Intravenous Methocarbamol for Acute Pain After Spine Surgery

The IMAPSS study is a target trial emulation evaluating the effect of intravenous (IV) methocarbamol on postoperative pain management and opioid consumption in adult patients undergoing elective spine surgery. The study employs a target trial emulation design to simulate the conditions of a randomized controlled trial using observational data extracted from electronic medical records. The main focus is to determine if administering IV methocarbamol within two hours after surgery reduces postoperative pain scores and opioid use.

Patients will be tracked from the point of their first recorded pain score in the Post-Anesthesia Care Unit (PACU) after surgery to assess for treatment assignment eligibility. Data will be collected at 15-minute intervals, which corresponds to the standard assessment frequency in Phase I PACU care, for up to 12 hours postoperatively for each patient. The primary analysis will use time-varying propensity score matching (TV-PSM) to control for baseline and time-varying confounders. Pain levels will be measured using a time-weighted average (TWA) of pain scores, and opioid consumption will be quantified in oral morphine equivalents (OME).

The primary analysis will utilize TV-PSM, with propensity scores estimated through a Cox proportional hazards model. Matching will create balanced treatment groups at each 15-minute interval. Generalized estimating equations (GEE) will then estimate the effect of IV methocarbamol on the primary outcome (TWA pain) and secondary outcome (cumulative OME) over a 6-hour postoperative period. As a sensitivity analysis, a marginal structural model (MSM) using inverse probability weighting (IPW) will assess the robustness of the findings to the time-varying confounder adjustment.

This study's findings will aim to inform evidence-based recommendations for postoperative pain management and opioid-sparing strategies, potentially providing safer alternatives to conventional opioid-based regimens.

Inclusion criteria

• 18 years of age or older
• Undergoing one of the following three elective spine surgeries: (i) Anterior Cervical Discectomy and Fusion (ACDF), (ii) Posterior Spinal Fusion, or (iii) Laminectomy, Laminotomy, or Discectomy
• At least 8h of valid pain scores recorded postoperatively

Exclusion criteria

• Emergency Surgery
• Pregnancy
• Sensitivity to methocarbamol (allergy)
• End-stage renal disease (ESRD)
• Myasthenia gravis
• Missing a valid electronic anesthesia record