Different case definitions for carpal tunnel syndrome have been used in epidemiological studies, based on different combinations of symptoms, physical examination, and nerve conduction studies. When we tested different case definitions in the same study population, we found widely varying estimates of prevalence, yet a relatively high degree of quantitative concordance between case definitions, with relatively low rates of misclassification.
Not surprisingly definitions based on non-specific hand symptoms only led to the highest prevalence of disease, while more restrictive definitions requiring specific hand symptoms plus median nerve conduction abnormalities resulted in the lowest prevalence. Results of population surveillance studies are clearly sensitive to the case definition (2
). The proportion of misclassification between more restrictive and less restrictive case definition of CTS was relatively low.
The selection of the case definition papers was based on a literature search. We decided to include only those papers suggesting a case definition for use by other investigators, rather than testing the much larger group of different case definitions used in epidemiological studies (3
). We did not study definitions based on insurance or medical treatment claims of carpal tunnel syndrome nor self-reports of treatment or diagnosis, though such case definitions are useful for some surveillance and epidemiological studies (28
). We believe that our choice of case definitions is representative of the spectrum of definitions that have been used.
One limitation of our study approach could be in the mapping of our study data to the case definitions described in the literature. When authors described a symptom or a sign not recorded in the study, we selected the closest item in our study. Only a few items were different: motor loss in physical examination, choice of sensory examination, and the time period for symptoms in the Sluiter et al. case definition. The Sluiter time period, that incorporates a timeframe, frequency and duration, were included in their document in order to differentiate common aches and pain from work related musculoskeletal disorders, and did not serve to define the type of the disease (24
). As noted earlier, the difference in time frames between Sluiter and our study probably made only minor differences in the prevalence of CTS. Motor loss in our study was evaluated by inspection (thenar atrophy). The study was based on screening a large population for clinically unreported CTS and no atrophy was found, which is not surprising. In a study of active workers, the prevalence of motor loss corresponding to severe CTS is expected to be low, even if assessed by physical examination of motor strength. Different results might be seen in a clinical population with a higher prevalence and greater severity of CTS (32
Another potential limitation of our study was our definition of abnormal median nerve conduction. In both clinical settings and population studies, the determination of normative values for nerve conduction is complicated, with different possible cut-offs depending on the studies’ purposes (11
). None of the case definitions selected from the literature defined cut-points for nerve conduction. We chose nerve conduction cut-offs that have been proposed for use in a large multi-center study of CTS in working populations, and applied these same criteria to all case definitions. More stringent criteria for abnormality may have resulted in slightly different study results, with fewer subjects rated as abnormal (33
Our study compared concordance between different case definitions, but did not propose a “best” case definition for CTS - even in clinical settings, there is no gold standard for establishing a diagnosis of CTS (35
). Different authors have described different methods to study the clinical diagnosis of CTS, with different results (35
). We could conclude some definitions are more conservative than others. Their use depends on the purpose of the study and their feasibility (43
). We found a fair degree of agreement between different case definitions in a general working population. These results suggest that comparison of risk factors for CTS across studies may not be greatly biased by misclassification errors due to differences in case definitions of CTS, though it is important to note that the prevalence of disease is likely to be different when using different case definitions.