Distance to first symptoms measured by the 6-min walking test differentiates between treatment success and failure in patients with degenerative lumbar disorders

European Spine Journal https://doi.org/10.1007/s00586-021-07103-9 ORIGINAL ARTICLE Distance to first symptoms measured by the 6‑min walking test differentiates between treatment success and failure in patients with degenerative lumbar disorders Anna M. Zeitlberger1 · Marketa Sosnova1 · Michal Ziga1 · Oliver P. Gautschi3 · Luca Regli4 · Oliver Bozinov1 · Astrid Weyerbrock1 · Martin N. Stienen1,4 · Nicolai Maldaner1,2,4 Received: 21 February 2021 / Revised: 12 December 2021 / Accepted: 21 December 2021 © The Author(s) 2022 Abstract Purpose The smartphone-based 6-min walking test (6WT) is an established digital outcome measure in patients undergoing surgery for degenerative lumbar disorders (DLD). In addition to the 6WTs primary outcome measure, the 6-min walking distance (6WD), the patient’s distance to first symptoms (DTFS) and time to first symptoms (TTFS) can be recorded. This is the first study to analyse the psychometric properties of the DTFS and TTFS. Methods Forty-nine consecutive patients (55 ± 15.8 years) completed the 6WT pre- and 6 weeks (W6) postoperative. DTFS and TTFS were assessed for reliability and content validity using disease-specific patient-reported outcome measures. The Zurich Claudication Questionnaire patient satisfaction subscale was used as external criterion for treatment success. Internal and external responsiveness for both measures at W6 was evaluated. Results There was a significant improvement in DTFS and TTFS from baseline to W6 (p < 0.001). Both measures demonstrated a good test–retest reliability (β = 0.86, 95% CI 0.81–0.90 and β = 0.83, 95% CI 0.76–0.87, both p < 0.001). The DTFS exceeded the 6WD capability to differentiate between satisfied (82%) and unsatisfied patients (18%) with an AUC of 0.75 (95% CI 0.53–0.98) vs. 0.70 (95% CI 0.52–0.90). The TTFS did not demonstrate meaningful discriminative abilities. Conclusion Change in DTFS can differentiate between satisfied and unsatisfied patients after spine surgery. Digital outcome measures on the 6WT metric provide spine surgeons and researchers with a mean to assess their patient’s functional disability and response to surgical treatment in DLD. Keywords 6-min walking test · 6WT · Functional self-assessment · Objective functional impairment · Distance to first symptoms Abbreviations 6WT 6-minute walking test 6WD 6-minute walking distance app Application * Nicolai Maldaner 1 Department of Neurosurgery, Clinical Neuroscience Center, Kantonsspital St. Gallen, St. Gallen, Switzerland 2 University Hospital Zurich, University of Zurich, Frauenklinikstrasse 10, 8091 Zurich, Switzerland 3 Neuro‑ and Spine Center, Hirslanden Clinic St. Anna, Lucerne, Switzerland 4 Department of Neurosurgery, Clinical Neuroscience Center, University Hospital Zurich, University of Zurich, Zurich, Switzerland CI Confidence interval COMI Core outcome measures index DLD Degenerative lumbar disorders hrQoL Health-related quality of life ICC Intraclass correlation coefficient LDH Lumbar disc herniation LSS Lumbar spinal stenosis MCID Minimum clinically important difference m Metres PROMs Patient-reported outcome measures SPWT Self-paced walking test SRM Standardized responsive mean VAS Visual analogue scale ZCQ Zurich Claudication Questionnaire 13 Vol.:(0123456789) European Spine Journal Introduction Methods The management of patient with degenerative lumbar disorders (DLD) requires reliable measures of functional impairment. Today, patient-reported outcome measures (PROMs) are used as a gold standard for the outcome assessment in spine surgery [1–3]. Apart from subjective PROMs, objective measures of function find increasing attention in research and clinical practice as they help to monitor and compare treatment results over time and across populations [4]. The 6-min walking test (6WT) is increasingly applied as an objective outcome measure in patients with lumbar DLD [5]. We recently developed a smartphone app-based version of the 6WT, which demonstrated excellent reliability [6, 7]. The 6WT can be self-performed by the patient in his/her home environment, and results are standardized with respect to age and sex [6, 8, 9]. By providing the ability to monitor patients from afar, digital outcome measures are invaluable tools for physicians and patients. In a recent study, three out of four patients favoured the smartphone-based 6WT over traditional paper-based PROMs for the assessment of spine-related symptoms [10]. This is a trend that will only accelerate in a time when a global pandemic hampers avoidable physical “face-toface” consultations as those might endanger elderly or particularly fragile patients [11, 12]. The 6WT’s primary outcome measure is the maximum distance a patient can walk within 6 min (6WD = 6-min walking distance, measured in metres) [6]. In addition, the 6WT-app provides patients with the possibility to push a “flash button” and records both the time (TTFS = time to first symptoms, measured in seconds) and distance (DTFS = distance to first symptoms, measured in metres) when first symptoms of neurogenic claudication appear. While the 6WD expresses the result of walking restrictions that occur over the whole duration of the test time, TTFS and DTFS may give more granular information about the severity or urgency of patient’s symptoms. Studies have proven the 6WD to be a reliable, valid, and responsive measure of functional impairment [6]. The added value of both TTFS and DTFS in addition to the 6WD, however, is yet unclear. This study aims to analyse the psychometric properties of DTFS and TTFS as determined by the 6WT. We hypothesize that the pre- to postoperative change in both measures may help to differentiate between treatment successes in patients with lumbar DLD and compare the responsiveness to the traditional 6WD outcome. All adult patients with lumbar DLD scheduled for elective spine surgery between May 2019 and March 2020 with one of the following diagnosis (1) lumbar disc herniation (LDH), (2) lumbar spinal stenosis lumbar (LSS) or (3) DLD with or without instability requiring lumbar fusion were prospectively screened for study enrolment at the XX, XX, XX. A detailed prescription of the app-based outcome measures and PROMs used in this study is provided as Online Resource 1. 13 Inclusion and exclusion criteria for the study cohort Patients fulfilling all of the following inclusion criteria were considered for this study: • Male or female subject ≥ 18 years; • Written informed consent. Patients were not enrolled if any of the following exclusion criteria were met: • Pregnancy; • Inability to walk (extreme pain or severe neurological deficits); • Severe chronic obstructive lung disease (COPD) corre- sponding to ≥ Gold III; • Severe heart failure corresponding to ≥ NYHA III; • Lung cancer and diffuse parenchymal lung disease; • Other medical reasons interfering with the patient’s abil- ity to walk and perform the 6WT (e.g. osteoarthritis (...truncated)