Ultrasound-guided lumbar plexus block is utilized most frequently for surgical anesthesia of the lower extremity. This technique is occasionally utilized as a diagnostic maneuver when performing differential neural blockade on an anatomic basis in the evaluation of groin and lower-extremity pain. The technique may be utilized in a prognostic manner if destruction of the lumbar plexus is being contemplated to demonstrate the degree of motor and sensory impairment that the patient may experience. Ultrasound-guided blockade of the lumbar plexus is also useful as a therapeutic maneuver when treating inflammatory conditions involving the lumbar plexus including idiopathic, diabetic, and viral plexitis. The technique is also used to palliate acute pain emergencies, including groin and lower-extremity trauma or fracture, acute herpes zoster, and cancer pain including tumor invasion of the lumbar plexus while waiting for pharmacologic, surgical, and antiblastic therapies to become effective. Destruction of the lumbar plexus is indicated for the palliation of cancer pain, including invasive tumors of the lumbar plexus and the tissues that the plexus innervates. More selective techniques such as radiofrequency lesioning of specific lumbar paravertebral nerve roots may cause less morbidity than lumbar plexus neurolysis.

The lumbar plexus comprises fibers from the ventral roots of the first four lumbar nerves and, in most patients, a contribution from the 12th thoracic nerve (Fig. 111.1). The lumbar plexus lies within the posterior substance of the psoas muscle where it is amenable to ultrasound-guided neural blockade (Fig. 111.2). The nerves lie in front of the transverse processes of their respective vertebrae, and as they course inferolaterally, they divide into a number of peripheral nerves. The ilioinguinal and iliohypogastric nerves are branches of the L1 nerves, with an occasional contribution of fibers from T12. The genitofemoral nerve is made up of fibers from L1 and L2. The lateral femoral cutaneous nerve is derived from fibers of L2 and L3. The obturator nerve receives fibers from L2 to L4, and the femoral nerve is made up of fibers from L2 to L4. The lumbar plexus also provides interconnecting branches to the sacral plexus via the lumbosacral trunk (see Fig. 111.1).

Ultrasound-guided lumbar plexus block can be carried out by placing the patient in the lateral decubitus position (Fig. 111.3). A total of 20 to 25 mL of local anesthetic is drawn up in a sterile syringe. Given the large volume of local anesthetic required for this technique, the clinician must carefully calculate the total milligram dosage of local anesthetic to be injected to avoid local anesthetic toxicity. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. The midline of the lumbar spine is identified by palpation as is the iliac crest, and a line is drawn from each of these anatomic landmarks (see Fig. 111.3).

After preparation of the skin with antiseptic solution, a curvilinear low-frequency ultrasound transducer is placed in the transverse plane ˜3 cm laterally from the midline along the line drawn from the iliac crest, and an ultrasound survey is taken (Figs. 111.4 and 111.5). This should place the transducer at the L2-L3 level. Note that the transverse process blocks ultrasound visualization of the lumbar plexus (Fig. 111.6). Once the transverse process is identified, the ultrasound transducer is slowly moved in a cephalad direction to identify the acoustic window between two adjacent transverse processes (Fig. 111.7). Once the acoustic window between the adjacent transverse processes is identified, the lateral aspect of the ultrasound transducer is rocked anteriorly to identify the intervertebral foramen and the lateral margin of the vertebral body (Figs. 111.8 and 111.9). The lumbar plexus can then be seen within the psoas muscle, which is just lateral to the vertebral body and intervertebral foramen (see Figs. 111.9 and 111.10). A color Doppler image is then obtained to identify adjacent vasculature to avoid inadvertent intravascular injection (Fig. 111.11).