Defining minimal clinically important differences in pain and disability outcomes of patients with chronic pain treated with spinal cord stimulation

Abstract

OBJECTIVE Minimal clinically important difference (MCID) thresholds for a limited number of outcome metrics were previously defined for patients with failed back surgery syndrome (FBSS) at 6 months after spinal cord stimulation (SCS). This study aimed to further define MCID values for pain and disability outcomes. Additionally, the authors established 1-year MCID values for outcome measures with previously defined metrics commonly used to assess SCS efficacy. METHODS Preoperative and 1-year postoperative outcomes were collected from 114 patients who received SCS therapy for FBSS, complex regional pain syndrome, and neuropathic pain. MCID values were established for the numerical rating scale (NRS), Oswestry Disability Index (ODI), Beck Depression Inventory (BDI), McGill Pain Questionnaire (MPQ), and Pain Catastrophizing Scale (PCS). Four established anchor-based methods were utilized to compute MCID values with two anchored questions: “Are you satisfied with SCS therapy?” and “Would you have SCS surgery again?” For each question, patients were categorized as responders if they answered “yes” or as nonresponders if they responded “no.” The methodologies utilized to compute MCID scores included the average change method, minimum detectable change approach, change difference calculation, and receiver operating characteristic (ROC) analysis. Area under the ROC curve (AUC) analysis has been shown to inform the accuracy at which the MCID value can distinguish responders from nonresponders and was analyzed for each instrument. RESULTS For the first time, ranges of MCID values after SCS were established for MPQ (1–2.3) and PCS (1.9–13.6). One-year MCID values were defined for all indications: NRS (range 0.9–2.7), ODI (3.5–6.9), and BDI (2–5.9). AUC values were significant for NRS (0.78, p < 0.001), ODI (0.71, p = 0.003), MPQ (0.74, p < 0.001), and PCS (0.77, p < 0.001), indicating notable accuracy for distinguishing satisfied patients. CONCLUSIONS This was the first study to successfully determine MCID values for two prominent instruments, MPQ and PCS, used to assess pain after SCS surgery. Additionally, previously established MCID values for ODI, BDI, and the visual analog scale for patients with FBSS at 6 months after treatment were explored at 12 months for the most common indications for SCS. These data may better inform physicians of patient response to and success with SCS therapy.