Breast cancer is one of the most frequently occurring cancers in women. Surgical excision is the primary treatment for breast cancer, even in stage IV cases [1]. Surgery increases the survival rate, while a high percentage of patients who undergo mastectomy suffer from postsurgical pain [2]. Inadequate pain control causes high opioid consumption, leading to several complications, such as nausea, vomiting, and impaired patient recovery and quality of life [3]. In addition, severe acute pain can lead to persistant postoperative pain after breast surgery [4].

The thoracic paravertebral block (TPVB) is a well-established regional analgesic technique for breast surgery. This block can reduce postoperative pain and opioid consumption [5]. However, when employing the conventional TPVB using the anatomical landmark technique, it can be technically difficult to identify the thoracic paravertebral space (TPVS). This analgesic technique can cause complications, such as inadvertent pleural puncture, unintended epidural spread, accidental vascular injection of local anesthetic, intrathecal injection, and postural headache [6]. Although recent studies on ultrasound-guided TPVB reported a low incidence rate (0.7%) of complications [7], serious problems such as intrathecal injection or pleural puncture could occur even when using ultrasound [8,9]. Moreover, the failure rate of this block with ultrasound guidance was similar to that of the landmark technique (5.3% vs. 5.6%) [5]. Also, the ultrasound-guided procedure might require some proficiency compared to the landmark method.

The retrolaminar block (RLB), which was first introduced by Pfeiffer et al. [10], is a modified paravertebral block that administers local anesthetic between the lamina of the thoracic vertebra and the erector spinae muscles, using landmark technique or under ultrasound guidance, rather than entering the needle into the TPVS directly. Moreover, real-time ultrasound guidance can help identify the lamina and monitor the spread of local anesthetic [11]. Previous studies indicated that the RLB could be an easy, safe, and effective analgesic technique for breast surgery [11-14]. However, there have been a few randomized, double-blinded studies on the efficacy of RLB for analgesia after breast surgery [13,14]. These studies did not apply ultrasound guidance for RLB. We hypothesized that ultrasound-guided RLB could reduce postoperative pain and the opioid requirement after breast surgery. Therefore, we evaluated the analgesic efficacy of a single injection of ultrasound-guided RLB for postoperative pain management following breast surgery.

A total of 64 patients were assessed for inclusion in this study. Ten patients who did not meet the inclusion criteria were excluded (anticoagulant use, n = 3; pulmonary disease, n = 3; anxiety disorders, n = 2; heart disease, n = 1; noncommunicable foreigner, n = 1). Further, four patients declined participation in the study. Thus, we included 50 patients in the study and divided them into two groups. One patient in group C and two patients in group R discontinued PCA on their own because of nausea, and one patient in group R due to dizziness. Accordingly, 24 patients in group C and 22 patients in group R completed the final analysis (Fig. 2).

The demographic and clinical characteristics of both the groups are presented in Table 1. There were no significant differences in ASA, Apfel score, age, height, weight, body mass index, duration of surgery, propofol consumption, and the type of surgery between the two groups. The total intraoperative dose of remifentanil was higher in group C than in group R (P = 0.025). The incidence of adverse events and patient satisfaction were not significantly different between the two groups (Tables 2, 3).

The VAS scores in the postoperative period decreased in both the groups with time (Fig. 3). However, the VAS scores during resting and coughing 1 hour postoperatively were higher in group C than in group R (P = 0.011, P = 0.004). The total consumption of morphine with IV-PCA at 1, 6, 24 and 48 hours postoperatively did not differ between the two groups (P = 0.63, Fig. 4).

This clinical study evaluated the efficacy of a single injection of ultrasound-guided RLB for postoperative pain management in patients undergoing breast surgery. The RLB group had a lower requirement of remifentanil than the control group intraoperatively. In addition, the VAS scores during resting or coughing 1 hour postoperatively were lower in the RLB group than in the control group. However, the total consumption of morphine 24 hours after the surgery did not show a statistically significant difference between the two groups, and the occurrence of adverse events, such as PONV did not differ considerably between the groups. Therefore, the addition of a single injection of ultrasound-guided RLB to conventional treatment, such as IV- PCA, did not show clinically significant improvement in postoperative pain management in patients undergoing breast surgery.

In our study, a single injection of ultrasound-guided RLB resulted in lower pain scores immediately after surgery, but it did not have the advantage of reducing morphine consumption postoperatively. This result is in agreement with previously published studies regarding the classic TPVB and RLB. A retrospective study by Agarwal et al. [18] showed that a single injection of the classic TPVB for mastectomy could decrease pain scores in the immediate postoperative period, but did not show a beneficial effect 24 hours postoperatively. A prospective randomized study by Onishi et al. [14] found that the RLB group had lower pain scores than the control group until 2 hours after surgery, but the requirement of the analgesic drug was similar after 12 hours postoperatively in both groups. However, Kairaluoma et al. [19] reported that a single administration of the TPVB for breast surgery reduced postoperative opioid consumption 1 hour after surgery, and that less overall pain intensity was observed 24 hours after the surgery. Previous studies on the TPVB reported that multi-level injection resulted in greater numbers of anesthetized dermatomes [20,21]. However, the postoperative pain scores and morphine consumption were comparable between the single and multi-level injection groups [20]. Therefore, we postulated that the multi-level doses of RLB did not have a significant impact on postoperative analgesia compared to a single injection.

The principal mechanism of action in the RLB for analgesia can be explained by local anesthetic spreading anteriorly through the superior costotransverse ligaments into either the paravertebral space, epidural spaces, or intervertebral foramen [22]. A human cadaveric study suggested that the paravertebral space could not be a closed triangular space, and the superior costotransverse ligament could be a passage for the diffusion of local anesthetic [23]. Another cadaveric study, using magnetic resonance imaging and anatomical dissection, demonstrated that the RLB with 20 mL of injectate produced spread into the epidural and neural foramen spaces [24]. However, the pattern of injectate spread was variable [25], and the extent of paravertebral distribution could be volume-dependent [26]. Furthermore, most of these human anatomical studies had in common that the RLB was consistently associated with a large distribution of injectate posterior to the back muscle and with limited spread to the paravertebral block [24,25,27]. Thus, we postulated that the RLB could present a similar or less analgesic effect than the classic TPVB.

Our findings show that a single injection of ultrasound-guided RLB could be insufficient to reduce postoperative analgesic requirement in breast surgery. The analgesic effect of the RLB was limited possibly because of the following: insufficiency of local anesthetic spreading into TPVS, the delivery method of local anesthetics (single injection), and the complexity of breast innervation. The terminal elimination half-life of ropivacaine was approximately 4.2 hours following epidural administration, and anesthesia time in this study was approximately 3 hours (180.0 ± 33.8 min) in the RLB group [28]. Therefore, to maximize the effect of RLB and to effectively control postoperative pain, we suggest that this block should be performed at the end of the surgery. Considering a recent study on the erector spinae plane block, which is similar to the RLB [29], the continuous infusion method in the RLB could be a good option for adequate postoperative analgesia following breast surgery.

This study had some limitations. First, the exclusion criteria, regarding history of trauma or surgery to the thoracic vertebrae and structural abnormalities in the pleural space, which could affect local anesthetic spread into the TPVS, was not included. Nevertheless, none of the patients enrolled in our study had these problems. Second, the regional analgesic technique may give trauma to the patient if it is performed during general anesthesia, but ultrasound-guided RLB is safe and simple, so we performed it after the induction of general anesthesia to reduce patients’ discomfort and to ensure double-blindness. However, the procedure during general anesthesia reduced the ease of the RLB and prevented a pinprick test for dermatomal distribution. Third, we calculated the sample size using data from the TPVB because there was a lack of studies on applying the RLB to breast surgery. Fourth, we used IV-PCA with morphine alone to evaluate the effect of RLB on analgesic requirements. Evidence supports the combination of non-opioid systemic analgesics and opioids in PCA to improve analgesic efficacy and reduce opioid-related adverse events [30]. The use of acetaminophen or non-steroid anti-inflammatory drugs could reduce the overall pain score and analgesic requirements in both the groups.

In conclusion, a single injection of ultrasound-guided RLB could not provide a postoperative opioid-sparing effect in breast surgery. The optimal timing and delivery method of local anesthetics for RLB require further investigation.