Chapter 38: Sonography of the Lumbar Paravertebral Space and Considerations for Ultrasound-Guided Lumbar Plexus Block

INTRODUCTION

Traditionally, lumbar plexus block (LPB) is performed using surface anatomical landmarks to identify the site for needle insertion and eliciting quadriceps muscle contraction in response to nerve electrolocalization, as described in the nerve stimulator-guided chapter. The main challenges in accomplishing LPB relate to the depth at which the lumbar plexus is located and the size of the plexus, which requires a large volume of local anesthetic for success.¹ Due to the deep anatomical location of the lumbar plexus, small errors in landmark estimation or angle miscalculations during needle advancement can result in needle placement away from the plexus or at unwanted locations. Therefore, monitoring the needle path and final needle tip placement should increase the precision of the needle placement and the delivery of local anesthetic. Although computed tomography and fluoroscopy can be used to increase precision during LPB, these technologies are impractical in the busy operating room environment, costly, and associated with radiation exposure. It is only logical, then, that ultrasound (US)-guided LPB be of interest because of the ever-increasing availability of portable machines and the improvement in the quality of the images obtained.^2,3

ANATOMY AND GENERAL CONSIDERATIONS

LPB, also known as psoas compartment block, comprises an injection of local anesthetic in the fascial plane within the posterior aspect of the psoas major muscle, usually at the L3–4 level (occasionally at the L2–3 or L4–5 levels). Because the roots of the lumbar plexus are located in this plane, an injection of a sufficient volume of local anesthetic in the posteromedial compartment of the psoas muscle results in a block of the majority of the plexus (the femoral nerve, lateral femoral cutaneous nerve, and obturator nerve).¹ The anterior boundary of the fascial plane that contains the lumbar plexus is formed by the fascia between the anterior two-thirds of the compartment of the psoas muscle that originates from the anterolateral aspect of the vertebral body and the posterior one-third of the muscle that originates from the anterior aspect of the transverse processes. The lateral and dorsal borders of the psoas major muscle consist of the quadratus lumborum muscle and the erector spinae muscle, respectively. Considering the rich vascularity of the lumbar paravertebral area, such as the dorsal branch of the lumbar artery, the use of smaller-gauge needles and the avoidance of this block in patients on anticoagulants are prudent.^4,5 LPB in patients with obesity can be challenging.

TRANSVERSE IN-PLANE TECHNIQUE

Regardless of technique, the patient is placed in the lateral decubitus position with the side to be blocked uppermost. The operator should identify the transverse processes on a longitudinal sonogram (Figure 38–1a, b, c). One technique involves identifying the flat surface of the sacrum and then scanning proximally until the intervertebral space between L5 and S1 is recognized as an interruption of the sacral line continuity ...