Erector spinae plane block for postoperative analgesia in patients undergoing flexible ureteroscopy: a randomized controlled trial

Abstract

Background

Postoperative pain after flexible ureteroscopy (URS) is clinically significant. It involves both somatic and visceral components, especially from ureteral dilation and irritation by double-J ureteral stents. The erector spinae plane block (ESPB) is an interfascial technique with potential to modulate thoracolumbar nociceptive pathways. We aimed to assess the analgesic efficacy of ultrasound-guided ESPB during flexible URS.

Methods

In this prospective, randomized, assessor-blinded trial, 70 adults scheduled for elective flexible URS at a university hospital (February–August 2025) were randomized to receive either unilateral ESPB after general anesthesia induction or standard multimodal analgesia (Control). The primary outcome was postoperative pain (Numeric Rating Scale) in the first 24 h. Secondary outcomes included cumulative opioid use, time to first rescue analgesia, total rescue analgesic needs, and incidence of postoperative nausea and vomiting (PONV). Perioperative hemodynamic parameters were exploratory outcomes.

Results

Seventy patients were analyzed (ESPB, n = 35; Control, n = 35). Key findings included significantly lower resting Numeric Rating Scale (NRS) scores in the ESPB group from the sixth postoperative hour onward, with the greatest difference at 12 h (p < 0.001). The significant group-by-time interaction in pain scores was confirmed by nonparametric (nparLD, p < 0.001) and parametric (mixed ANOVA, p = 0.01) analyses. Movement-related pain scores were significantly reduced in the ESPB group at 6, 12, and 24 h. Although cumulative opioid use and time to first rescue analgesia did not differ, the ESPB group showed a lower incidence of PONV (5.7% vs. 34.3%; p = 0.01). Perioperative hemodynamic parameters remained comparable between groups.

Conclusions

Ultrasound-guided ESPB reduced postoperative pain in a time-dependent manner and decreased PONV after flexible URS, but did not have a clear opioid-sparing effect. The statistically significant pain reduction was close to the threshold of clinical relevance, so its practical impact may be limited and should be interpreted with caution.

Ethical committee approval

2024-KAEK-06-2502A05.

Trial registration

ClinicalTrials.gov NCT06826833, registered on 07 February 2025.

Introduction

Ureteroscopy (URS), also known as ureterorenoscopy, is widely used for treating renal stone disease. It is generally considered a minimally invasive procedure [1]. Nevertheless, postoperative pain following URS remains clinically significant. Patients often experience moderate pain in the early postoperative period [2]. Factors such as ureteral dilation, mucosal irritation, and intrarenal manipulation contribute to nociceptive input. This input originates from the thoracolumbar segments [3]. In addition, ureteral stent placement, most commonly a double-J ureteral stent, may worsen postoperative discomfort [4]. Poor pain control can increase opioid use, delay recovery, and reduce patient satisfaction [2, 5].

Building on this, postoperative pain following URS arises from both somatic and visceral components. Somatic pain is mainly mediated by segmental spinal innervation corresponding to the lower thoracic and upper lumbar dermatomes. Conversely, visceral discomfort is transmitted via sympathetic afferent pathways from the thoracolumbar region [3]. Considering this mixed pain profile, effective analgesic strategies should address both somatic and visceral pathways within a multimodal framework.

The mechanisms underlying postoperative discomfort after URS are similar to those of acute renal colic: both involve ureteral distension, smooth muscle spasm, and local inflammatory responses that activate visceral afferent fibers projecting to thoracolumbar spinal segments [3]. Ureteral stent placement, often performed during ureteroscopy, may further increase sustained visceral irritation. Consequently, the pain seen after URS–especially in patients with ureteral stents–may mimic the visceral pain of acute renal colic. Ultrasound-guided erector spinae plane block (ESPB) has shown significant analgesic effects in patients with acute renal colic in emergency settings [6–12], supporting its potential application for perioperative pain management in ureteroscopy. However, most available evidence comes from small trials, pilot studies, case series, and recent reviews, which suggest a potential benefit but are limited by methodological heterogeneity and low certainty. Therefore, the role of ESPB in structured perioperative multimodal analgesia for ureteroscopy remains to be unclear.

Prior studies comparing ESPB with traditional analgesics in acute renal colic have reported mixed results. Randomized and pilot studies suggest ESPB may provide better short-term pain relief than intravenous (IV) non-steroidal anti-inflammatory drugs (NSAIDs) or opioids. It may also reduce the need for rescue analgesia [9, 10, 12]. However, these studies usually have small sample sizes and short follow-up periods. They also show heterogeneity in design and outcome measures. Recent systematic reviews note that ESPB seems to lower early pain scores, but overall evidence certainty remains low. Its effects on opioid use and recovery parameters are inconsistent [6, 7]. As a result, the comparative effectiveness of ESPB in multimodal analgesia remains uncertain.

ESPB is an ultrasound-guided interfascial plane technique. It allows for extensive cranio-caudal spread of local anesthetic deep to the erector spinae muscle. This spread can extend forward to involve the dorsal and ventral rami and sometimes the paravertebral space [13]. This is thought to modulate both somatic and visceral pain transmission. This may occur through variable anterior spread to the ventral rami and paravertebral or epidural compartments [14]. ESPB has shown analgesic benefits in thoracic [15] and abdominal [16] surgeries, as well as in selected urological surgeries [17, 18]. Still, its efficacy in flexible URS remains unproven in large randomized trials.

Within this context, to our knowledge, randomized controlled evidence specifically evaluating the analgesic efficacy of ESPB in patients undergoing flexible ureteroscopy remains limited. This prospective, randomized, assessor-blinded clinical trial aimed to determine whether a single ultrasound-guided ESPB improves postoperative analgesia in this population. We hypothesized that patients receiving ESPB would demonstrate an improved postoperative pain profile during the first 24 h compared with those receiving standard multimodal analgesia alone.

Methods

Trial design

This study was a prospective, single-center, randomized, assessor-blinded controlled trial at Tekirdağ Namık Kemal University Research Hospital. Ethical approval was obtained from the Trakya University Clinical Research Ethics Committee (Approval No: 2024-KAEK-06-2502A05) before the study began. The protocol was prospectively registered at ClinicalTrials.gov (Identifier: NCT06826833; registration date: 07 February 2025). Patient recruitment began on 20 February 2025.

The study adhered to the ethical principles outlined in the Declaration of Helsinki (DoH) (64th WMA General Assembly, Fortaleza, Brazil, 2013) and complied with Good Clinical Practice (GCP) standards [19]. Reporting conformed to the Consolidated Standards of Reporting Trials (CONSORT) 2010 guidelines [20].

Study participants

Adult patients aged 18 to 65 years with American Society of Anesthesiologists (ASA) physical status I or II, scheduled for elective flexible URS at Tekirdağ Namık Kemal University Research Hospital between February and August 2025, were considered for inclusion. Eligibility was determined according to predefined inclusion and exclusion criteria. Written informed consent was obtained from all participants prior to enrollment.

Exclusion criteria included body mass index greater than 35 kg/m², documented hypersensitivity to local anesthetics, coagulation disorders, anticoagulant therapy, active infection at the planned injection site, or the need for emergency surgical intervention.

Individuals with chronic opioid use, chronic pain conditions, or cognitive impairments that could interfere with reliable pain reporting were also excluded.

Randomization, allocation concealment, and blinding

Eligibility and written informed consent were confirmed before group assignment. Participants were randomized 1:1. An independent physician, not involved in care, created a computer-based randomization list with permuted blocks of four. Allocations were concealed in sequentially numbered, opaque envelopes stored in the operating suite. The anesthesiologist opened each patient’s envelope on surgery day, just before the intervention.

Seventy patients were enrolled and allocated equally to the ESPB group (n = 35), which received an ultrasound-guided erector spinae plane block after induction of general anesthesia, or the Control group (n = 35), which underwent general anesthesia without regional anesthesia. The anesthesiologist performing the block did not participate in intraoperative management, postoperative data collection, statistical analysis, or manuscript preparation.

Because the ESPB was performed after induction of general anesthesia without a sham procedure, the anesthesiologist performing the block was aware of group allocation, whereas participants remained unaware of their allocation, as all interventions occurred under general anesthesia. Postoperative outcome assessments were conducted by a blinded anesthesia nurse. Although surgeons and intraoperative anesthesia providers were not formally blinded, they were not involved in outcome assessment. Consequently, the study was conducted as an assessor-blinded randomized controlled trial.

Interventions

All patients received intravenous catheter placement and standard ASA monitoring, including electrocardiography, noninvasive blood pressure measurement, and pulse oximetry. In our institution, flexible ureteroscopy is routinely performed under general anesthesia according to the surgical team’s preference and institutional practice, to optimize patient immobility, surgical conditions, and procedural safety. Both groups received standardized general anesthesia per protocol.

In the intervention group, after induction with intravenous propofol (2–2.5 mg/kg; Propofol, Fresenius Kabi, Germany), fentanyl citrate (1–2 µg/kg; Talinat, Vem İlaç, Türkiye), and rocuronium (0.6 mg/kg; Esmeron, Merck Sharp & Dohme, USA), patients received an ultrasound-guided ESPB prior to the start of the surgery. In the control group, patients underwent surgery under general anesthesia only, with no regional anesthetic technique. In both groups, anesthesia maintenance used sevoflurane (Sevorane, AbbVie, USA) in an oxygen/air mixture, additional rocuronium as needed, and mechanical ventilation to maintain normocapnia.

Postoperative analgesia was standardized across both groups. The protocol included intravenous tenoxicam (20 mg; Tilcotil, Abdi İbrahim, Istanbul, Türkiye) after induction, and intravenous paracetamol (1 g; Parol, Atabay, Istanbul, Türkiye) plus ondansetron (4 mg; Zofran, GlaxoSmithKline, Istanbul, Türkiye) near the end of surgery. Paracetamol was repeated every 6 h postoperatively. Tramadol-based intravenous patient-controlled analgesia (PCA) was also provided. PCA contained 3 mg/mL of tramadol. The device delivered 15 mg (5 mL) per bolus, with a 60-minute lockout and a maximum of 50 mg in 4 h. The max daily tramadol dose was 300 mg. If the postoperative Numeric Rating Scale (NRS) score was ≥ 5 despite PCA, first-line rescue analgesia with intravenous NSAIDs was given as per protocol. If pain persisted after 30 min, intravenous pethidine was used as a second-line rescue, also according to protocol. All patients received a double-J ureteral stent (Percuflex Plus, Boston Scientific, Marlborough, MA, USA) at the end of routine.

ESPB technique

The ESPB was performed unilaterally as described by Forero et al. [13]. After induction of general anesthesia and endotracheal intubation, patients were placed in the lateral decubitus position. A low-frequency curved transducer (Samsung HS30 ultrasound system, Samsung Medison Co., Ltd.) was positioned longitudinally 2–3 cm lateral to the midline at T10 to identify the transverse process and overlying erector spinae muscle on the surgical side. The trial registry describes the ESPB technique in general terms: at the thoracic level (T8–T10) using a linear probe. In contrast, this manuscript details the procedure, specifying a single-level injection at T10 and the use of a curved transducer for better thoracic visualization. No protocol modification occurred during the study.

After antiseptic skin preparation, a 20-gauge, 100-mm sonovisible needle (Ultraplex 360, B. Braun, Melsungen, Germany) was used. The needle was inserted in-plane, cephalocaudal, until reaching the fascial plane beneath the erector spinae muscle.

After confirming correct needle positioning by hydrodissection with saline and negative aspiration, 20 mL of 0.25% bupivacaine (Marcaine, AstraZeneca, Istanbul, Türkiye) was injected incrementally with intermittent aspiration. Visualization of a linear hypoechoic spread beneath the erector spinae muscle confirmed correct placement.

Outcomes

Primary outcome

The primary endpoint was postoperative pain intensity measured on the 11-point NRS (0 = no pain, 10 = worst pain) over 24 h after surgery. Assessments were made in the post anesthesia care unit (PACU), and at 2, 6, 12, and 24 h postoperatively. Pain at rest was recorded in PACU; at later times, both resting and movement pain scores were documented. The main analysis used longitudinal NRS scores throughout. Early postoperative pain was clinically relevant due to ureteroscopic manipulation and stent placement, causing early discomfort [2].

Secondary outcomes

Prespecified secondary outcomes were: cumulative opioid use within 24 h, time to first rescue analgesia, total rescue analgesic need, and rate of postoperative nausea and vomiting (PONV) in 24 h. All opioids, including IV tramadol by PCA and IV pethidine as rescue, were converted to morphine milligram equivalents (MME) using standard Table [21]. Additional variables, like total rescue analgesic counts and perioperative hemodynamics, were collected as exploratory outcomes.

Intraoperative and early postoperative hemodynamics – heart rate and mean arterial pressure – were recorded at baseline, after induction, at 1 h, at 2 h, at the end of surgery, and in PACU.

Missing data management

Data completeness was checked before analysis. No missing values were found for primary or secondary outcome, including postoperative NRS scores, PCA use, rescue analgesia, and PONV.

Intraoperative hemodynamic data at 1 h were missing for 22 patients (31.4%) whose surgeries lasted less than 60 min. These values were not collected after the procedure finished. Thus, 1-hour hemodynamic data were analyzed only for operative times 60 min ≥ (n = 48).

Hemodynamic data at 2 h were available for only 7 patients (10.0%) who underwent surgeries lasting over 120 min. These data were excluded from analysis due to the small sample size.

In the time-to-first rescue analgesia analysis, patients who did not require rescue analgesia within the 24-hour observation period (n = 16, 22.9%) were treated as right-censored observations at 24 h in the Kaplan–Meier analysis.

Given that all primary and secondary outcome variables were complete and the observed missingness was structurally related to operative duration, no imputation procedures were applied.

Sample size calculation

Sample size was calculated a priori using G*Power V3.1 (Heinrich-Heine-Universität Düsseldorf, Germany). The estimation relied on preliminary clinical observations and published data from comparable ESPB trials [17]. For the purpose of sample size estimation, the between-group difference in resting NRS score at postoperative hour 2 was used as a reference time point. The 2-hour assessment was selected because it represents a clinically relevant early postoperative period, during which ureteroscopic manipulation and ureteral stent placement are known to generate significant visceral discomfort, and it is commonly used as a reference time point in similar analgesia studies.

Although the 2-hour time point was used for sample size estimation, the primary outcome of the study was defined as the overall postoperative pain trajectory over 24 h, analyzed using repeated-measures methods to provide a more comprehensive evaluation of the temporal analgesic profile. This approach reflects a pragmatic design choice combining a clinically interpretable time-point–based power calculation with a longitudinal assessment of pain.

Assuming a clinically meaningful difference of 1.5 points, a standard deviation of 2.0, a two-sided alpha level of 0.05, and 80% power, the required sample size was 29 patients per group based on an independent two-sample t-test. To account for potential dropouts and missing data during postoperative follow-up, 35 patients were enrolled per group, yielding a total sample size of 70.

Statistical analysis

Statistical analyses were conducted using Jamovi version 2.6.44 (The Jamovi Project, Sydney, Australia), JASP version 0.18 (University of Amsterdam, Amsterdam, The Netherlands), and Python version 3.11 (Python Software Foundation, Wilmington, DE, USA). A two-sided p-value less than 0.05 was considered statistically significant.

The Shapiro–Wilk test was used to assess the normality of continuous variables. Variables with normal distribution were reported as mean ± standard deviation and compared using Student’s t-test or Welch’s t-test, as appropriate. Non-normally distributed variables were reported as median (interquartile range) and compared using the Mann–Whitney U test. Categorical variables were analyzed using the χ² test or Fisher’s exact test.

The primary analysis of the repeated NRS measurements used the nonparametric longitudinal data analysis method (nparLD, F1-LD-F1 design). This rank-based factorial model for repeated measures was used to assess the main effects of group and time, as well as the group-by-time interaction.

A parametric mixed ANOVA was conducted as a sensitivity analysis to assess the robustness of the findings under parametric assumptions.

Post hoc pointwise comparisons between groups at each time point were performed using the Mann–Whitney U test with Bonferroni correction and were considered secondary to the primary longitudinal analysis.

Time to first rescue analgesia was evaluated using Kaplan–Meier survival analysis and compared via the log-rank test. Effect sizes with 95% confidence intervals were reported when appropriate.

Results

Eighty-six patients were assessed for eligibility. Sixteen were excluded before randomization. The remaining seventy patients were randomized equally to the ESPB group (n = 35) and the Control group (n = 35). All randomized patients completed the 24-hour follow-up and were included in the final intention-to-treat analysis (Fig. 1).

Fig. 1.

CONSORT flow diagram of the study. Seventy eligible patients were randomized to the erector spinae plane block (ESPB) group (n = 35) or the Control group (n = 35). All randomized patients completed the 24-hour follow-up and were included in the intention-to-treat analysis

Baseline demographic and clinical characteristics did not differ significantly between groups, including age, sex distribution, body mass index, ASA physical status, operation time, surgical side, and baseline hemodynamic parameters (Table 1).

Table 1.

Demographic and baseline clinical characteristics of the study groups

Variable	ESPB Group (n = 35)	Control Group (n = 35)	p	Effect Size [95% CI]
Age (years)	46.0 [39.5–58.5]	47.0 [39.5–55.5]	0.81	r = 0.03 [− 0.21–0.26]
Sex, n (%)			0.14	V = 0.18
Male	25 (71.4)	19 (54.3)
Female	10 (28.6)	16 (45.7)
BMI (kg/m²)	27.1 [24.8–29.0]	27.8 [25.1–32.6]	0.23	r = 0.14 [− 0.09–0.37]
ASA physical status, n (%)			0.60	V = 0.06
I	9 (25.7)	11 (31.4)
II	26 (74.3)	24 (68.6)
Operation time (min)	70.0 [62.5–92.5]	60.0 [55.0–80.0]	0.09	r = 0.20 [− 0.04–0.42]
Surgical side, n (%)			0.81	V = 0.03
Right	17 (48.6)	18 (51.4)
Left	18 (51.4)	17 (48.6)
Baseline hemodynamics
Heart rate (beats/min)	81.6 ± 12.4	78.9 ± 13.8	0.39	d = 0.21 [− 0.26–0.68]
MAP (mmHg)	98.0 [91.5–108.0]	98.0 [92.5–102.0]	0.57	r = 0.07 [− 0.17–0.30]

Continuous variables are presented as mean ± SD or median [interquartile range], depending on normality (Shapiro-Wilk test). Categorical variables are presented as n (%). Age, BMI, operation time, and baseline MAP were compared using the Mann-Whitney U test; baseline heart rate was compared using Student’s t-test; sex, ASA physical status, and surgical side were compared using the chi-square test. Effect sizes: Cohen’s d for parametric comparisons, r = Z/√N for nonparametric comparisons, Cramér’s V for categorical comparisons.

BMI body mass index, ASA American Society of Anesthesiologists, MAP mean arterial pressure, ESPB erector spinae plane block, CI confidence interval

For the primary outcome, resting NRS scores diverged between groups over time, favoring the ESPB group (Table 2; Fig. 2A). No statistically significant differences were observed at PACU (p = 0.54) or at the second postoperative hour (p = 0.05). The prespecified primary longitudinal analysis demonstrated a significant group-by-time interaction, indicating divergence in pain trajectories over time, despite no significant difference at the 2-hour point. From the sixth postoperative hour onward, resting NRS scores were significantly lower in the ESPB group: sixth hour (p = 0.01; r = 0.33), twelfth hour (p < 0.001; r = 0.52), and twenty-fourth hour (p < 0.001; r = 0.43). The largest effect size was at the twelfth hour.

Table 2.

Comparison of postoperative pain scores between groups

Time Point	ESPB Group (n = 35)	Control Group (n = 35)	p a	r [95% CI]
Resting NRS
PACU	3.0 [1.0–5.0]	3.0 [2.0–6.0]	0.54	0.07 [− 0.16–0.30]
2nd hour	3.0 [2.0–4.0]	4.0 [2.0–5.0]	0.05	0.23 [− 0.00–0.44]
6th hour	2.0 [1.0–3.0]	4.0 [2.0–5.0]	0.01	0.33 [0.10–0.52]
12th hour	2.0 [1.0–3.0]	4.0 [2.0–5.0]	< 0.001	0.52 [0.32–0.67]
24th hour	1.0 [0.0–2.0]	3.0 [2.0–4.0]	< 0.001	0.43 [0.21–0.60]

nparLD	ATS	df	p
Group effect	20.17	1.97	< 0.001
Time effect	144.79	2.70	< 0.001
Group × Time interaction	5423.21	2.70	< 0.001
Movement NRS
2nd hour	3.0 [2.0–4.0]	4.0 [3.0–5.0]	0.02	0.27 [0.04–0.48]
6th hour	3.0 [2.0–4.0]	4.0 [3.0–5.0]	0.002	0.36 [0.14–0.55]
12th hour	2.0 [1.0–3.0]	4.0 [2.0–5.0]	< 0.001	0.52 [0.33–0.68]
24th hour	1.0 [0.0–3.0]	3.0 [2.0–5.0]	< 0.001	0.43 [0.22–0.60]

nparLD	ATS	df	p
Group effect	45.49	1.91	< 0.001
Time effect	167.85	2.28	< 0.001
Group × Time interaction	3326.91	2.28	< 0.001

NRS values are presented as median [interquartile range]. NRS: 0 = no pain, 10 = worst imaginable pain. PACU resting NRS represents the earliest postoperative assessment; movement NRS was not assessed at PACU. ^aPointwise comparisons were performed using the Mann-Whitney U test with Bonferroni correction (significance threshold: p ≤ 0.010 for resting NRS [5 comparisons], p ≤ 0.013 for movement NRS [4 comparisons]). Effect size r = Z/√N; r = 0.10 small, r = 0.30 medium, r = 0.50 large. nparLD: nonparametric analysis for longitudinal data using F1-LD-F1 design. ATS: ANOVA-type statistic. Bold p-values indicate statistical significance

NRS numeric rating scale, PACU post-anesthesia care unit, ESPB erector spinae plane block, CI confidence interval, df degrees of freedom

Fig. 2.

Postoperative pain scores over time in the ESPB and Control groups. Panel A shows resting NRS scores measured in the post-anesthesia care unit (PACU) and at postoperative hours 2, 6, 12, and 24; Panel B shows movement-related NRS scores measured at postoperative hours 2, 6, 12, and 24. Data are presented as medians with interquartile ranges (IQR) as error bars. Pointwise between-group comparisons were performed using the Mann–Whitney U test with Bonferroni correction. The primary longitudinal inference was based on the nparLD analysis; mixed ANOVA was used as a sensitivity analysis

Longitudinal analysis using the nonparametric nparLD model showed significant effects of group, time, and the group-by-time interaction (all p < 0.001). This indicates different temporal trajectories of resting pain across groups. Sensitivity analysis using parametric mixed ANOVA also confirmed the significance of the group-by-time interaction under parametric assumptions (p = 0.01) (Fig. 3).

Fig. 3.

Forest plot of relative treatment effects (RTE) derived from the nparLD F1-LD-F1 analysis for resting and movement NRS scores at each postoperative time point. Horizontal bars represent 95% confidence intervals. The dashed vertical line at RTE = 0.50 indicates no difference between groups; values below 0.50 favor the ESPB group

Movement NRS scores were consistently lower in the ESPB group at all assessed time points (Table 2; Fig. 2B). After Bonferroni correction, statistically significant between-group differences were found at the 6th (p = 0.002; r = 0.36), 12th (p < 0.001; r = 0.52), and 24th postoperative hours (p < 0.001; r = 0.43).

The nparLD model, a nonparametric statistical test, demonstrated significant group effects (differences between groups), time effects (changes over time), and group-by-time interaction effects (differences in how groups changed over time) for movement pain (all p < 0.001). In contrast, the parametric mixed ANOVA sensitivity analysis, which assumes normally distributed data, did not confirm the group-by-time interaction (p = 0.06). Thus, evidence for a different temporal pattern in movement pain between groups appears less robust under parametric assumptions.

Among secondary outcomes, the most pronounced effect was for PONV (Table 3). The incidence of PONV was 5.7% (2 out of 35) in the ESPB group and 34.3% (12 out of 35) in the Control group. This represents a statistically significant reduction (p-value = 0.01; odds ratio (OR) = 0.12, 95% confidence interval (CI) = 0.02 to 0.57).

Table 3.

Comparison of secondary outcome measures between groups

Variable	ESPB Group (n = 35)	Control Group (n = 35)	p	Effect Size [95% CI]
Opioid consumption
Total PCA consumption, (mg MME)	90.0 [60.0–135.0]	105.0 [75.0–150.0]	0.13	r = 0.18 [− 0.06–0.40]
Postoperative nausea and vomiting
PONV incidence, n (%)	2 (5.7)	12 (34.3)	0.01	OR = 0.12 [0.02–0.57]
Rescue analgesic requirement
First rescue time (hours)‡	5.0 [4.0–6.0]	5.0 [3.0–6.0]	0.25	r = 0.16 [− 0.12–0.41]
Median pain-free duration (hours, 95% CI)‡	6.0 (5.0–9.0)	6.0 (5.0–6.0)	0.63	HR = 0.88 [0.52–1.50]
Total rescue count at 24 h	1.0 [1.0–2.0]	1.0 [1.0–2.0]	0.41	r = 0.10 [− 0.14–0.33]
Rescue-free rate at 24 h, n (%)	8 (22.9)	8 (22.9)

Continuous variables are presented as median [interquartile range]. Categorical variables are presented as n (%). ‡Patients who did not require rescue analgesia within 24 h were treated as censored observations in the Kaplan-Meier analysis (ESPB: n = 8, Control: n = 8). The first rescue time is reported for patients who received rescue analgesia only (ESPB: n = 27, Control: n = 27). Pain-free duration and rescue-free rates were estimated using the Kaplan-Meier method; groups were compared using the log-rank test (χ² = 0.231, df = 1). PCA consumption and rescue count were compared using the Mann-Whitney U test. PONV incidence was compared using Fisher’s exact test. Bold p-values indicate statistical significance (p ≤ 0.05)

PCA patient-controlled analgesia, MME morphine milligram equivalent, PONV postoperative nausea and vomiting, CI confidence interval, OR odds ratio, HR hazard ratio, ESPB erector spinae plane block

Total PCA opioid use was lower in the ESPB group (median 90.0 vs. 105.0 mg), but this was not significant (p = 0.13; r = 0.18). Both groups had a median time to first extra pain medicine of 6.0 h, with no difference (log-rank p = 0.63; HR = 0.88 [95% CI: 0.52–1.50]) (Fig. 4).

Fig. 4.

Kaplan–Meier curves for time to first rescue analgesia over the 24-hour postoperative period. Patients who did not require rescue analgesia were censored at 24 h (ESPB: n = 8; Control: n = 8). Groups were compared using the log-rank test (p = 0.63); hazard ratio (HR) = 0.88 (95% CI: 0.52–1.50)

Similarly, the number of rescue pain medications administered in 24 h (p = 0.41) and the percentage of patients who did not require rescue medication at 24 h (22.9% in both groups) were similar. No patient needed second-line intravenous pethidine, and no block-related complications or side effects occurred in either group during follow-up.

Collectively, these results indicate that ESPB was associated with reduced postoperative pain intensity and a much lower incidence of PONV, while not significantly affecting opioid consumption or the timing of rescue analgesia.

Perioperative hemodynamic parameters were similar between groups (Table 4), as heart rate and mean arterial pressure remained consistent across baseline, post-induction, intraoperative, and PACU measurements. Although the Control group had a higher post-induction heart rate (85.1 ± 13.6 vs. 79.8 ± 13.2 beats/min), this difference was not statistically significant (p = 0.11, where p is the probability that the difference was due to chance; d = − 0.39, where d indicates the effect size of the magnitude of the difference). Overall, these findings suggest that ESPB does not adversely affect perioperative hemodynamic stability.

Table 4.

Perioperative hemodynamic parameters

Time Point	ESPB Group (n = 35)	Control Group (n = 35)	p	Effect Size [95% CI]
Heart rate (beats/min)
Baseline (admission)	81.6 ± 12.4	78.9 ± 13.8	0.39	d = 0.21 [− 0.26–0.68]
Post-induction	79.8 ± 13.2	85.1 ± 13.6	0.11	d = − 0.39 [− 0.87–0.08]
1st hour*	74.8 ± 11.5	72.2 ± 10.5	0.44	d = 0.23 [− 0.35–0.81]
End of surgery	76.5 ± 9.6	76.2 ± 11.6	0.92	d = 0.02 [− 0.44–0.49]
PACU	79.3 ± 15.4	77.1 ± 12.9	0.50	d = 0.16 [− 0.31–0.63]
MAP (mmHg)
Baseline (admission)	98.0 [92.0–110.0]	98.0 [93.0–103.0]	0.57	r = 0.07 [− 0.17–0.30]
Post-induction	83.8 ± 13.8	81.4 ± 7.4	0.36	d = 0.22 [− 0.25–0.69]
1st hour*	87.9 ± 12.7	83.4 ± 12.1	0.23	d = 0.36 [− 0.22–0.95]
End of surgery	94.1 ± 14.5	90.7 ± 8.8	0.25	d = 0.28 [− 0.19–0.75]
PACU	97.8 ± 14.7	95.7 ± 7.9	0.46	d = 0.18 [− 0.29–0.65]

Values are presented as mean ± SD or median [interquartile range] according to normality (Shapiro-Wilk test). *1st hour data were available only in patients with operation time ≥ 60 min (ESPB: n = 29, Control: n = 19). The 2nd hour data were excluded due to insufficient sample size (n = 7). Heart rate at all time points was compared using Student’s t-test. Baseline MAP was compared using the Mann-Whitney U test (ESPB group non-normal, p = 0.030). Post-induction, end of surgery, and PACU MAP values were compared using Welch’s t-test (unequal variances). The 1st hour MAP was compared using a Student’s t-test. Effect sizes: Cohen’s d for parametric comparisons, r = Z/√N for nonparametric comparisons. Bold p-values indicate statistical significance (p ≤ 0.05)

Hemodynamic comparisons were considered exploratory and were not adjusted for multiplicity

MAP mean arterial pressure, PACU post-anesthesia care unit, ESPB erector spinae plane block, CI confidence interval

Discussion

The principal finding of this randomized controlled trial is that ultrasound-guided ESPB after flexible ureteroscopy exhibits a time-dependent analgesic effect, with benefits becoming more apparent over time rather than immediately after surgery. ESPB may also reduce discomfort from ureteral stents. Although the sample size was based on the 2-hour NRS score, no statistically significant difference was observed at that point. Predefined longitudinal analysis revealed a significant group-by-time interaction, highlighting the evolving pain trajectory. This difference reflects the limitations of a single time-point power calculation compared to a repeated-measures approach. While longitudinal analysis provides a broader picture of postoperative pain progression, the study was not specifically powered for repeated-measures modeling, which should be considered when interpreting the findings.

Resting NRS scores diverged from postoperative hour 6, with the greatest between-group separation at 12 h. This pattern suggests a delayed but sustained analgesic effect rather than an immediate peak response.

Pain scores were lower, but cumulative opioid use remained similar between groups. This may be due to the standardized multimodal analgesic regimen and variability of PCA use. These factors complicate translating subjective pain improvement into measurable opioid reduction.

ESPB led to statistically significant reductions in postoperative pain scores at later time points; however, the size of the reduction, particularly at 12 h, was close to the lower threshold of clinical significance (minimal clinically important difference for NRS). Importantly, these improvements did not result in significant reductions in total opioid use or the need for rescue analgesics. Therefore, the overall additional clinical benefit of ESPB within a standardized multimodal analgesic regimen appears modest.

Several factors may explain the difference between reduced pain and unchanged opioid use. All patients received a standardized multimodal analgesic regimen, possibly creating a ceiling effect and limiting further opioid reduction. Also, PCA opioid use may not directly reflect real-time pain due to behavioral variability. Finally, protocol-driven rescue analgesia may have minimized group differences. These are known limitations of PCA-based measures, where opioid use can reflect patient behavior, not just nociceptive burden.

To the best of our knowledge, there is limited randomized controlled evidence for ESPB in flexible ureteroscopy. In our study, all patients received double-J ureteral stents, which are known contributors to postoperative stent-related symptoms [4, 22, 23]. This irritation can lead to ongoing visceral afferent signaling from thoracolumbar segments for several hours after the surgery. Because stents were used in both groups, the comparison between the groups is valid, and our findings suggest that ESPB may help modulate visceral afferent input. However, we found that the overall clinical impact of ESPB in this context was modest.

The observed delayed analgesic effect matches the proposed ESPB spread patterns. Local anesthetic beneath the erector spinae muscle can extend cranially, caudally, or anteriorly to the dorsal and ventral rami, and may enter the paravertebral space. Imaging studies confirm paravertebral and neural foraminal spread after ESPB, supporting its role in modulating the thoracolumbar visceral pathway. Since ureteral dilation and stent irritation activate afferent fibers projecting to thoracolumbar spinal segments, this pathway likely explains the greater separation in resting pain scores observed [3, 13, 14, 24].

Most ESPB evidence for upper urinary tract visceral pain comes from studies involving patients with acute renal colic in emergency settings. In a randomized pilot trial, Aydin et al. [11] found greater short-term pain relief and less rescue opioid use with ESPB compared to NSAIDs. Similarly, Torabi et al. [9] reported better analgesia with ESPB versus intravenous fentanyl in refractory renal colic patients. Other reports support these results. Both Aydin et al. [12] and Noble et al. [10] reported marked reductions in pain scores after thoracic-level ESPB in cases of renal colic.

A recent meta-analysis by Candel et al. [7] included randomized and observational studies. It showed ESPB significantly reduced pain at 30 and 60 min. However, overall certainty was very low due to heterogeneity, small sample sizes, and methodological limitations. Barrett and Kerr [6] reviewed these results, stressed ESPB’s promise, but noted considerable study weaknesses. They call for higher-quality research.

Together, these studies show ESPB may modulate visceral pain in acute renal colic. This study extends that evidence to perioperative URS, supporting ESPB’s effect on post-stenting pain.

Reduced PONV was the most notable secondary outcome in this study. PONV was much lower in the ESPB group despite similar opioid exposure. This aligns with evidence suggesting that ESPB reduces PONV across different surgeries [25]. Modulation of visceral afferent input or autonomic pathways may also contribute to the antiemetic effect [26]. However, this mechanism is speculative and was not directly assessed in this study. The disconnect between opioid use and PONV reduction suggests other mechanisms may be at play. Given the importance of PONV after ureteroscopy, this result may be a useful benefit of ESPB.

ESPB did not disrupt perioperative hemodynamic stability. Heart rate and mean arterial pressure were similar in both groups, with no clinically significant hypotension or bradycardia. This aligns with the safety of interfascial plane blocks, which typically have limited sympathetic effects [27].

Collectively, these results suggest ESPB’s main benefits are time-dependent pain relief and reduced PONV, rather than opioid sparing, within a multimodal regimen.

Future multicenter trials with longer follow-up are needed to confirm these results. They should assess longer-term recovery outcomes. These studies should clarify the link between pain reduction and opioid use. Comparative research on PCA versus other analgesic methods may define ESPB’s opioid-sparing potential within multimodal approaches.

This study has several limitations. First, it was a single-center trial at a university hospital. This may limit generalizability to other sites or patient groups. Multicenter studies are needed to confirm these results.

Second, postoperative outcome assessment was blinded. All interventions were performed under general anesthesia, keeping participants unaware of their group assignment. However, the anesthesiologist performing the ESPB was not blinded. This limitation in regional anesthesia studies may introduce performance bias.

Third, follow-up was limited to 24 h, covering the effect of a single injection of ESPB and the key early postoperative interval. Long-term outcomes, such as satisfaction, recovery, and discharge readiness, were not measured.

Fourth, we used standardized general anesthesia and a set multimodal protocol. However, there may be individual differences in intraoperative anesthetic needs. This could have introduced confounding that we could not fully control.

Lastly, even though pain scores dropped significantly, opioid use did not differ between groups. This gap may be due to variability in PCA use or to the ceiling effect of multimodal analgesia. Further research is needed.

Conclusions

In patients undergoing flexible ureteroscopy, a single ultrasound-guided ESPB was associated with a time-dependent reduction in postoperative pain intensity and a reduction in PONV, without a clear opioid-sparing effect. The magnitude of pain reduction approached the lower threshold of clinical relevance and should be interpreted with caution. ESPB may provide selective benefits within a multimodal analgesic approach, particularly for pain modulation over time and reduction of PONV. Further multicenter studies with extended follow-up are needed to better define its broader clinical impact.

Abbreviations

URS

Ureteroscopy

IV

Intravenous

ESPB

Erector Spinae Plane Block

ASA

American Society of Anesthesiologists

NRS

Numeric Rating Scale

PACU

Post-Anesthesia Care Unit

MME

Morphine Milligram Equivalent

PONV

Postoperative Nausea and Vomiting

NSAID

Nonsteroidal Anti-Inflammatory Drug

Authors’ contributions

Ayhan Şahin contributed to study conception, methodology, data collection, and drafting of the manuscript. Onur Baran contributed to study design, statistical analysis, data interpretation, and manuscript writing. Çağrı Doğan and Mehmet Fatih Şahin contributed to data collection and project administration. Cavidan Arar contributed to study supervision, data interpretation, and critical revision of the manuscript. All authors read and approved the final version of the manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Trakya University Clinical Research Ethics Committee (Approval No: 2024-KAEK-06-2502A05). Written informed consent was obtained from all individual participants included in the study. The study was conducted in accordance with the ethical principles of the Declaration of Helsinki.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.