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Correlating median nerve cross sectional area with distal sensory and distal motor nerve latencies
Benson Pulikkottil, MD; Micah Schub, BS; Tiffany Kadow, MD; William Wang, BS; John R. Fowler, MD
University of Pittsburgh Medical Center, Pittsburgh, PA

CTS is the most common nerve compression syndrome. Currently, aside from clinical exam, the main diagnostic aid is EMG/NCS. However, they are costly, uncomfortable, and time-consuming. Ultrasound measurement of median nerve cross sectional area has been shown to confirm the diagnosis of carpal tunnel syndrome with better specificity and equal sensitivity as compared with electrodiagnostic testing

Hypothesis: There is a direct measurable correlation between ultrasonographic median nerve cross sectional area (CSA) and both distal motor latency and distal sensory latency in patients referred for nerve conduction studies (NCS) due to carpal tunnel syndrome (CTS) symptoms.

Methods: A prospective study was carried out on 91 wrists/52 patients referred by an orthopedic surgeon for nerve conduction studies due to clinical manifestations of CTS. Demographic information including age, gender, race, height and weight was recorded. Ultrasound examinations were performed using a 15-6 MhZ linear array transducer. Median nerve CSA was measured at the carpal tunnel inlet 3 times and the results averaged with greater than 10mm as the cutoff for CTS. Accuracy of CSA measurements is to 0.1mm. All NCS were performed according to the guidelines of the American Association of Electrodiagnostic Medicine and no treatment was administered between the ultrasound test and the NCS.Median nerve CSA was compared to both distal sensory and distal motor latency using Pearson correlations. Pearson correlations were run again controlling for BMI. Sensitivity and specificity were calculated for CSA compared to both latencies

Results: Of the 91 wrists included in the study, all were used in the analysis of distal motor latency, while 4 were excluded in the analysis of distal sensory latency due to missing data. Correlation was significant between CSA and distal motor latency (R=0.493, p<0.005), as well as between CSA and distal sensory latency (R=0.352, p=0.001). When controlling for BMI, the correlations were also significant for CSA and distal and sensory motor latencies. CSA compared to distal motor latency had a sensitivity of 87% and a specificity of 50%. CSA compared to distal sensory latency had a sensitivity 92% and a specificity of 49%.

Conclusions: The significant correlation and high sensitivity between CSA and nerve conduction studies indicate that diagnosis of carpal tunnel syndrome using ultrasonographic median nerve cross sectional area is a promising complementary and alternative diagnostic tool to the costly and uncomfortable nerve conduction studies; however the low specificity may preclude it from being a confirmatory test.


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