Ultrasound Imaging of the Sural Nerve Improves Near-Nerve Recording

Introduction: The near-nerve technique is a particularly sensitive method for the detection of demyelination of peripheral nerves. However, the quality of the results critically depends on the position of the recording electrode. Therefore we used B-mode ultrasound to quantitatively study how the properties of sensory nerve action potentials vary with the distance between the needle electrode and the nerve. Subjects and Methods: We examined both legs of 24 healthy people with a standard ultrasound system equipped with a linear 13.5MHz array. At the beginning of each investigation the sural nerve was located lateral to the gastrocnemius tendon and approximately 15cm proximal to the ankle. In transversal image planes the nerve was distinguished from vessels by both its speckled appearance and the absence of blood flow. The neurography of the sural nerves was done with the near-nerve technique following standard methods. The distance between the recording needle electrode and the outer diameter of the nerve, as measured by ultrasound, was intentionally varied between 0.1 and 15mm, and compound action potentials were recorded. Results: Sural nerves could be identified by ultrasound in all subjects. With 2 exceptions, the nerve was located lateral to the posterior tibial vein in a depth of 4 to 10.4mm. Its circumference varied between 5.3µm and 12.2µm. Neither the latency nor the nerve conduction velocity changed significantly with distance to the nerve. The amplitude varied between 4µV and 70µV. This variation was largely due to the variation of the distance between the electrode and the nerve, and followed a non-linear inverse relationship (p<0.001). Comments: With ultrasound it is feasible to locate the sural nerve and to position a needle electrode precisely. This should improve both the feasibility and the reliability of the results of near-nerve recordings. Especially, it is possible to control the variation in amplitude that is caused by a variation of the needle-to-nerve distance.