This is the first comprehensive systematic and critical review on NP prevalence and we therefore provide reference data for future studies on NP. Although considerable heterogeneity in prevalence estimates was found, two trends are evident: first, the average NP prevalence estimates increase with longer prevalence periods; second, in nearly all studies women reported more NP than men.
Differences in prevalence estimates could be a result of several factors. First, wording of the questions and use of different manikins may affect the results [37
]. In the majority of studies self-developed questionnaires were used and this may explain some of the observed variation in the prevalence estimates. Second, the anatomical definition varies between studies (i.e. including or excluding the shoulder region). Interestingly, no general differences between studies including or excluding the shoulder region were seen, and it has been questioned whether neck and neck/shoulder pain can actually be clearly distinguished from each other [53
]. Finally, methodological quality of a study may affect the outcome. Surprisingly, this was not the case in our review. In fact, two studies with very different quality scores showed roughly the same prevalence estimates [1
], indicating that estimates of NP prevalence could be regarded as independent of the quality of individual studies.
This lack of correlation between study–quality and outcome estimates may, however, be true for our scoring system only, but, since no evaluation of the appropriateness of quality criteria for epidemiological studies on the prevalence of musculoskeletal disorders such as NP exists, this remains unknown. Lebeouf-Yde and Lauritsen [39
] developed a comprehensive set of quality criteria for studies assessing low-back pain prevalence, whereas Loney and Stratford [45
] developed a less detailed scoring system for this purpose. However, no consensus exists as to what quality scoring system should be used when evaluating prevalence studies. It is our opinion that scoring systems should be kept as simple as possible and be easy to use. Our quality criteria cover basic issues that should be considered in any epidemiological study. Nevertheless, more work is needed in this area.
Previous attempts at reviewing the literature on NP prevalence also showed wide prevalence ranges [3
]. However, these reviews included very few papers. We included 56 papers due to a more comprehensive search strategy and different inclusion/exclusion criteria. More than one database should be included in a comprehensive search and one paper was not retrieved in any database. Therefore, screening of reference lists must be performed for complete retrieval of all relevant literature.
The results of this review should be interpreted in the light of several limitations. Our search strategy was broad and resulted in many potentially relevant papers. However, since most of the included studies had other primary objectives than to study NP prevalence, the broad search strategy was necessary in order to retrieve all relevant studies. We did not conduct a search of journals by hand, as prevalence studies are reported in many different journals. For example, the 56 included papers in our review were retrieved from 29 different journals.
Although no prior training with regards to applying the quality assessment was conducted, no substantial disagreements between the raters were seen for any of the papers. Any initial divergence was based merely on interpretations of the papers, and in fact the third reviewer was never involved in the quality assessment.
The mean prevalence estimates should be regarded with caution, as our review showed a great heterogeneity among the included studies. For example, the pain definition and duration of pain varied, typically by including the shoulder region or extending the duration of pain (i.e. pain lasting more than 3 months). Also, upper and lower age ranges were not identical between studies. For instance, some studies included 15-year-olds in the adult group, whereas others defined adulthood from 20 years of age and onward.
Reviewer bias cannot be ruled out, as we did not attempt to blind reviewers. This was because some studies were already known to the reviewers, hence making a fully blinded assessment was impossible.
We found that women consistently report more NP than men and this is in agreement with other reviews dealing with NP [3
]. In fact, women appear to report more musculoskeletal pain than men, and it has been suggested that this is based on different physiological mechanism for pain perception between the sexes [40
]. No other review has included studies specifically on children or elderly populations.
Homogeneity is a key issue when comparing results from different studies. Some of the requirements in studies on prevalence are: sufficiently large and unbiased study samples with non-responder analysis, uniform and valid anatomical definitions, and precise outcome measures. Our systematic review clearly indicates a lack of homogeneity in prevalence studies on NP. This is also apparent in other smaller reviews on NP prevalence where the variation in the definitions and the durations of NP [3
], as well as the sample source and age distribution have been emphasised as explanations for the large variation in the NP prevalence estimates [3
Knowledge about prevalence does not in itself inform about the impact of NP on individuals and on society at large. To assess the impact on individuals, information such as pain intensity, influence on daily performance, general health status, care seeking, and co-morbidities should be available. To assess the impact of NP on society at large, both direct costs (e.g. use of medication and/or health care providers) and indirect costs (e.g. number of sick days, decreased daily performance) should be provided. Such clinically and sociologically relevant information is difficult to obtain, and this may in part explain the diversity in questionnaires used.
Time has come to reach a consensus on these issues and develop new standardised instruments that entail more relevant outcome measures based on our present knowledge. This would facilitate comparisons between different countries and cultures, and, most importantly, provide clinicians, researchers, and politicians with a relevant and detailed picture of both occurrence and impact of NP in the population.