The Neck Disability Index: a study of reliability and validity.
Purpose of Study
The Neck Disability Index (NDI) was developed by Vernon and Mior in the late 1980’s and published in 1991. It remains the most widely used outcome instrument for the assessment of patients with neck related disorders.
The NDI was developed to provide an objective measure to assess activities of daily living (ADL) in patients with neck pain and represents a revised form of the well-known Oswestry Low Back Pain Index. At present, it has been translated into over 20 different languages.
This paper by Vernon and Mior from 1991 provides the medical community with the first review of reliability and validity of the NDI. Based on the authors view, the NDI was shown to have a high rate of internal consistency and test reliability.
Multiple studies have taken a further look at the NDI and agree that it is a valid, internally consistent tool to measure patient reported disability for those with neck pain. It provides an objective baseline before starting any type of treatment and can be used post-treatment to assess progress. Based on further published studies, the minimally detectable change (MDC) is estimated to be somewhere between 5 and 10 points. The minimally clinically important difference (MCID) was found to be 3.5 up to 7.5 depending on the condition being studied. There is also a reported floor/ceiling effect at 0 to 10 and 40 to 50, where it becomes difficult to detect improvement or worsening of a condition (Westaway et al 1998, Wheeler et al. 1999).
Despite the longevity and widespread use of the NDI, there are several modern day criticisms of its design. Aillet et al (2013) expressed concerns that medication use can have an impact on scoring as medication usage is not factored into the design of the NDI. Furthermore, as NDI represents a disability assessment, computer work and sports activities should be included in the ratings process since these are part of modern day living. Hoving et al (2003) suggested the NDI may be an incomplete outcome measurement when evaluating patients with whiplash type disorders.
As a patient reported outcome, the NDI is an objective measure that allows us to assess and monitor a patient’s progress with treatment. Despite several concerns associated with specific cervical conditions, the development of the NDI, as a valid outcome measure, by Vernon and Mior should be considered one of the most significant contributions to the spinal literature as this has helped provided a comparative basis of measuring outcomes in hundreds, if not thousands of studies.