Ultrasound-Guided Anterior Sciatic Nerve Block Using a Longitudinal Approach

Written By: Sam Savage

Published Date: May 15, 2008 Last Edited: May 15, 2008

By Tsui, Ban C H Ozelsel, Timur J-P

To the Editor: Ultrasound-guided anterior sciatic nerve block has been described using a medially positioned curved array probe to scan the nerve transversely at the anterior proximal thigh (Fig I).1 The ultrasound image of sciatic nerve in cross section is typically seen as an oval-to-circular hyperechoic structure deep to the adductor muscles (Fig 2).1,2,3 It is often vaguely delineated or appears isoechoic to the surrounding muscles (anisotropic), the latter particularly if using a tangential ultrasound beam plane.2 In our experience, identification of the sciatic nerve with the sole use of transverse scanning is usually difficult if not impossible in most patients, particularly in those who are obese and when using a portable ultrasound system (MicroMaxx, SonoSite Inc., Bothell, WA). This may be due to the fact that the sciatic nerve is deep to relatively thick muscles and is potentially hidden behind the acoustic shadow of the femur, depending on the rotation of the leg. We felt that the imaging technique for this block needed improvement to increase the reliability of nerve localization.

Fig 1. Schematic diagram of the probe positioning and movements during scanning to identify the sciatic nerve. (A) The probe (rectangle) is placed to capture a transverse view of nerve. (B) The sagittally placed probe can be tilted medially or laterally to optimize the longitudinal view of the nerve.

The sciatic nerve is the largest nerve of the body; however, blocking this deep nerve remains one of the most challenging procedures for an ultrasound-guided approach. The nerve exits through the sciatic notch and passes anteriorly to the piriformis muscle to then lie between the ischial tuberosity and the greater trochanter of the femur. It curves caudally and descends the posterior thigh adjacent and almost parallel to the femur (Fig 1). From the anterior aspect, the sciatic nerve continues its longitudinal course deep to the adductor magnus muscle, superficial to the biceps femoris muscle, and immediately adjacent to the lesser trochanter of the femur. Based on the distinct anatomical course of the sciatic nerve from the anterior perspective, we have discovered that the nerve can be more consistently visualized along its longitudinal axis when compared with its transverse axis. In contrast to the small oval shadow in transverse view, the nerve appears as a long cable-like structure with a characteristic fascicular pattern in longitudinal view. This maximizes the amount of nerve exposed under the ultrasound beam. In this view, we find it easier to identify the sciatic nerve not only as a more hyperechoic, linear structure deep to the clearly delineated adductor magnus muscle, but also without the influence of the position of the lesser trochanter (Fig 2).

To perform a longitudinal ultrasound-guided approach for an anterior sciatic nerve block, one needs to use a low-frequency curved ultrasound transducer (e.g., C-60, MicroMaxx, Sonosite, Bothell, WA). In general, a longitudinal view of the nerve can be immediately obtained by directly placing the probe sagittally/ longitudinally over the medial side of the upper midthigh at a level similar to the typical anterior block needle insertion site, in the gap between the sartorius and rectus femoris muscles.4 The image can be further optimized by tilting the scanner medially and laterally (Fig 1). The nerve is typically located medial to the femur bone and approximately 6 to 10 cm deep to the skin. If uncertainty remains as to where the muscle gap is, rotating the probe to the transverse axis may be helpful. In this transverse view, one can simply place the hyperechoic femur shadow at the lateral edge of the screen and the transducer is then rotated back 90 degrees to obtain a longitudinal image of the target structure. With this maneuver, a clear view of the distinct cable-like fascicular structure crossing from one side to the other is often obtained with ease.

Fig 2. Images of the sciatic nerve (indicated by arrowheads) in the anterior thigh. (A) Cross-sectional and (B) longitudinal views of a thigh in a cadaver. The sciatic nerve (SN) is medial to the femur (F) and its consistent cable-like structure deep to the adductor magus (AM) muscle continues during the majority of its course in the thigh. Ultrasound images from a live adult using corresponding (C) transverse and (D) longitudinal views. The large cable-like structure of the sciatic nerve in the longitudinal view is more readily identifiable than the small oval shadow in the transverse view. (A) and (B) were generated with permission using Visible Human Visualization Software.3

Once accurately positioning the needle in plane (aligned) to the ultrasound beam’s longitudinal axis, one is able to view the entire needle during its trajectory (to improve the safety of the block) and then confirm the nerve’s identity using an optimal nerve stimulation threshold (0.4-0.5 mA). From experience, we have had an easier time following the course of the needle tip when directing the needle from cephalad to caudad. More often than not, a fascial “click” can be felt when the needle tip passes through the posterior fascia of the adductor magnus. After nerve localization, local anesthetic solution can be injected. In this longitudinal view, it is also very reassuring to be able to observe the spread of local anesthetic solution along the sciatic nerve. We have found this approach to be very practical and now have little trouble performing these blocks using in plane real time ultrasound guidance. As technology advances and we gain more experience with the application of ultrasound in regional anesthesia, there will be a continual search for “expanding the view” using different angles or approaches in visualizing the target nerve.

References

1. Chan VW, Nova H, Abbas S, McCartney CJL, Perlas A, Xu DQ. Ultrasound examination and localization of the sciatic nerve: A volunteer study. Anesthesiology 2006;104:309-314.

2. Chantzi C, Saranteas T, Zogogiannis J, Alevizou N, Dimitriou V. Ultrasound examination of the sciatic nerve at the anterior thigh in obese patients. Ada Anaesthesiol Scand 2007;51:132.

3. Visible Human Visualization Software, courtesy Ecole Polytechnique Federale de Lausanne, http://visiblehuman.epfl.ch.

4. Meier G, Buettner J. Proximal sciatic nerve block. In: Meier G, Buettner J, eds. Peripheral Regional Anesthesia: An Atlas of Anatomy and Techniques. New York: Thieme; 2006:126-158.

Accepted for publication November 28, 2007.

doi: 10.1016/j.rapm.2007.11.008

Ban C.H. Tsui, M.D., M.Sc., F.R.C.P.(C)

Timur J.-P. Ozelsel, M.D., D.E.S.A.

Department of Anesthesiology and Pain Medicine

University of Alberta Hospital

Edmonton, Alberta, Canada

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