Ropivacaine Spinal Anesthesia for IELD: Dose-Finding Pitfalls and Scoring Inconsistency [Letter]

Dear editor

We read with interest the study by Zhang et al determining the MEC₉₀ of ropivacaine for spinal anesthesia in interlaminar endoscopic lumbar discectomy using a biased coin design.1 While the authors address a clinically relevant question, several methodological issues warrant careful consideration.

The most serious flaw lies in a fundamental self-contradiction in the directional definition of the Bromage score. The Methods section explicitly defines a reverse scale—higher scores indicate less motor blockade (1 = complete block, 6 = no block)—whereas the Plain Language Summary states that “higher Bromage scores” reflect more severe motor blockade, a directly opposing interpretation. This contradiction is systemic: if one adopts the reverse scale defined in Methods, the negative correlation between concentration and Bromage score (r = −0.235) implies that “higher concentration → lower score → greater motor blockade”, which is pharmacologically coherent, yet the Plain Summary entirely inverts this direction; conversely, if one accepts the direction implied by the Plain Summary (higher score = greater block), then the scale definition in Methods is erroneous, and the explanatory chain in the Discussion—that “lower concentrations produce lighter motor blockade”—collapses accordingly. Whichever end is credited, at least one key statement in the paper must be incorrect.

Even more concerning, the reported median Bromage score at the end of surgery was 4.5 (IQR: 3.0–5.0). Under the authors’ own reverse scale, a score of 3 denotes “able to move only the knee joint”—meaning that approximately one-quarter of patients exhibited substantial motor blockade at the conclusion of surgery. This directly contradicts the authors’ central thesis in the Discussion that the regimen “maximally preserves motor function, thereby facilitating timely intraoperative detection of nerve injury”. The data themselves demonstrate that the protocol failed to achieve its professed safety objective. The authors neither address this contradiction nor explain why their data run counter to their core argument, leaving readers with a fundamental doubt regarding both the safety of the proposed regimen and the reliability of the conclusions.

The definition of a “positive outcome” as VAS = 0 represents an exceptionally stringent criterion that may systematically overestimate the required MEC₉₀. In biased coin design dose-finding studies for regional anesthesia, successful block is conventionally defined as adequate surgical anesthesia allowing procedure completion without rescue analgesia, rather than complete absence of any pain sensation.2 The authors’ own data demonstrate that all nine negative cases (VAS≥1) ultimately completed surgery successfully—three with local anesthesia supplementation and five with additional intravenous analgesics. This indicates that VAS = 0 is not a clinically meaningful threshold for determining the minimum effective concentration, as patients with VAS = 1–2 may still have entirely acceptable surgical conditions. The MEC₉₀ of 9.474 mg/3mL derived from this overly strict definition may therefore represent an unnecessarily high clinical recommendation that could increase motor blockade and urinary retention without commensurate analgesic benefit.

Despite recruiting patients across ASA I–III, the authors did not stratify the MEC₉₀ estimation by ASA class, although Figure 2 shows that ASA III patients received higher ropivacaine doses. This heterogeneity may have inflated the overall MEC₉₀ estimate if higher-risk patients disproportionately contributed to the upper end of the dose distribution. Given that ASA III patients represent nearly one-quarter of the cohort, the absence of subgroup estimation limits the clinical applicability of a single population-wide recommendation.3

In summary, the internal contradiction in Bromage score interpretation fundamentally undermines the study’s central safety argument, while the overly stringent VAS = 0 criterion and the absence of ASA-stratified analysis collectively limit the clinical applicability of the proposed 9.5–10.5 mg/3mL recommendation. We suggest the authors re-examine their data under a consistent Bromage scoring framework, consider redefining clinical success as surgery completion without rescue intervention rather than VAS = 0, and provide ASA-stratified MEC estimates to better inform individualized dosing. Addressing these issues would substantially strengthen the validity and translational value of this otherwise valuable dose-finding study.

Disclosure

None of the authors have any conflicts of interest in this communication.