Clinimetric properties of the Turkish translation of a modified neck disability index

Neck pain is a common condition [1–8]. It becomes chronic at the rate of 30-50%, thus representing one of the most important reasons of disability and workforce loss [3, 4, 6, 8]. Neck pain has been shown to affect a person's activities of daily living [9–14]. Chronic neck pain results in greatly increased treatment costs and as well as decreases in work capacity. As such, it is important in the early diagnosis and follow-up of neck pain to assess a patient's level of self-rated disability.

Various scales have been developed for the evaluation self-rated disability in neck pain patients. The Neck Disability Index (NDI) [15] is the first such scale published. It was modified from the Oswestry Low Back Pain Disability Questionnaire [16] by Vernon and Mior [15]. The NDI is the most widely used scale for evaluating neck pain related disability throughout the world [17], and it has been translated into many languages such as Brazilian Portuguese [18], Greek [19], Chinese [20], Farsi [21], Dutch [22], Korean [23], French [24]. It has also been translated and used in studies in Turkey [25] along with a Turkish version of the Neck Pain Disability Index [26, 27]; however, neither of these studies employed factor analysis of the translated and modified scales. This is important in cross-cultural studies because there is controversy about the factorial structure of the NDI [28, 29], and cultural differences may play a role in the variation observed in different studies.

The aim of the present study was to investigate the clinimetric properties of a Turkish translation of a modified NDI in order to evaluate self-rated disability in patients with neck pain.

In the translation-pilot study, in the first item on "pain intensity" a modification was made as to "your neck pain". In the third item, 'lifting', a statement of "having weight equal to that you lift when you don't have neck pain" was added for the purpose of providing clarification regarding lifting heavy loads. In item seven, 'work', a statement of "Please check the option G if you are not employed," was added to make it clear that it was the work-life that was evaluated.

In the pilot study, the most frequently asked question by participants was associated with item 10. Since "recreation" was better understood as "leisure time activities", it was modified in this way. In the pilot study, there were 9 persons who were non-drivers, and problems arose in answering item 8. Furthermore, to a lesser extent, since there were participants who were non-workers and who did not perform leisure time activities, the option of "never done" was added to sections 7, 8 and 10. The Turkish version of the modified NDI was also used in validity study and, 120 participants (64%) scored item 8 as "never done".

In general, patient comments were that the questions were clear. The time for completing the Turkish version of modified NDI was 5.5 minutes. When there were questions that were not understood, explanation was provided by the observer.

For the purpose of evaluating subjects with neck pain in the Turkish society, a Turkish translation and adaptation of NDI was performed, and the validity and reliability of its use was demonstrated. Our study group for the test which was performed on patients from polyclinic through randomized selection comprised of patients with neck pain. This was a homogeneous population regarding age and gender.

In our study, the duration for completing Turkish version of modified NDI was similar to that in the original study [15, 17]. Unclear questions were explained by the observer.

As with other studies, there were non-drivers, unemployed and those who did not have leisure time activities [42, 43]. Ackelman et al. modified this part of the index wherein they added the explanation of "not applicable" [44]. It was considered appropriate to add the option of "never done" to the test also in our study.

As with most other studies, [18, 19, 21, 24, 42–45] test-retest for all NDI domains was found to be high (0.87-1) in our study. In the study by Ackelman et al. the result (0.97) that was found in the test-retest reliability performed with a 2-day interval was high [44], which was attributed to the additional explanations provided by the investigators. These additions were also provided in our study. The study by Cleland et al., determined an ICC of 0.50, which was the lowest value reported in the literature [46]. However, this study involved patients with cervical radiculopathy. In the adaptation study conducted in the Netherlands, test-retest ICC was found lower as 0.53 in personal care domain [22].

The most important comparison of our results is with the work of Aslan et al. [25]. They also reported very high test-retest reliability (r = 0.98).

As with our study, in most similar studies, a duration of a day or two was given for test-retest [18, 19, 21, 24, 42–45]. In pain studies, a retest interval of one week was reported to be not suitable as treatment administration and would be unethical [18].

In NDI internal consistency studies, values were found to be between 0.74-0.93 by investigators. The internal consistency we determined in Turkish version of modified NDI was consistent with those found in the studies by these investigators [18, 19, 21, 23, 24, 42, 45]. Aslan et al. [25] did not report on the internal consistency of their Turkish translation of the NDI.

In our study, factor analyses revealed one dimension. Similar factorial structures of the questionnaire were observed in its Greek, Brasilian, Canadian [47], Spanish versions. But, in French version, two factor were found. The percentage of explanation of the single factor was higher than in the Greek version, but similar to that found in the Brasilian and Canadian versions (84%) [18, 47]. Aslan et al. [25] did not report on the factorial structure of their Turkish translation of the NDI.

In validity studies, pain was generally evaluated using VAS. Similarly, the results of our study was found good [21, 23, 45]. Mousavi et al. found a correlation with VAS as r = 0.71 [21]. In these studies methodological basis was similar. In addition, results are parallel with those found in many studies that followed different methodologies [23, 45]. Aslan et al. reported only moderately good (r = 0.51-0.62) correlations with the pain VAS [25].

The SF-36 was used for validity also in other studies [18, 19, 21, 44, 45, 48]. In the Portuguese translation study conducted in Brazil, no correlation was found with physical role, emotional role and pain subtitles of SF-36 [18]. Similarly, no correlation was found with physical role in our study, but there was a weak correlation with emotional role and pain. This could be associated with the number of participants. Riddle et al. found equivalence at strong correlations with physical and mental parts of SF-36 [48]. In the translation study for the scale conducted in Iran, only emotional role was found not to be correlated [21]. This result also supports our study.

The HAD test was used in the translation study conducted in France. Investigators, who found correlation between HADS depression scores and NDI, stated that sense of pain was closely associated with psychology [24]. In our study, HADS was found to be correlated with both anxiety and depression scores. In studies conducted on patients with neck pain, it was shown that anxiety and stress might either be the cause or the result of neck pain [49, 50].

In the comparison with Aslan et al., [25] no correlations with self-rated questionnaires for any other important health-related variables was reported. They did report a high correlation with the Turkish version of the Neck Pain Disability Index, which would be expected.

With respect to the work of Aslan et al. [25], in summary, our study has extended that work by including analyses of internal consistency and factorial structure as well as analyses of convergent validity with the pain VAS, HAD and SF-36.

This study has limitations. As this was a study of a modified version of the Turkish NDI, an Item Response Theory (IRT) approach [31] was not adopted; however, with regard to the reliability study, individual item analyses were undertaken. With regard to validity, only current pain, anxiety/depression and quality of life were assessed for concurrent validity. This was deemed to be an appropriate profile of separate constructs with which to evaluate the modified version of the Turkish NDI. Most of the correlations with these instruments did conform to our moderately high predictions. Additional testing with other constructs such as catastrophization or fear-avoidance beliefs would be interesting.

An additional limitation was the lack of assessment of measurement error and responsiveness. We intend to pursue this is a separate study involving a treatment phase.

The modified NDI-Turkish version has been found to be a reliable and valid test and is suitable for daily practise for characterizing concurrent disability. Its use in determining clinical response has not yet been determined.