Geriatrics Going to Ground : Ultrasound-Guided Femoral Nerve Block

CASE

It would be unusual to go too many shifts as an emergency medicine provider without seeing the classic elderly female patient with hip pain after a fall. On exam, the patient would likely be uncomfortable with their hip appearing shortened and externally rotated on exam. Before you even view the X-ray, you know they likely fractured their hip. You ascertain whether it was a mechanical fall, if there were other injuries, if the injury is open or closed and if the patient is neurovascularly intact. You proceed to order your imaging and consider your plan for pain control. Sound familiar?

DIAGNOSIS

Hip fracture

DISCUSSION

A femoral nerve block is a great option for pain control in the elderly patient with a hip fracture who you are concerned about respiratory depression from opioids, or you are having trouble getting comfortable. The femoral nerve provides sensory innervation to the hip, knee, and ankle joints, the anterior and medial thigh, along with the medial aspect of the leg and foot. The femoral nerve also provides motor innervation to the muscles that flex the hip and extend the knee (Figure 1). The most common pathologies for which a femoral nerve block is utilized include hip and femur fractures. Contraindications to a nerve block include allergy to the anesthetic, infection at the site of injection, or need for frequent neurovascular checks.

The risks of a nerve block include bleeding, infection, allergic reaction, neuropathy, and local anesthetic systemic toxicity (LAST). These risks can be minimized by utilizing clean or sterile technique, visualization of the needle tip at all times during the procedure, accurate identification of vascular structures, aspirating prior to injection, and by minimizing the amount of anesthetic used. The femoral nerve block has proven to be safe with minimal incidence of adverse events and has been shown to significantly decrease dosing of parental analgesic medications [1]. Using ultrasound guidance for this procedure increases the duration of effect and decreases the volume of anesthetic used [2].

Figure 1: Osteotomal and dermatomal distribution of the femoral nerve [3]

Procedure Overview

Select the high frequency linear transducer probe, which will maximize resolution for this superficial structure, and cover with a sterile probe sheath. Draw up approximately 10-20cc of local anesthetic of your choice based on preference of duration and side effect risk profile. Be sure to calculate your maximum dosage based on patient weight (Figure 2). Common choices include 1-2% lidocaine, which has a shorter duration, and 0.5% bupivacaine, which has a longer duration. We recommend lidocaine for novice users because it has a higher safety profile with inadvertent vascular injection. There are many acceptable options for needle type including a regular needle, a special nerve block needle or a spinal needle. The syringe containing the anesthetic can be attached directly to the back of the needle, or you can place standard IV tubing between the needle and the syringe. The advantage of the latter set-up is that you can hold the needle itself rather than the end of the syringe and it gives the operator more control, however it requires a second operator to draw back and inject.

Figure 2: Local anesthetic dosing guide [4]

Position the patient supine with the femoral crease exposed and the ultrasound machine positioned where you can easily view the screen (likely on the opposite side of the patient from the operator). The leg should be straightened, slightly abducted, and externally rotated.

Disinfect the skin and apply the probe in the transverse position along the femoral crease with the indicator pointing to the left of the operator. The anatomic structures here from lateral to medial include the femoral nerve, femoral artery, femoral vein, and lymphatics (as is often remembered with the NAVL mnemonic). The femoral nerve is lateral to the femoral artery, contained deep to the hyperechoic fascia lata and fascia iliaca. The nerve, which is more easily identified proximal to the bifurcation of the femoral artery, appears hyperechoic and triangular or oval (Figure 3) and is described in appearance like honeycomb or the surface of the moon (Figure 4). If you have trouble locating the nerve, you can apply compression to the probe -- the nerve is less compressible as compared to the surrounding muscle and fat.

Figure 4: Nerve appearance on ultrasound

Insert the needle from the lateral side of the transducer utilizing an in-plane approach and create a skin wheal for superficial analgesia. The needle must then pass through both the fascia lata and the fascia iliaca to enter the correct tissue plane (Figure 5).

Figure 5: Approach and anatomic diagram [5]

Prior to injection, aspirate to ensure the needle tip is not intravascular. The anesthetic injection should enter the tissue plane smoothly and with minimal resistance. You should see the hypoechoic anesthetic spreading under direct visualization from the needle tip - if you do not see this, your needle tip is not where you think, and you should stop injecting immediately and relocate your needle tip. The goal is to dissect the tissue plane below the fascia iliaca and surround the nerve in anesthetic without needing to direct the needle tip too close to the nerve. This block contrasts with the fascia iliaca plane block (also known as the 3-in-1 block), which targets the same tissue plane but also affects the obturator and lateral femoral cutaneous nerves and requires higher volumes of anesthetic for dispersion along the tissue plane.

Symptoms of local anesthetic systemic toxicity (LAST) include tinnitus, perioral numbness, metallic taste, agitation, and dysarthria, amongst a myriad of other symptoms. Toxicity may be indicated by tachycardia and hypertension, thereafter devolving into bradycardia, hypotension, and ventricular arrythmia. If LAST is suspected, treatment includes intravenous benzodiazepine, ACLS protocol, and lipid emulsion therapy.

TAKE-AWAYS

The femoral nerve block is a powerful tool for multimodal analgesia in the Emergency Department.
This short but efficacious procedure is relatively low risk and can greatly reduce the need for escalating doses of opioid analgesics and the risk of subsequent respiratory depression.
Next time a hip or femur fracture rolls into the Emergency Department be sure to consider reaching for the ultrasound!

Keywords:

Analgesia Femoral Nerve Block Procedure

AUTHOR: Patrick Wasserman is a fourth year medical student at The Warren Alpert Medical School of Brown University.

FACULTY REVIEWER: Kristin Dwyer, MD is an attending physician at The Warren Alpert Medical School of Brown University/Rhode Island Hospital.

References:

Turner AL, Stevenson MD, Cross KP. Impact of ultrasound-guided femoral nerve blocks in the pediatric emergency department. Pediatric Emergency Care. 2014 Apr;30(4):227-9.
Oberndorfer U, Marhofer P, Bösenberg A, Willschke H, Felfernig M, Weintraud M, Kapral S, Kettner SC. Ultrasonographic guidance for sciatic and femoral nerve blocks in children. Br J Anaesth.2007 Jun;98(6):797-801. Epub 2007 Apr 21
New York School of Regional Anesthesia -- https://www.nysora.com/techniques/lower-extremity/ultrasound-guided-femoral-nerve-block/
Highland EM Ultrasound -- http://highlandultrasound.com/med-guide
Highland EM Ultrasound -- http://highlandultrasound.com/new-femoral-updated
Kasibhatla RD, Russon K. Femoral nerve blocks. J Perioper Pract. 2009 Feb;19(2):65-9.
Sykes Z, Pak A. Femoral Nerve Block. [Updated 2021 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK546704/