported any major complications with the use of ESPB. No patient had local anesthetic toxicity, nerve injury, pneumothorax, or vascular injury in the ESPB group. This could be credited to the safety of ESPB where the needle penetration path and position are away from major neurovascular structures [29].

The results of our review are similar to the past meta-analysis of Bhushan et al [10] wherein they too did not find any significant analgesic effect of ESPB for liver surgeries. However, their study could include only six RCTs and the authors also included a comparison of ESPB with other blocks like quadratus lumborum block. Inclusion of a mix of placebo, spinal and other blocks in the control group results in biased evidence decreasing the credibility of the results. In the current review, we excluded comparisons with other regional blocks, updated the literature search and included four more RCTs, and also conducted a separate analysis of ESPB vs. control and ESPB vs. spinal analgesia to provide high-quality evidence on the subject.

The lack of effectiveness of ESPB in liver surgery could be related to the anatomy of the block. The ESPB is primarily a paraspinal fascial plane block wherein the local anesthetic is injected between the erector spinae muscle and the thoracic transverse processes. It predominantly blocks the posterior rami of the thoracic and abdominal spinal nerves with little effect on the anterior rami resulting in minimal analgesia beyond the mid-axillary line [4]. While the thoracic epidural is technically more difficult, it may still be the preferred approach in patients undergoing liver surgeries.

There are limitations to this meta-analysis. The primary drawback is the extremely high heterogeneity noted in the analysis. Indeed, despite including a very specific cohort of liver surgery patients, there were several methodological differences in the included studies. Variations in the type of liver surgery, invasiveness of the procedure, type of local anesthetic, its concentration and volume, the level of the injection, type of drug in PCA, and postoperative standard analgesic protocol were noted among the studies which could have led to such high heterogeneity. Secondly, while most of the trials used single injections of ESPB, two of the studies used continuous blocks. Due to limited data, we could not differentiate the outcomes of single vs. continuous ESPB blocks. Thirdly, despite an updated literature search, only nine RCTs were available for the meta-analysis and most of them had a small sample size. Also, the division of studies based on the control group protocol further reduced the number of trials in each meta-analysis. Lastly, the trials were from a few specific countries and the results should be generalized with caution.

Our results have clinical significance. Based on current evidence, routine use of ESPB cannot be recommended for patients undergoing liver surgeries. Secondly, as nursing personnel are closely involved in the perioperative and postoperative management of patients, patients receiving ESPB should not be deprioritized during control of postoperative pain. Nursing personnel should maintain a high index of suspicion even for those receiving ESPB till further evidence establishes the efficacy of this block in liver surgery patients.