Fig. 24.1
Surface anatomy and landmarks for posterior lumbar plexus block
The patient lies in the lateral decubitus position with the operative side up and both hips and knees flexed after general anesthesia has been induced. Surface landmarks include:
Horizontal line:
Intercristal line – a line connecting the upper border of the iliac crests, corresponding to the level of the L4/L5 intervertebral space
Vertical line:
A line perpendicular to the intercristal line and parallel to the spinous processes, crossing through the ipsilateral posterior superior iliac spines (PSIS)
The site of needle insertion is approximately at the intersection of the two lines described above for Winnie’s technique; however, ultrasound imaging may identify a slightly different needle insertion site.
Clinical Pearl: Chayen’s Technique
A more medial and caudal (L5 or below) needle puncture site may be used, following a technique modification described by Chayen and colleagues [1], which places the needle midway between the L5 spinous process and the PSIS.
Patients often experience bilateral blockade indicative of epidural spread. This block may be advantageous in cases where epidural anesthesia is contraindicated, such as in patients with scoliosis.
Redirections of the needle due to bone contact (iliac bone or vertebral body) may be necessary.
The plexus may be reached at a significantly greater depth than when using Winnie’s approach.
Motor responses to nerve stimulation may be present in both the thigh and lower leg (ankle and foot).
The volume of local anesthetic should likely be reduced to that recommended for epidural injections.
24.3 Nerve Stimulation Technique
The authors recommend combining nerve stimulation with ultrasound guidance for lumbar plexus blocks as the nerve roots/plexus can be difficult to visualize, especially for older children.
24.3.1 Needle Insertion
A flowchart illustrating the needle insertion site and procedures is shown in Fig. 24.2.
Fig. 24.2
Flowchart of needle insertion and procedures for lumbar plexus blocks
Insert an insulated 22–25G, short-bevel needle perpendicular to the skin.
The needle will pass through the quadratus lumborum muscle before reaching the psoas major muscle and finally the lumbar plexus. The skin-plexus distance correlated strongly with children’s weight rather than age and height in a study of children between the ages of 3 and 12 and ranged from 1.24 to 1.74 mm/kg depending on age and the level at which the lumbar plexus was visualized with ultrasound imaging (Table 24.1) [2].
Table 24.1
Distance of skin-plexus at different ages at L3/L4 and L4/L5
Levels where lumbar plexus is delineated
>3–5 years old
>5–8 years old
>8–12 years old
L3/4
1.68 mm/kg
1.55 mm/kg
1.24 mm/kg
L4/5
1.74 mm/kg
1.6 mm/kg
1.31 mm/kg
If the needle contacts the bone (transverse process) at a moderate depth, the needle should be reinserted slightly cranial or caudad. This will occur more frequently if using the needle puncture site described by Chayen et al. [1] (see Clinical Pearl above).
24.3.2 Current Application and Appropriate Responses
Figure 24.3 illustrates the procedure for employing nerve stimulation techniques for lumbar plexus block.
Fig. 24.3
Flowchart of nerve stimulation techniques for lumbar plexus blocks
Initially, set the nerve stimulator to apply 1–2 mA current (2 Hz), and aim to elicit motor twitches of the quadriceps muscle at a current intensity threshold of 0.5 mA. See Table 14.3 for expected motor responses during nerve stimulation.
24.3.3 Modifications to Inappropriate Responses (Table 24.2)
Table 24.2
Responses and recommended needle adjustments for use with nerve stimulation during lumbar plexus blocks
Correct response from nerve stimulation |
Quadriceps muscle twitch (palpable or visual) at 0.5–1.0 mA intensity |
Other common responses and needle adjustment |
Muscle twitches from electrical stimulation |
Paraspinal (local twitch from direct stimulation) |
Explanation: needle tip too superficial |
Needle adjustment: advance needle tip |
Hamstring (roots of sciatic nerve) |
Explanation: needle inserted too caudally |
Needle adjustment: withdraw completely and reinsert 3–5 cm cranially |
Thigh flexion (quite deep; psoas major muscle stimulation) |
Explanation: needle tip too deep (close to peritoneal cavity) |
Needle adjustment: withdraw needle and follow protocol |
Bone contact |
Transverse process |
Explanation: close placement; angle slightly off |
Needle adjustment: withdraw to subcutaneous tissue and reinsert with an angle of 5° more cranially or caudally |
No response despite deep placement |
Past transverse process and lumbar plexus |
Explanation: needle tip too deep |
Needle adjustment: withdraw completely and reinsert according to protocol |
An algorithm of modifications to inappropriate responses to nerve stimulation is shown in Fig. 24.4.
Fig. 24.4
Flowchart of modifications to inappropriate responses to nerve stimulation during lumbar plexus blocks
24.4 Ultrasound-Guided Technique
For a summary of ultrasound guidance techniques in lumbar plexus blocks, see Fig. 24.5.
Fig. 24.5
Flowchart of ultrasound-guided techniques in lumbar plexus blocks
Major anatomical structures in the lumbar plexus as captured by MRI and VHVS images are shown with the corresponding ultrasound image in Fig. 24.6.
Fig. 24.6
(a) VHVS and MRI images of major anatomical structures in the lumbar plexus region. (b) Ultrasound image of major anatomical structures in the lumbar plexus region. Blue rectangle indicates position of ultrasound probe
Prepare the needle insertion site and skin surface with an antiseptic solution. Prepare the ultrasound probe surface by applying a sterile adhesive dressing to it prior to needling as discussed in Chap. 4.
24.4.1 Scanning Technique
Traditionally, the approach for the lumbar plexus block has been at the L4–L5 level to avoid renal hematoma or other complications at the level of the kidney (L2–L3). This low approach often results in absence of analgesia in the ilioinguinal/iliohypogastric nerves and occasionally the lateral femoral cutaneous nerve. In the future, ultrasound visualization of the kidneys and vascular structures may allow needle insertion at a more cephalad level (L1–L4) to provide more consistent blockade of these nerves. It must be noted that the lower pole of the kidney can reach down even to the L4–L5 level in young children and infants (Fig. 24.6a).
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In infants and young children:
Place a 6–13 MHz “hockey stick” transducer in the posterior longitudinal (parasagittal) plane just lateral to the spinous processes, starting at a caudal level, to view the hyperechoic line and corresponding bony shadowing from the cephalad portion of the sacrum.
Identify the first transverse process (L5) and place the probe at the L4/L5 interspace. The L3/L4 interspace may allow higher resolution in some children.
Rotate the probe 90° to capture a transverse axis view (Fig. 24.7) in order to visualize (from medial to lateral) the spinous process, the erector spinae muscles, and the quadratus lumborum muscle. Use these landmarks to identify the plexus within the deeper psoas major muscle.
