Letter to the Editor Regarding "Impact of Posterior Quadratus Lumborum Block on Acute Pain Relief and Chronic Pain Prevention in Breast Cancer Surgery".

To the Editor,
By conducting a prospective, randomized, patient-blinded, controlled clinical trial in a total of 120 patients who underwent breast cancer surgery under general anesthesia, Wang et al. [1] demonstrated that, compared with conventional intravenous analgesia, the addition of ultrasound-guided erector spinae plane block (ESPB) and posterior quadratus lumborum block (PQLB) significantly decreased the incidence of moderate to severe postoperative pain within 24 h postoperatively. Furthermore, a single-injection PQLB compared with a single-injection ESPB significantly reduced the occurrence of chronic pain at 3 months postoperatively. However, there are several issues in the methodology of this study that may affect the interpretation of the results.
First, the primary outcome of this was postoperative pain levels assessed by the Brief Pain Inventory scale with 0–10 points. The postoperative analgesia target was designed to maintain a pain score of less than 4, but it was unclear why a postoperative analgesia target with a pain score of less than 3 was used for sample size estimation. In fact, the use of a postoperative analgesia target being smaller than that in the actual research plan may result in an underestimated sample size. Furthermore, in the sample size evaluation section, Wang et al. [1] described that the effect sizes of 0.4, 0.85 and 0.72 for the conventional care, PQLB, and ESPB groups were selected based on prior studies. As Wang et al. [1] did not provide the references of these prior studies, it was unclear if these effect sizes were really suitable for the participants of this study, that is, the female patients undergoing breast cancer surgery under general anesthesia.
Second, based on a 0- to 10-point scoring system, postoperative pain levels are generally classed into no pain (0 point), mild pain (1–3 points), moderate pain (4–6 points) and severe pain (7–10 points) [2]. To ensure postoperative comfort and good experience of patients and achieve a rapid postoperative recovery, optimizing postoperative pain is actually one of the primary targets achieved by the current enhanced recovery after surgery practices [3]. As mentioned above, the designed postoperative analgesia target of this study was to maintain a pain score of less than 4, that is, keeping no pain to mild pain. According to the data of Table 2 of Wang et al.’s article [1], median pain scores at rest and during movement at all time points postoperatively in three groups were 2 or less, indicating that most of the patients only had mild postoperative pain. In Table 3 of Wang et al.’s article [1], however, the incidence of moderate to severe postoperative pain within postoperative 24 h was 16.7–46.2%. Because Wang et al. [1] did not provide the diagnostic criteria of moderate to severe postoperative pain in their Methods, it is somewhat difficult for the readers to understand these results. Given that poorly controlled early postoperative pain is an independent risk factor for the development of postoperative chronic pain [4], we believe that clarifying the above issues would be helpful for comprehensively understanding their findings.
Finally, Wang et al. [1] reported the incidences of postoperative chronic pain at 3 months postoperatively, but did not provide the related diagnostic criteria, especially for definitions of neuropathic pain. Most important, the result of Wang et al.’s study that the PQLB was superior to the ESPB in term of reducing the occurrence of postoperative chronic pain was not in agreement with the findings of a recent meta-analysis regarding the analgesic efficacy of different regional block techniques after breast cancer surgery [4]. Because preoperative pain severity, anxiety, and depression, and the number of axillary lymph node dissection have been associated with an increased risk of chronic pain after breast cancer surgery [5, 6], we would like to know whether the three groups were comparable with respect to these risk factors. The use of preoperative chemoradiotherapy was not significantly different among the three groups, but it demonstrated a trend of progressive decrease from the conventional care group to the ESPB group and then to the PQLB group, which is similar to the trend of postoperative chronic pain occurrence in the three groups. Available literature indicates that preoperative chemoradiotherapy is a significant risk factor of chronic pain after breast cancer surgery [6]. Thus, we argue that these unknown factors would have biased their results regarding the incidences of postoperative chronic pain in the three groups.