To understand the potential use of ultrasound imaging for lumbar plexus block, detailed anatomy and background must be reviewed.
The lumbar plexus consists of contributions from the anterior rami of the L1 through L4 roots (and sometimes T12 or L5). The lumbar plexus forms the subcostal, iliohypogastric, ilioinguinal, lateral femoral cutaneous, genitofemoral, femoral, and obturator nerves. Anatomic dissections have revealed that the lumbar plexus lies within the psoas muscle, with the lateral femoral cutaneous nerve and femoral nerve in the same fascial plane. However, the obturator nerve can be found within a distinct muscular fold in about half of anatomic specimens. The lumbosacral trunk is formed from the anterior rami of L4 and L5. It contributes to the sacral plexus and enters the pelvis apart from the lumbar plexus.
Patient Position: Lateral decubitus with the operative side up or in the sitting position.
Indications: Surgery on the hip, thigh, or knee.
Needle Size: 9-cm, 22-gauge needle.
Surface Landmarks: The posterior spinous process of the fourth lumbar vertebra.
Ultrasound Landmarks: The transverse processes of the third and fourth lumbar vertebrae and the psoas muscle.
Transducer Type: Linear or curved array, 3 to 7 MHz
Transducer Position: Sagittal plane, 4 to 5 cm lateral to the posterior spinous process of the fourth lumbar vertebra (Fig. 37-1).
Figure 37-1. External photograph illustrating the position of the ultrasound transducer for imaging of the lumbar transverse processes and the puncture location.
Volume: 20 to 30 mL.
Technique: Controversy surrounds the use of ultrasound imaging to guide lumbar plexus block. Some authors have argued that because current ultrasound imaging does not provide direct lumbar plexus imaging in adults, alternative methods for block guidance such as loss-of-resistance or nerve stimulation are necessary. Other authors have used ultrasound imaging for lumbar plexus blocks in pediatric patients where more favorable imaging is possible. In our own clinical practice, we have reserved the use of ultrasound for offline imaging of surrogate landmarks of the lumbar plexus in adult patients. The usual level of the block procedure is the intertransverse space between L4 and L5. The best method of localization has been achieved by counting the transverse process echoes from the sacrum upward and marking their location with indelible ink (Fig. 37-2). The lumbar plexus is approximately 2 cm deep to the transverse process of L4 off its caudal edge (median value 18 mm for adults of either gender). The lumbar nerve roots that contribute to the lumbar plexus lie 5 to 6 mm deep to the intertransverse ligament. With the offline technique (scanning prior to needle insertion), nerve stimulation is often used to confirm correct location of the block needle tip prior to injection. Scanning prior to offline needle insertion may be especially useful in obese patients or those with spinal deformities.
Figure 37-2. Longitudinal sonogram of the intertransverse spaces from L3 to L5 with a 3 MHz transducer. Between the bright reflections of the posterior surface of the transverse process the psoas muscle and underlying bowel can be seen.
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Kirchmair L, Enna B, Mitterschiffthaler G, et al. Lumbar plexus in children. A sonographic study and its relevance to pediatric regional anesthesia. Anesthesiology 2004;101:445–450.
Kirchmair L, Entner T, Wissel J, et al. A study of the paravertebral anatomy for ultrasound-guided posterior lumbar plexus block. Anesth Analg 2001;93:477–481.