Nerve conduction test results in prospective epidemiological studies of carpal tunnel syndrome: Are they a cause or are they an effect?

Introduction
Criteria for defining carpal tunnel syndrome (CTS) in epidemiological studies have commonly used a combination of symptoms, median nerve conduction studies (NCS), and/or physical examination findings. Modeling of CTS risk factors depends, in part, on how it is defined and the investigators’ understanding of its pathogenesis. The authors of a widely cited CTS consensus document noted previously that 1) “…an abnormal electrodiagnostic study in an asymptomatic worker is not predictive of future risk of carpal tunnel syndrome…” and 2) use of NCS alone to define CTS was “not recommended” (Rempel 1998). The present paper explores these recommendations in light of more recent published findings.

Methods
We review studies that used NCS to predict risk of CTS and discuss how they may impact case definitions of CTS and decisions for constructing models of CTS risk factors.

Results
The first statement above was an accurate reflection of scientific knowledge when published (Werner 1997). However, recent studies (Werner 2001; Nathan 1998; Franzblau 2004) have demonstrated that CTS occurs more frequently among persons who were previously asymptomatic and had abnormal NCS. Because baseline NCS results are associated with future CTS risk, investigators may question whether or how to use baseline NCS when modeling CTS outcomes collected prospectively. In addition, at least one prospective study has found median NCS to be adversely affected by workplace exposures (Kearns 2000), suggesting that NCS alone may be a useful surrogate for CTS in occupational epidemiology.

Discussion
The consensus recommendations of Rempel et al (1998) remain largely relevant today, but the role of NCS in defining CTS as an outcome, and the use of baseline NCS as a covariate in modeling of prospective studies of CTS, is likely more complex than previously believed.