Cross-cultural adaptation of the Neck Disability Index and Copenhagen Neck Functional Disability Scale for patients with neck pain due to degenerative and discopathic disorders. Psychometric properties of the Polish versions

Annually about 30% of the population experience neck pain (NP), 14% of whom report complaints lasting longer than 6 months [1, 2]. Many authors' experiences indicate that subjective assessment of pain intensity and how it influences the extent to which everyday activities are executed is becoming increasingly significant and an important component of clinical practice, however it is vital that, the tools used in the evaluation of the level to which everyday activities are carried out are valid and reliable [1].

There are several questionnaires available measuring NP that have been developed and published in English-speaking countries: Neck Disability Index (NDI) [3, 4], Neck Pain and Disability Scale (NPDS) [5], the Northwick Park Neck Pain Questionnaire (NPNPQ) [6], the Copenhagen Neck Functional Disability Scale (CDS) [7]. These are classed as region-specific functional outcome questionnaires, which concentrate on specific parts of the body, therefore providing more detailed data on its function within a defined disease entity with greater responsiveness compared to a questionnaire of a general nature such as the Short Form-36 [SF-36] [8]. SF-36 is a multi-purpose, short-form health survey with 36 questions and yields an 8-scale profile of functional health and well-being scores as well as psychometrically-based physical and mental health summary measures [8].

A number of authors [6, 9‐11] highlight the need to adapt recognized and widely applied assessment tools in research rather than developing a new scale leading to the multiplication of outcome measures lacking the comparison of populations [1].

We decided to evaluate the Polish versions of the Copenhagen Neck Functional Disability Scale (CDS-PL) and the Neck Disability Index (NDI-PL). The NDI is the scale most commonly applied, extensively tested and translated. The English version of the NDI has shown moderate differences in reliability and validity with different patient populations [12]. The responsiveness of the NDI is unknown, however concurrent validity when compared with the Visual Analog Scale has been reported [4].

The NDI has been shown to be a valid and reliable instrument to measure disability related to neck pain in studies conducted for French, Brazilian-Portuguese, Iranian, Greek, Finnish, Spanish, Turkish, Korean, Dutch, Chinese, Swedish-speaking patients [1, 13‐21]. The CDS focuses on quality of life and the scores for each item as rated by the patient can be easily transferred and interpreted in terms of their clinical relevance [7, 22]. Despite its many advantages, to our knowledge, the CDS has only been successfully translated and validated into French [22].

The objective of this prospective study was to translate and culturally adapt the NDI and CDS into Polish and to validate their use among Polish-speaking patients with NP. To our knowledge, no questionnaire assessing disability in everyday activities in Polish-speaking patients with neck pain has ever been evaluated and tested for its psychometric properties. Our hypothesis was that if we adapt the NDI and CDS to the Polish cultural conditions and test psychometric properties of the NDI-PL and CDS-PL, such as internal consistency, test-retest reliability, concurrent validity, ceiling or floor effects and analyses of the item-total correlation, then we will achieve assessment tools that are equivalent to the original English- language questionnaires. As a result, we aimed to achieve tools that would help us properly assess pain intensity and the related limitations of cervical spine function during the execution of everyday activities in Polish conditions.

The NDI questionnaire was designed by Vernon and Mior in 1992 to assess pain intensity and the related limitations of cervical spine function during the execution of everyday activities [4]. The NDI is based on the Oswestry Disability Index and is composed of 10 questions: pain intensity, personal care, lifting, reading, headaches, concentration, work, driving, sleeping, and recreation [4]. Each item is scored from 0 (no disability) to 5 (total disability). The maximum possible score is 50. However, the sum of the scores obtained is often doubled to give a percentage score out of 100. The interpretation is as follows: 0-20 normal, 21-40 mild disability, 41-60 moderate, 61-80 severe and 80 or over (complete or exaggerated disability). Since the questionnaire is straightforward, the average patient needs approximately 5 minutes to complete it [4, 23].

The CDS consists of 15 items that evaluate the impact of neck pain. Three items evaluate pain severity directly, including the patient's perception of the future impact of neck pain, eight items evaluate disability during everyday activities and four items focus on social interaction and recreation [7, 22]. There are three possible answers to select from each item; "yes" (2 points), "occasionally" (1 point), and "no" (0 points). For items 1 - 5 however the scoring is reversed and here "yes" carries a score of 0, "occasionally" 1 and "no" 2. The highest score attainable is 30, indicating worst possible impact, the lowest is 0 where no impact of neck pain can be identified. 90 seconds is invariably sufficient in order to complete the questionnaire [7].

To our knowledge, this is the first study of a Polish version of a questionnaire assessing disability in everyday activities in patients with neck pain. Even though there are several region-specific functional outcome questionnaires measuring neck disorders available, no Polish version has ever been validated. Our study indicated that NDI-PL and CDS-PL are valid and reliable methods for measuring disability in Polish patients with neck pain.

There have been numerous studies on the reliability and validity of the Neck Disability Index for patients with neck pain [3, 4, 23]. Vernon and Mior obtained a high degree of test-retest reliability in patients with post-traumatic neck pain, using the Pearson correlation coefficient [4].

The internal consistency of each questionnaire was assessed using Cronbach's alpha. We also assessed the item-total correlations. Our study indicated that NDI-PL and CDS-PL are internally consistent. In comparison to the French version of CDS [22], where all items had good or fair correlations with the total score, almost all items of CDS-PL had very good correlations with the general result. The Cronbach's alpha coefficient of the total score of CDS-PL was excellent and higher, compared to the French version of CDS. However, the Cronbach's alpha in our study was exactly the same as in the original version of the CDS (0.90) [7].

Excellent internal consistency of the NDI-PL (0.90) is comparable with the results from other studies - Spanish, Finnish, Iranian or Brazilian versions of NDI [13, 14, 16, 17]. It is higher than the value obtained by Vernon and Mior (0.8) in the original NDI [4]. Pearson correlation coefficients between individual NDI-PL item scores and the total ranged from 0.43 to 0.77 (test) and from 0.56 to 0.81 (retest) and, as in the original version of the NDI [3, 4] no item dominated with an especially high correlation and no item appeared to be redundant.

We assessed the test-retest reliability of NDI-PL and CNFDS-PL using the Intraclass Correlation Coefficients (ICCs). We decided to choose a retest interval of 2 days, similarly to the authors of the Spanish or Iranian version of NDI, in order to avoid variations in the clinical status of the patients and to avoid the patients remembering their previous answers. As suggested by Holt et al. [26], a long interval period may be inappropriate for a test-retest study of health measures because too many changes in the patient's health status can occur. Opinions regarding the appropriate interval have varied from 1 hour to 1 year, but a retest interval of 2 to 14 days is generally accepted [27]. The very good test-retest reliability of NDI-PL is comparable with the results of the Iranian (0.90) and Greek (0.93) versions of the NDI [16, 19]. Our findings are also similar to the original version of NDI (0.90) [3]. Furthermore, the NDI-PL test-retest reliability values were found to be slightly higher than those achieved during the initial trials for the original version when study participants completed the questionnaires with an interval of 2 days between the first and second test as occurred in our study (0.80) [4].

For the NDI-PL and CDS-PL the expected good concurrent validity was observed. As in the Spanish version of the NDI, a good correlation with the VAS score is evident, which may signify, as highlighted by Andrade et al [13], that the NDI is designed to assess not pain levels as such but rather disability due to pain experienced. The results of our study showed that the correlation between NDI-PL and CDS-PL is very high, showing a clear association between these two measures, which seem to measure similar constructs.

What is more, our examinations proved that the degree of spinal cord compression and spinal cord ischemic changes, expressed as changes of MRI signal intensity, correlate with the disability scales NDI-PL and CDS-PL. Intervertebral cervical disc herniation and degenerative changes are associated with narrowing of the sagital diameter of the spinal canal. Progressive compression may lead to spinal cord ischemia, leading to histopathological changes of the spinal cord. Nonetheless, these changes may or may not be symptomatic [28, 29].

Our study presents an analysis of the psychometric properties of two region-specific functional disability scales for patients with degenerative changes in the cervical spine. As far as we know, this article describes the first attempt at translation and validation of questionnaires appropriate for Polish-speaking patients with neck pain. We have indicated, that the present versions of the NDI-PL and CDS-PL, the first to be published in Polish, have proven to be reliable and valid.

The NDI-PL and CDS-PL have excellent internal consistency and test-retest reliability, and good concurrent validity. The adapted questionnaires showed a strong inter-correlation both in the test and in the retest. No ceiling or floor effects were detected in the NDI-PL and CDS-PL. The NDI-PL and CDS-PL are comparable with other versions and can be recommended and used in international comparative studies.