Introduction
Evidence & Methods
The Neck Pain Disability Index (NPDI) is a popular standardized questionnaire providing a self-report of perceived neck symptoms and impact at one point in time.
The authors found that baseline NPDI scores declined more in subjects who felt, compared to baseline, they were globally “better,” than in subjects who did not feel they were “better.” But there was considerable overlap. The statistically determined minimum detectable change (MDC), calculated from the standard error of measure, appeared to be larger than the minimal clinically important difference in the NPDI.
What constitutes meaningful improvement in spinal disorders remains a controversial area, and statistically derived values based upon a patient's global report of being “better” apparently varies with method of calculation and the population being studied.
—The Editors
It is estimated that up to 54% of the population has experienced neck pain within the past 6 months [1], with up to 42% seeking care from general practitioners [2]. Fifteen percent of a physical therapist's caseload consists of patients with neck pain [3]. Several self-report functional outcome or disability measures have been developed for the assessment of disability in patients with neck pain [4], [5], [6], [7], [8], [9], [10]. Of interest to clinicians is the clinical utility of self-report measures to accurately reflect patient-perceived status and identify when that status has changed through a course of treatment.
The Neck Disability Index (NDI), originally modeled after the Oswestry Low Back Pain Disability Questionnaire [4], [11], is the most studied and well established of the outcome measures for neck pain [12] and assesses both subjective symptoms and activities of daily living. Several researchers [4], [8], [13], [14] have assessed the reliability and validity of the NDI. Three studies [8], [13], [15] have reported on the responsiveness of the scale or the ability of the scale to accuretly detect when change has indeed occurred [16]. Responsiveness is often reported in two ways. The first is the minimum detectable change (MDC), that is defined as a change in a patient's score that is greater than measurement error [17]. The second is the minimum clinically important difference (MCID) or the smallest change in an instrument that is perceived to be beneficial by the patient and thus would bring about a change in patient management [18]. Prior MDC scores for the NDI have been reported to be between 4.2 and 9.8 raw score points [8], [13], [15]. Prior MCID scores for the NDI have been reported to be between 5 and 9.5 raw score points [13], [15].
Sixty-five percent of patients with pain referable to the cervical spine report both axial and upper extremity (UE) radicular symptoms [19]. However, prior reports of the psychometric properties of the NDI have yet to be reported in a sample which approximates this high percentage of patients with neck pain and concomitant UE referred symptoms. Therefore, the primary purpose of this study was to report the psychometric properties of the NDI in a large sample of patients with a primary complaint of mechanical neck pain, presenting with or without concomitant UE radicular symptoms. A secondary purpose was to assess if there is a difference in the amount of change in NDI scores relative to baseline scores between patients with and without unilateral UE symptoms who were rated as improved, as well as between those rated as stable.