The Speed of Sound: Ultrasound Leads the Way for Nerve Blocks : Emergency Medicine News

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The Speed of Sound

The Speed of Sound

Ultrasound Leads the Way for Nerve Blocks

Butts, Christine MD

doi: 10.1097/01.EEM.0000898188.74017.67
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    ultrasound, nerve block:
    Image 3. The out-of-plane approach to a posterior tibial nerve block.
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    Figure

    A 25-year-old woman presented to the ED with a laceration on the sole of her foot. She was walking outside when she cut her foot on a broken bottle. An examination revealed a jagged laceration with multiple pieces of glass visible.

    Directly applying anesthesia to the sole is extremely painful, but a nerve block is ideal in this situation. The posterior tibial nerve supplies sensation to most of the sole and heel. Using ultrasound takes some of the guesswork out of locating the nerve compared with using landmarks alone, and it can also be used to guide anesthetic placement.

    Select a high-frequency transducer, preferably one with the smallest footprint available. The transducer should be placed just posterior to the medial malleolus in the transverse plane. (Image 1.) Begin by looking for the posterior tibial vessels, which make an easily identifiable landmark. They are frequently found just posterior to the malleolus.

    The nerve will be located posterior to the vascular structures and is usually very superficial. It is typically oval and often described as having a honeycomb appearance—white with some stippling. (Image 2.) Try sliding the transducer superiorly up the medial leg if the nerve does not jump out at you immediately. This technique often helps the nerve become more obvious.

    Once the nerve has been identified, you can use an in-plane or out-of-plane technique to guide the needle and instill your anesthetic of choice. Given the awkward location of the nerve on the posterior medial aspect of the ankle, the out-of-plane method may be more practical. (Image 3.) Sliding the transducer more superiorly may also identify an easier location to perform the block.

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    Figure:
    Image 1. Place the transducer just posterior to the medial malleolus in the transverse plane to identify the posterior tibial nerve.
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    Figure:
    Image 2. Sonographic appearance of the area posterior to the medial malleolus. The vessels (arrowhead) are often a good landmark because they are easily visible. The tendons (T) are more anterior (A), and the nerve (arrow) is seen as an oval honeycomb structure posterior (P) to the vessels.

    The predicted duration of your procedure will guide your choice of anesthetic, either lidocaine or a more long-acting agent, but typically 3-5 cc injected around but not into the nerve is sufficient to give an adequate effect.

    Dr. Buttsis the director of the division of emergency ultrasound and a clinical associate professor of emergency medicine at Louisiana State University at New Orleans. Follow her on Twitter@EMNSpeedofSound, and read her past columns athttp://bit.ly/EMN-SpeedofSound.

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