Distance measure error induced by displacement of the ulnar nerve when the elbow is flexed

Objectives: To determine the presence of ulnar nerve displacement at the elbow when it is flexed and to determine its effect on distance measurements using the conventional measurement method for nerve conduction studies (NCSs). Design: Comparing the ultrasonography-assisted distance measurement method with the conventional measurement method. Setting: An electrodiagnostic laboratory at a university hospital. Participants: Seventy-eight elbows of 39 healthy volunteers. Interventions: We used high-resolution ultrasonography in real time. Based on sonographic searching, we marked 3 points on the skin through the course of the displaced ulnar nerve when the elbow is flexed: (1) point A, 7cm above the elbow (from the midpoint between the medial epicondyle tip and olecranon in the postcondylar groove [point M]); (2) point B, 3cm below the elbow; and (3) point C, the point closest to the medial epicondyle tip. Main Outcome Measures: Distance measurements between points A, B, and C were taken. These values were compared with measurements obtained through conventional measurement methods. Results: Ulnar nerve displacement occurred in 24.3% (19/78) of the elbows; approximately 20.5% (16/78) were subluxation, and 3.8% (3/78) were dislocation. In the ulnar nerve displacement group, the distance between points A and C was 5.84±0.33cm (range, 5.10-6.30cm), and the distance between points B and C in the flexed position was 3.35±0.19cm (range, 3.10-3.70cm). When the conventional distance measurement was used, the ulnar nerve conduction velocity across the elbow was overestimated by approximately 5.33±2.29m/s in the ulnar nerve displacement group. Conclusions: This distance measurement error may be responsible for the decreased sensitivity found in NCSs that test for ulnar neuropathy at the elbow. If the NCS results are normal in patients who have clear symptoms of ulnar neuropathy, the possibility of ulnar nerve displacement at the elbow should be considered, and further investigation with ultrasonography would be beneficial.