Lumbar Plexus Block
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The lumbar plexus block is an advanced nerve block technique. The block has significant clinical applicability and because of this, it is used commonly in our practice. However, this block has a relatively higher potential for complications and should be practiced only after appropriate training. Due to the placement of the needle in the deep muscle beds, the potential for systemic toxicity is greater than in many other techniques. In addition, the proximity of the lumbar nerve roots and epidural space carries a risk of an epidural spread. For these reasons, care should be taken when selecting the type, volume, and concentration of local anesthetic, particularly in elderly and frail patients. In addition, due to the depth of the needle placement, this block is best avoided in very obese patients. A lumbar plexus block provides anesthesia or analgesia to the entire distribution of the lumbar plexus, including the anterolateral and medial thigh, knee, and the saphenous nerve below the knee. When combined with a sciatic nerve block, anesthesia of the entire leg can be achieved. |
The lumbar plexus consists of five nerves on each side, the first of which emerges between the first and second lumbar vertebra and the last one between the last lumbar vertebra and the base of the sacrum. As soon as the L2, L3, and L4 roots of the lumbar plexus split off their spinal nerves and emerge from the intervertebral foramina, they become embedded in the psoas major muscle. This is because the psoas is attached to the lateral surfaces and transverse processes of the lumbar vertebrae. Within the muscle, these roots then split into anterior and posterior divisions, which then reunite to form the individual branches (nerves) of the plexus. The major branches of the lumbar plexus are the genitofemoral, lateral femoral cutaneous, femoral, and obturator nerves. The femoral nerve is formed by the posterior divisions of L2-4 and descends from the plexus lateral to the psoas muscle. The anterior divisions of the same roots unite to form the other major branch of the lumbar plexus, the obturator nerve. |
The femoral nerve supplies motor fibers to the quadriceps muscle (knee extension), skin of the anteromedial thigh, and the medial aspect of the leg below the knee and foot. The obturator nerve sends motor branches to the adductors of the hip and a highly variable cutaneous area on the medial thigh or knee joint. The lateral femoral cutaneous and genitofemoral nerves are purely cutaneous nerves. |
The patient is in the lateral decubitus position with a slight forward tilt. The foot on the side to be blocked should be positioned over the dependent leg so that twitches of the patella can be seen easily. It is equally important that the anesthesiologist assume a position from which the anterior thigh is seen so that the contractions of the quadriceps muscle are visible. TIP: A slight forward pelvic tilt in the patient position allows for a more ergonomic position for the anesthesiologist. However, this however slight forward position should be kept in mind by the anesthesiologist when advancing and redirecting the needle. |
A standard regional anesthesia tray is prepared with the following equipment:
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There are only two surface anatomy landmarks that are important for determining the insertion point for the needle:
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Landmarks for the lumbar plexus block include:
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Local anesthetic skin infiltration
After a cleaning with an antiseptic solution, the skin is anesthetized with by infiltrating local anesthetic subcutaneously at the determined needle insertion site. TIP: Because the needle can be inserted at any point on the line 4-cm lateral and parallel to the midline, local infiltration should be done alongside this line, rather than at a single projected needle insertion point. This allows for needle reinsertion at another point (if necessary) without the need to anesthetize the skin again. |
Needle insertion
The fingers of the palpating hand are firmly pressed against the paravertebral muscles to stabilize the landmark and decrease the skin-nerve distance. The palpating hand should not be moved during the entire block placement procedure to allow for precise redirections of the angle of the needle insertion when necessary. The needle is inserted at a perpendicular angle to the skin. The nerve stimulator should be initially set to deliver 1.5 mA current. |
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As the needle is advanced, local twitches of the paravertebral muscles are obtained first at a depth of a few cm. The needle is then advanced further until twitches of the quadriceps muscle are obtained (usually at the depth of 6-8 cm). After the twitches are obtained, the current should be lowered to obtain stimulation between 0.5 mA and 1.0 mA. At this point, 25-35 mL of local anesthetic is slowly injected with frequent aspiration to rule out inadvertent intravascular placement of the needle.
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Visible or palpable twitches of the quadriceps muscle at 0.5-1.0 mA current.
TIPS:- Successful lumbar plexus blockade depends on the dispersion of the local anesthetic in the fascial plane (psoas muscle) where the roots of the plexus are situated. Thus, the goal of the nerve stimulation is to identify this plane by eliciting stimulation of one of the roots.
- Stimulation at currents less than 0.5 mA should not be sought when using this technique. Dural sleeves thickly envelop the roots of the lumbar plexus. Motor stimulation with a low current may indicate placement of the needle inside a dural sleeve. An injection inside this sheath can result in tracking of the local anesthetic toward the epidural or subarachnoid space, with consequent epidural or spinal anesthesia.
Failure to obtain quadriceps twitch on first needle pass
When insertion of the needle does not result in quadriceps muscle stimulation, the following maneuvers should be performed:
- Withdraw the needle to the skin level, redirect 5º-10º cranially, and repeat the procedure.
- Withdraw the needle to the skin level, redirect 5º-10º caudally, and repeat the procedure.
- Withdraw the needle to the skin level, redirect 5º-10º medially, and repeat the procedure.
- Withdraw the needle to the skin level and reinsert 2-cm caudally or cranially and repeat the procedure.
- Failure to obtain a quadriceps muscle twitch on the first needle insertion is common, even with proper needle placement, simply because the needle tip may pass between the two roots and thus stimulation is not obtained.
- Using a higher stimulating current (e.g., 4 mA) to decrease the chance of missing the roots is only marginally beneficial. In addition, there is a drawback that such stimulation is very uncomfortable for the patient (burning) and much deeper sedation/analgesia is required.
Some common responses to nerve stimulation and the course of action to take to obtain the proper response:
| Response Obtained | Interpretation | Problem | Action |
| Local twitch of the paraspinal muscle | Direct stimulation of the paraspinal muscles | Too shallow placement of the needle | Continue advancing the needle |
| Needle contacts bone at 4-6 cm depth; no twitches are seen | The needle advancement is stopped by the transverse process | This indicates proper needle placement, but requires redirection of the needle | Withdraw the needle to the skin level and redirect 5o cranially or caudally |
| Twitches of the hamstring muscles are seen; needle inserted 6-8 cm | This is the result of stimulation of the roots of the sciatic plexus (sciatic nerve) | The needle is inserted too caudally | Withdraw the needle and reinsert 3-5 cm cranially |
| Flexion of the thigh at the depth of > 6-8 cm | This subtle and often missed response is caused by direct stimulation of the psoas muscle | The needle is inserted too deep (missed the lumbar plexus roots); further advancement may place the needle intraperitoneally | Stop advancing the needle; withdraw the needle and reinsert using the protocol outlined in the technique description |
| The needle is placed deep (10 cm) but twitches were not elicited and bone is not contacted | The needle missed the transverse process and roots of the lumbar plexus | Too deep placement of the needle | Withdraw the needle and reinsert using the protocol outlined in the technique description |
Lumbar plexus blockade requires a relatively large volume of local anesthetic to achieve anesthesia of the entire plexus. The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or pain management. Due to the highly vascular nature of the area and the potential for inadvertent intravascular injection, rapid absorption from the deep muscle beds, and epidural spread, it can be used as a vascular marker.
| Onset (min) |
Anesthesia (hrs) | Analgesia (hrs) | |
| 1.5% Mepivacaine (plus HCO3; plus epinephrine) | 10-20 | 2-3 | 3-4 |
| 2% Lidocaine (plus HCO3 + epinephrine) | 10-15 | 2-3 | 3-4 |
| 0.5% Ropivacaine | 15-25 | 3-5 | 8-12 |
| 0.75% Ropivacaine | 10-15 | 4-6 | 12-18 |
| 0.5% Bupivacaine (plus epinephrine) | 15-25 | 4-6 | 12-18 |
| 0.5% I-Bupivacaine (plus epinephrine) | 12-25 | 4-6 | 12-18 |
- Always assess the risk-benefit ratio of using large volumes and concentrations of long-acting local anesthetic for lumbar plexus blockade.
- Lumbar plexus block carries a higher risk of local anesthetic toxicity due its deep location and vicinity of muscles. For that reason, we avoid high concentrations of long-acting local anesthetic with this block.
- Smaller volumes and concentrations can be used successfully for analgesia (e.g., 20 mL); however, for surgical anesthesia - 30 mL is necessary to achieve a dense blockade of the entire lumbar plexus.
- Consider using a less toxic local anesthetic (e.g., alkalinized 3% chloroprocaine) for skin infiltration to avoid a large cumulative dose of local anesthetic.
A lumbar plexus block can be associated with significant patient discomfort during nerve localization due to the needle passage through multiple muscle planes. Adequate sedation and analgesia are necessary to ensure a still and tranquil patient. Typically, we use midazolam 4-6 mg after the patient is positioned and alfentanil 500-1000 µg just before needle insertion. A typical onset time for this block is 15-25 minutes, depending on the type, concentration, and volume of local anesthetic and the level at which the needle is placed. For example, although an almost immediate onset of anesthesia in the anterior thigh and knee can be achieved with an injection at the L3 level, additional time is required for local anesthetic to block the lateral thigh (L1) or obturator nerve (L5). The first sign of the onset of blockade is usually the loss of sensation in the saphenous nerve territory (medial skin below the knee).
TIP: Inadequate skin anesthesia despite the apparent timely onset of the blockade can occur. It can take up to 30 minutes for full sensory-motor anesthesia to develop. Local infiltration at the site of the incision by the surgeon is often all that is needed to allow the surgery to proceed.
The lumbar plexus block is an advanced technique with a significant clinical applicability but also with a potential for serious complications. The table provides some general and specific instructions on possible complications and methods to avoid.
| Infection | - A strict aseptic technique is used |
| Hematoma |
- Avoid multiple needle insertions, particularly in anticoagulated patients - Continuous lumbar plexus blocks are best avoided in anticoagulated patients - However, once the catheter is placed, we use it even in patients on prophylactic thromboembolic prophylaxis, such as low molecular weight and subcutaneous heparin therapy - The use of antiplatelet therapy is not a contraindication for this block in the absence of spontaneous bleeding |
| Vascular puncture |
- Vascular puncture is not common with this technique - However, deep needle insertion should be avoided (vena cava, aorta) |
| Local anesthetic toxicity |
- Systemic toxicity after sciatic blockade may be more common than then with plexus blocks due to the location of the plexus in the proximity of large muscle beds - Higher volumes result in more solid, complete and faster blockade, however, it carries a higher risk of toxicity - Large volumes of long-acting anesthetic should be reconsidered in older and frail patients - Careful and frequent aspiration should be performed during the injection - Avoid forceful, fast injection of local anesthetic |
| Nerve injury |
- The risk of nerve injury after lumbar plexus block is low - Local anesthetic should never be injected when the patient complains of pain or abnormally high pressure on injection is met - When stimulation is obtained with current intensity of < 0.5 mA, the needle should be pulled back to obtain the same response with a current of 0.5mA before injecting local anesthetic to avoid injection into the dural sleeves and the consequent epidural or spinal spread |
| Hemodynamic consequences |
- Lumbar plexus blockade results in unilateral sympathectomy; as such, significant hypotension is rare - Spread of the local anesthetic to the epidural space may result in significant hypotension and occurs in as many as 15% of the patients - Every patient receiving a lumbar plexus block should be monitored to the same extent as patients receiving epidural anesthesia |
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