NC-stat as a Screening Tool for Carpal Tunnel Syndrome

Abstract

To the Editor: We read with interest the article by Dr Katz1 on use of the NC-stat instrument in an industrial population and commend the author for investigating the complex issue of nerve conduction studies in industrial populations.2 We agree with the author in the conclusion that the NC-stat median nerve distal motor latency (DML) is comparable to the same measurement performed in traditional electromyography settings and that nerve conduction parameters in some industrial populations may be shifted relative to other populations.2 However, we think that some of the statements as to the implications of this particular data set are overstated. Although Dr Katz appears to be under the impression that the NC-stat normal values and methods are proprietary and have not been published, that is not the case. The methods and correction factors for the median DML and F-wave latency have been described.3 Furthermore, we have shared our normative data with many physicians and organizations. We would be pleased to provide this data to Dr Katz. It is clear from the presented data (Fig. 1) that a significant number of job applicants had an abnormal NC-stat DML, as would be expected in applicants to heavy industry jobs. This data and similar results by others2 clearly validate the need to carefully and quantitatively interpret abnormal DMLs in this population.4 However, we think that the accompanying analysis overstates the effect. First, the mean (3.81 msec) and standard deviation (0.57 msec) are both quite high relative to both the NC-stat and other published reference ranges.5 Although Dr Katz takes the position that the DMLs are normally distributed, the peak of the distribution appears to be closer to 3.6 msec, which suggests a substantial effect of the positive tail. However, most importantly, if this population is to be used to “judge” the specificity of the NC-stat and comparable5 reference ranges, the subject population must be known to be free of disease. As pointed out by Dorfman and Robinson,6 asymptomatic and disease-free are not equivalent. It is not clear that this study population withstands such scrutiny. Applicants could have falsely denied symptoms and could have had asymptomatic median neuropathy at the wrist due to prior occupational exposure. In addition, it is not clear that applicants with overt or subclinical polyneuropathies were excluded. Because applicants fitting into any one of these groups would likely have prolonged median DMLs, we are not convinced that the 95th percentile value of 4.75 msec is a valid operating point for assessing specificity in this population. In both the Abstract and Conclusion sections, Dr Katz comments on the “sensitivity” of the NC-stat. However, the data presented in this study do not support an evaluation of sensitivity. As defined in the Methods section, all subjects were deemed to be asymptomatic and therefore not to have carpal tunnel syndrome. In the absence of a positive case definition, sensitivity cannot be determined. By contrast, in two prior studies,3,7 the NC-stat demonstrated high DML sensitivity relative to an electromyography laboratory case definition. We agree with Dr Katz that sensory nerve conduction, which has now been validated for the NC-stat,8 and ulnar nerve measurements are important in the clinical assessment of patients with upper extremity symptoms using the NC-stat. The author concludes by stating that “NeuroMetrix significantly overdiagnoses CTS in an asymptomatic population of industrial workers.” This statement is factually incorrect and requires clarification. NeuroMetrix, Inc. is a medical device company that manufactures equipment. The company does not examine, diagnose, or treat patients or workers in any way. Physicians purchase our equipment and use it as an extension of their history and clinical examination. The NC-stat is an electrodiagnostic instrument with a high level of standardization and automation.9 Like with any nerve conduction study, the data must be interpreted in the context of the subject’s presentation and study purpose. We trust that Dr Katz is aware of the manner in which the NC-stat equipment is used and the implication that NeuroMetrix or the NC-stat “makes diagnoses” was unintentional. J. Thomas Megerian, MD, PhD Xuan Kong, PhD Eugene Lesser, DO Shai N. Gozani, MD, PhD NeuroMetrix, Inc. Waltham, Massachusetts