Lumbar Plexus Block

The lumbar plexus is formed from nerve roots of L1-L4 with variable involvement from T12. The lumbar plexus travels within the psoas muscle and supplies motor and sensory innervations to the lower abdomen and proximal lower extremity as indicated in Table 20.10. Lumbar plexus block depends on placement of local anesthetic in the facial plane of the psoas muscle where the roots of the plexus are located. There are four major branches composing the lumbar plexus: genitofemoral nerve, lateral femoral cutaneous nerve, femoral nerve, and obturator nerve.

Table 20.10 Lumbar plexus innervation to the lower extremity.

Motor

Sensory

Quadriceps muscle

Skin of anteromedial thigh and medial aspect of the leg below the knee to the foot

Adductor muscles of the hip

Variable cutaneous distribution to the medial aspect of thigh and knee

(obturator distribution)

Articular branches to hip and knee

Cutaneous distribution from the lateral femoral cutaneous and genitofemoral

Indications

As the primary anesthetic and/or postoperative pain management with or without a continuous catheter.

Clinical Uses Surgical

• Acetabular fracture

• Anesthesia for the entire lumbar plexus distribution including the saphenous nerve below the knee, hip, knee, and anterolateral/medial thigh

Supplemental (in Combination with Sciatic Nerve Block)

• Total hip replacement

• Total knee replacement

• Anterior cruciate ligament repair

• Amputation of any component of the lower extremity

Postoperative Analgesia

For above-mentioned surgeries.

Technique Preparation

Arrange sterile towels, sterile gloves, gauze pads, marking pen, antiseptic solution, peripheral nerve stimulator, syringes, and needles for local infiltration and nerve block placement.

Dose

25-35 ml syringes of local anesthetic. Needles

25 g 1.5 in. needle for skin infiltration and 10 cm short bevel insulated stimulation needle.

Agents

3% chloroprocaine, 2% lidocaine, 0.5% ropivacaine, 0.5% bupivacaine. Surface Anatomy and Landmarks

Landmarks include iliac crests, midline of spinous processes of L2-L5, posterior superior iliac spine (Fig. 20.9). The patient is placed in a lateral decubitus position with a slight pelvic forward tilt, and the side to be blocked is nondependent. The foot of the extremity to be blocked should be positioned over the dependent leg so twitching of the patella and/or quadriceps muscle can easily be seen. The lumbar midline is identified starting at L4 spinous process and then draw a horizontal line connecting the tips of the palpable spinous processes from L1 to L5. A parallel line is then drawn 4-5 cm lateral to the midline on the non-dependent side (parasagittal line). Palpate the iliac crest (usually originating from the posterosuperior iliac spine) and draw a vertical line from the level of the highest point on the iliac crest down to the midline (bicrestial line). The intersection of the bicrestial line with the parasagittal line determines the site of insertion of the stimulating needle (approximately at the level of L4-L5). The skin is anesthetized by infiltrating local anesthetic subcutaneously after cleaning with an antiseptic solution. After connecting the 10 cm short bevel insulated needle to a nerve stimulator set at 1.5 mA, the needle is inserted through the skin wheal perpendicular to the skin and advanced slowly in search of visible quadriceps muscle twitch or contact of the transverse process. If the needle comes in contact with the transverse process (indicates proper needle orientation), it is withdrawn to the skin then redirected 5-10° caudally or cranially and again advanced until consistent twitching of the quadriceps muscle is obtained (6-8 cm depth) while decreasing the current of the nerve stimulator to 0.5-1.0 mA. After confirming

Figure 20.9 Anatomical landmarks for approach to the lumbar plexus block. PSIS posterior superior iliac spine. Vertical line #1 (bicrestal line) connects the right and left iliac crests, line #2 indicates the midline and the circles on line # 2 represent the palpable spinous processes, line #3 (parasagittal line) is parallel to the line #2 and 4-5 cm lateral to line #2. Line #3 should bisect line #1 at 90° and connect to the PSIS while remaining parallel to line #2.

Figure 20.9 Anatomical landmarks for approach to the lumbar plexus block. PSIS posterior superior iliac spine. Vertical line #1 (bicrestal line) connects the right and left iliac crests, line #2 indicates the midline and the circles on line # 2 represent the palpable spinous processes, line #3 (parasagittal line) is parallel to the line #2 and 4-5 cm lateral to line #2. Line #3 should bisect line #1 at 90° and connect to the PSIS while remaining parallel to line #2.

negative aspiration for blood, local anesthetic is injected slowly with repeated aspiration for blood after every 3-5 ml of injected solution.

Pitfalls and Pearls Pitfalls

• Position patient so as to avoid spinal rotation

• Patient discomfort associated with twitching of large muscle groups

• Increased risk of local anesthetic toxicity

• Hemodynamic consequences (unilateral sympathectomy)

• Quadriceps muscle stimulation at currents less than 0.5 mA may indicate needle tip placement within a dural sleeve permitting spread of local anesthetic solution toward the epidural or subarachnoid space potentially resulting in a high spinal or epidural blockade

• Takes longer for effectiveness to take effect

Pearls

• If twitching of paravertebral/paraspinal muscles is observed, then the needle needs to be introduced deeper

• Twitching of hamstrings means that the needle is introduced to caudally

• Flexion of thigh (stimulation of the psoas muscle) indicates that the needle is inserted too deep

• Needle contacts bone (transverse process at 4-6 cm depth), but NO twitches are seen; redirect the needle 5° caudally or cranially after withdrawing back to the skin

• Needle placed deep (>10 cm), but without proper quadriceps stimulation; then withdraw needle to skin and confirm protocol defined in surface anatomy and landmarks

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