In this study the accuracy of neuromuscular ultrasound as a screening tool for the diagnosis of CTS in a large population was assessed prospectively, and it was compared with nerve conduction studies, a more traditional diagnostic test for CTS. Neuromuscular ultrasound measurement of a single parameter (median nerve cross-sectional area at the wrist) demonstrated similar sensitivity and specificity to nerve conduction studies in which the median sensory nerve conduction velocity was compared with that of the ulnar nerve. This indicates that neuromuscular ultrasound, which is a painless, quick, inexpensive, and radiation-free imaging modality, can be considered for screening large populations at risk for CTS in a manner similar to what has been done previously with nerve conduction studies.9,19
Interestingly, neither ultrasound nor nerve conduction studies were particularly accurate in this study, which may have been due to several factors. First, and most importantly, the reference standard in this study was the Katz hand diagram. Although the hand diagram has a high negative predictive value, it is only moderately accurate itself for the diagnosis of CTS when used in large epidemiological studies, with a sensitivity of 64% and specificity of 73%.20
The moderate accuracy of this reference standard likely resulted in lowered accuracy of the nerve conduction studies and neuromuscular ultrasound, which was a limitation acknowledged during conceptualization of this study. Although it would be ideal for each participant to have a detailed history and examination for CTS, and to use this conclusion as the reference standard, it was not feasible from a time and financial perspective. Participants already underwent a 1-hour-long interview, conducted by a non-clinician researcher, that focused on many aspects of their health and occupation, so further detailed assessment for CTS was not feasible. Using a different reference standard for diagnosis, such as one involving nerve conduction studies, would not have allowed for direct comparison of the accuracy of nerve conduction studies and ultrasound.
Other potential causes of decreased diagnostic accuracy of both modalities included limitations of the testing environment, such as electrical noise and a bright room. The ideal of using the most advanced electrodiagnostic and ultrasonographic equipment in electrically shielded dark rooms, with time to employ the most sophisticated comparative techniques (such as mixed-palmar or multidigit studies for electrodiagnosis and wrist-to-forearm ratio for ultrasound21
) were likewise not feasible. Fortunately, the limitations, were systematic and unlikely to have affected 1 modality more than the other. In addition, both nerve conduction studies and neuromuscular ultrasound have demonstrated much higher accuracy when used in populations in which the diagnosis of CTS was established through traditional history and physical examination.22
This suggests that both modalities likely would have higher accuracy as screening tests in this population if the reference standard was more accurate.
The sensitivity of neuromuscular ultrasound for the diagnosis of CTS greatly improved when other parameters were included in addition to nerve cross-sectional area. In those with at least 1 abnormality in median nerve cross-sectional area, echogenicity, mobility, or vascularity, the sensitivity of ultrasound increased to 89%. Another way to maximize sensitivity was to decrease the cut-off of the cross-sectional area to <8 mm2
, which raised the sensitivity to 96%. Conversely, a cross-sectional area cut-off of >18 mm2
increased the specificity to 97%. Therefore, future screening studies could accurately include or exclude those with CTS based on the goals of the study using solely a single, quickly obtained parameter (median nerve cross-sectional area). In fact, a recently published study addressed just this issue and found that a median nerve cross-sectional area at the wrist >9 mm2
resulted in a sensitivity of 99% for the diagnosis of CTS, so the investigators suggested a potential change in the typical testing paradigm for CTS may be warranted, with ultrasound used as the initial screening modality.23
The strengths of this study include the prospective data collection, large number of participants, blinding of all examiners, inclusion of a broad spectrum of participants at risk for CTS, use of an appropriate and pre-specified gold standard, and generation of measures of diagnostic accuracy (sensitivity and specificity). As stated previously, the main limitation is that the Katz hand diagram is only moderately accurate for the diagnosis of CTS, but it is an appropriate reference standard. Other modest limitations are that the study involved a homogeneous population (Latino manual laborers); older nerve conduction study equipment was used; and ultrasonographic measures of mobility, echogenicity, and vascularity lack the quantitative rigor of the cross-sectional area measurement. Despite these limitations, the strengths of the study design would qualify this as a Class I study based on the American Academy of Neurology criteria for rating an article for diagnostic accuracy.24