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Ultrasound Guided Nerve Blocks

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Illustration 1:
Distribution of anesthesia

Figure 1:
Probe position for popliteal sciatic nerve block.

Figure 2:
Overview of anatomy in the popliteal fossa.

Video 1:
Popliteal block after nerve bifurcation.

 

Leonard V. Bunting, M.D., Emilie J.B. Calvello, M.D., M.P.H.

Popliteal Sciatic Nerve Block

The ability to block the sciatic nerve is a powerful tool in the emergency setting.  Accessing the nerve proximally is made challenging by its deep lie and will not be covered in this chapter.  However, the nerve becomes superficial as it approaches the popliteal fossa, allowing for relatively easy blockade.  At this distal site, the procedure still covers part of the knee and distal leg, making it excellent for injuries to the distal leg, especially the ankle.  The saphenous nerve provides innervation to the skin of the medial lower extremity and is not included in a sciatic nerve block. Therefore, consider also blocking the femoral nerve for injuries involving the whole distal lower extremity, such as bi-malleolar fractures.

Anatomy

The sciatic nerve originates from the lumbar and sacral plexi (L4 – S3) and travels deep in the posterior thigh.  Proximal to the popliteal fossa, the sciatic nerve lies more superficial, between the long head of the biceps femoris laterally and the semimembranous muscles medially.  In this location, the nerve generally bifurcates into the common peroneal and tibial nerves. These nerves continue into the popliteal fossa where they lie superficial and lateral to the popliteal vessels.[1-3]

From the popliteal fossa, the tibial nerve continues down the back of the leg to wrap around the posterior medial malleolus. It provides innervation to the majority of the posterior lower extremity, knee joint and plantar surface of the foot. The common peroneal nerve passes posterior to the head of the fibular and then winds anterior, where it divides into deep and superficial branches.  The common peroneal nerve provides sensory innervation to the dorsal lateral foot and ankle, and posterolateral portion of the distal lower extremity via its branches.

 

Illustration 1:
Distribution of anesthesia

Scanning Technique

Ideally, the patient is prone on the stretcher for this block.  The block can be performed with the patient on their side or even supine, but probe and needle handling is more difficult.  The injured extremity is supported during position changes.

A high-frequency (6-18 MHz), linear array probe is used with appropriate depth adjustment, usually between 2 to 6 centimeters depth setting.  The probe is placed transversely across the popliteal fossa at the popliteal crease.  The probe indicator is towards the patient’s left if they are in the standard prone position.  The femur is first identified by its’ characteristic hyperechoic stripe and dense posterior shadowing.  Next, the pulsatile popliteal artery and corresponding vein are located superficial and medial to the femur.  At this level, the sciatic nerve has generally bifurcated into the tibial and common peroneal nerves.  The tibial nerve is more easily visualized than the common peroneal nerve and is found superficial and lateral to the artery.  The common peroneal nerve is found lateral to the tibial nerve at a simiar depth.  If visualization of the nerves is difficult, the probe is fanned or rocked to find the ideal angle of incidence for nerve visualization. If visualization is still challenging, the patient should plantar flex the ankle to elevate the peroneal nerve and dorsiflex the ankle to elevate the tibial nerve if possible.  This is commonly referred to as the “seesaw” sign.[4] Once either or both of the nerves are identified, they are followed proximally until they form the sciatic nerve. If the sciatic nerve is well visualized and not too deep, this will be the target for the nerve block.  If the sciatic nerve is not visualized, consider both the tibial and common peroneal nerves as the targets.

Nerve Block

With the probe transversely in the popliteal fossa, the target nerve(s) is placed in the center of the screen.  After skin anesthesia, the block needle is inserted in an in-plane approach from the side of the probe.  The needle is inserted approximately 1 cm and the probe is slid proximally and distally to identify the needle tip.  Once the needle is found, it is advanced to the deep border of the sciatic nerve.  If blocking the tibial and common peroneal nerves after the bifurcation, the nerve furthest from the needle entry site is blocked before blocking the closer one.  The goal is to surround the nerve(s) with local anesthetic.  Block volumes range from 15-30 cc’s and anesthesia should occur within 15-20 minutes.

Figure 1:
Probe position for popliteal sciatic nerve block.

Figure 2:
Overview of anatomy in the popliteal fossa.

Video 1:
Popliteal block after nerve bifurcation.

 

Pearls and Pitfalls

Ensure that all nerves are blocked.  The tibial nerve can be large and easily mistaken for the sciatic nerve leading to no anesthesia in the region of the peroneal nerves.

Maintain a shallow needle angle.  It will greatly enhance needle visualization.

If blocking the femoral nerve as well, monitor the combined doses to avoid toxicity.

In this region, the nerve’s appearance is affected by the angle of the probe.  If the nerve does not appear hyperechoic to surrounding structures, rock or tilt the probe.

Avoid injecting too much local anesthetic into the surrounding muscle.  It will make the muscle hyperechoic and decrease contrast from the nerve.

 

References

1.  Sinha A, Chan V W.
Ultrasound imaging for popliteal sciatic nerve block. Reg Anesth Pain Med. 2004;29:130-134.

2.  Gray A T, Huczko E L, Schafhalter-Zoppoth I.
Lateral popliteal nerve block with ultrasound guidance. Reg Anesth Pain Med. 2004;29:507–509.

3.  Sites B D, Gallagher J D, Tomek I, Cheung Y, Beach M L.
The use of magnetic resonance imaging to evaluate the accuracy of a handheld ultrasound machine in locating the sciatic nerve in the popliteal fossa. Reg Anesth Pain Med. 2004;29:413–416.

4.  Schafthalter-Zoppoth I.
The “see-saw” sign:  improved sonographic identification of the sciatic nerve. Anesthesiology. 2004;101:808-809.


 

 

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