Is the neck disability index an appropriate measure for changes in physical function after surgery for cervical spondylotic myelopathy?

Background: The Neck Disability Index (NDI) is a 10-item questionnaire about symptoms relevant to cervical spine pathology, originally validated in the physical therapy literature. It is unclear if all of the items apply to spine surgery. The purpose of this study was to determine if improvements in the composite NDI score or specific NDI domains are appropriate measures for tracking changes in physical function after surgical intervention for cervical spondylotic myelopathy (CSM). Methods: A retrospective cohort review of patients treated at a major academic medical center was undertaken. Baseline and postoperative standardized outcome measurement scores, including composite NDI, NDI subdomain, and SF-12 physical component score (PCS), were collected. Wilcoxon signed-rank test was used to determine whether patients exhibited improvement in each of the outcome measures included. Multiple linear regression was performed to determine whether change in NDI composite or subdomain scores predicted change in physical function after surgery for CSM-compared with the well-validated PCS score-controlling for factors such as age, sex, etc. Results: Baseline data were collected on 118 patients. All outcome measures exhibited significant improvement after surgery based on the Wilcoxon signed-rank test. On linear regression, work (β =-2.419 [-3.831, -1.006]; P = .001) and recreation (b =-1.354 [-2.640, -0.068]; P = .039), as well as the NDI composite score (β =-0.223 [-0.319, -0.127]; P , .001), were significant predictors of change in physical function over time. Conclusions: Although the NDI composite score did predict change in PCS over time, only 2 of the 10 NDI subdomains were found to be associated with change in physical function over time. Based on these results, the item bank and composite scoring of the NDI are inappropriate for evaluating quality of life in studies of surgically treated cervical spondylotic myelopathy patients.