Ultrasound-Guided Electrodes for Conduction Studies of the Saphenous Nerve

Purpose: Saphenous nerve conduction studies are difficult, because the nerve is hard to localize and evoked responses are small. Ultrasound imaging may assist in the accurate localization and optimal positioning of surface (SE) and needle electrodes (NE). Methods: The study population included 39 subjects and was divided into two groups. Group A consisted of 20 healthy subjects, whereas group B of 19 patients with polyneuropathies. Orthodromic conduction was measured by distal supramaximal nerve stimulation. Surface electrode and NE recordings were compared. Results: In the control group, SEs recorded responses in 17 of 20 healthy subjects, whereas NEs in 19. In the patients' group, SEs recorded responses in 7 of 19 patients, whereas NEs in 16. In all healthy subjects and patients, sensory nerve action potentials recorded by NEs were significantly larger than those obtained by SEs (healthy subjects: 5.85 +/- 3.01 mu V vs. 1.98 +/- 1.37 mV, P < 0.0001; patients 3.05 +/- 2.35 mV vs. 0.71 +/- 1.14 mV, t-test P < 0.0001). Conclusions: Ultrasound guidance allows precise electrode positioning for saphenous nerve electrophysiological testing. Amplitudes of the recorded sensory nerve action potentials are clearly higher with ultrasound-guided needle than with surface recordings.