Focus On: Ultrasound-Guided Forearm Nerve Blocks

ACEP News
October 2011

 

By Aparajita Sohoni, M.D., Andrew Herring, M.D., Mike Stone, M.D., and Arun Nagdev, M.D. 

Contributors
Dr. Sohoni, Dr. Stone, and Dr. Nagdev are Attending Faculty, Alameda County Medical Center, Highland General Hospital, Department of Emergency Medicine. Dr. Herring is Chief Resident, Alameda County Medical Center, Highland General Hospital, Department of Emergency Medicine. Medical Editor Dr. Robert C. Solomon is core faculty in the emergency medicine residency at Allegheny General Hospial, Pittsburgh, and Assistant Professor in the Department of Emergency Medicine at Temple University School of Medicine. Nancy Calaway is an ACEP staff member who reviews and manages the ACEP Focus On series.

Disclosures
In accordance with the Accreditation Council for Continuing Medical Education (ACCME) Standards and American College of Emergency Physicians policy, all individuals in control of content must disclose to the program audience the existence of significant financial interests in or relationships with manufacturers of commercial products that might have a direct interest in the subject matter.

Dr. Sohoni, Dr. Harring, Dr. Stone, Dr. Nagdev, Dr. Solomon and Ms. Calaway have disclosed that they have no significant relationships with or financial interests in any commercial companies that pertain to this article. There is no commercial support for this activity.

This activity has been planned and implemented in accordance with the Essential Areas and Policies of the Accreditation Council for Continuing Medical Education (ACCME). The American College of Emergency Physicians is accredited by the ACCME to provide continuing medical education for physicians.

The American College of Emergency Physicians designates this enduring material for a maximum of 1 AMA PRA Category 1 CreditTM. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

"Focus On: Ultrasound-Guided Forearm Nerve Blocks" is approved by the American College of Emergency Physicians for one ACEP Category I credit.

Disclaimer
ACEP makes every effort to ensure that contributors to College-sponsored programs are knowledgeable authorities in their fields. Participants are nevertheless advised that the statements and opinions expressed in this article are provided as guidelines and should not be construed as College policy. The material contained herein is not intended to establish policy, procedure, or a standard of care. The views expressed in this article are those of the contributors and not necessarily the opinion or recommendation of ACEP. The College disclaims any liability or responsibility for the consequences of any actions taken in reliance on those statements or opinions.

Questionnaire Is Available Online
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The participant should, in order, review the learning objectives, read the article, and complete the CME post-test/evaluation form to receive up to 1 ACEP Category I credit and 1 AMA PRA Category 1 CreditTM. You must score at least 70 percent to receive credit. You will be able to print your CME certificate immediately.

This article was published online October 1, 2011. The credit for this CME activity expires September 30, 2014.

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Learning Objectives
After reading this Web-based article, the physician should be able to:
  • Recognize and treat uncommon presentations of common pathology and common presentations of rare pathology so that physicians have exposure to these rare conditions.
  • Learn the innervation of the hand.
  • Describe how bedside ultrasound can accurately locate nerves of the upper extremity.
  • Learn the proper procedure for placing anesthetic precisely around nerves.
 

 

Introduction 
Ultrasound-guided nerve blocks in emergency care are an expanding area of interest and research. While traditional "blind" or landmark-based nerve blocks (such as radial, median, and ulnar blocks performed at the wrist using anatomic landmarks) are frequently performed in emergency care settings,1,2 the use of ultrasound to perform real-time injection of an anesthetic agent around a target nerve is a newer application in the emergency department (ED). Ultrasound guidance for abdominal wall,3 sciatic,4 tibial,5 sural,6 intercostal,7 brachial plexus,8 and forearm nerve blocks has been shown to be efficacious in emergency care settings.9 Ultrasound guidance is also used with increasing frequency in the pediatric population.10 This article will focus on performing ultrasound-guided radial, median, and ulnar nerve blocks, together termed the "forearm nerve blocks."

For background, a review of the basic motor and sensory functions of the radial, median, and ulnar nerves and the appropriate clinical examination to test their function is helpful (Table 1).Basic Nerve Functions and Exams - Table 1 

Indications in Emergency Practice 
Forearm nerve blocks provide excellent anesthesia of the hand. Based on the sensory innervation described above, forearm nerve blocks are useful in cases of fractures, lacerations, or deep space infections of the hand. Unfortunately, the forearm nerve blocks described in this article do not provide anesthesia to the distal forearm or wrist and should not be performed for injuries affecting these areas.

Patient Selection 
Appropriate ED patient selection for regional anesthesia procedures is important. The following is a brief list of exclusion criteria for this type of procedure:

  • Patient unable to give consent (altered mental status, language barrier, seems unable to return to the ED if a complication from the nerve block occurred).
  • Pre-existing nerve injury, numbness, tingling or weakness in the affected extremity.
  • Injuries requiring urgent consultation or serial neurological examinations during admission (we recommend the ED provider communicate with the consultant before all blocks, since the sensory - and in some cases motor - exam will be altered).

Procedure 
Position the patient with the affected extremity held palm-up and resting comfortably on a table or other hard surface. Place the ultrasound machine in line with the practitioner's line of sight such that the practitioner has an unobstructed view of the ultrasound screen.

Forearm nerve blocks should be performed using the high-frequency linear (usually 10-5 MHz) transducer held in a transverse orientation. A soft-tissue or nerve preset if available is ideal to locate the nerves in the forearm. For consistency, we recommend pointing the probe marker toward the right side of the patient with the directional indicator on the left of the screen (as per standard emergency ultrasound convention). 

Specifics for Locating Each Nerve 
Systematically scan the forearm starting distally and moving proximally to find each nerve. Remember that the goal is to locate each nerve at a point where there is sufficient distance from vascular structures to avoid inadvertent vascular puncture. This must be balanced against selecting a site where nerve depth makes the procedure more difficult.

  • Median nerve. The median nerve does not have an associated vascular structure except in rare cases. To locate the median nerve, start at the wrist with the mid-point of the transducer over the middle of the wrist crease. Move the probe proximally and look for the nervous structure that exists at the junction of several fascial planes. The median nerve lies in the fascial plane between the flexor digitorum superficialis and profundus. As the probe moves more proximal, tendon structures will disappear, while the classic "honeycomb" bundle of the median nerve will persist (photo 1).
  • Radial nerve. The radial nerve is located radially to the radial artery. Starting at the wrist, locate the pulsatile radial artery. Immediately radial to the radial artery, a collection of nerve bundles is seen. Often, the radial nerve is difficult to identify as a separate structure at the wrist because of its close proximity to the artery. Trace the radial artery proximally, and by approximately mid-forearm the radial nerve should be located at a safe distance from the radial artery (photo 2).
  • Ulnar nerve. The ulnar nerve is located at the ulnar aspect of the ulnar artery. Starting at the wrist, locate the pulsatile ulnar artery. Immediately ulnar to the ulnar artery, a small nerve bundle can be visualized. Like the radial nerve, the ulnar nerve lies close to the ulnar artery. As the provider moves the probe in a more proximal manner, the ulnar nerve will separate from the artery, allowing an ideal location for injection while reducing risk of inadvertent arterial puncture (photo 3).

The examiner should locate the desired nerve at a location sufficiently separate from vascular structures (if possible), but not so deep that it cannot be reached with a standard needle.

Procedure Details 

  • Skin prep. The skin should be cleansed with alcohol or other cleansing agent (such as chlorhexidine).
  • Patient monitoring. We recommend placing the patient on continuous pulse oximeter monitoring to allow the operator to be aware of the patient's heart rate and oxygen saturation at all times.
  • Local anesthetic. A skin wheal of local anesthetic (1% lidocaine with or without epinephrine) should be injected with a (30-gauge) tuberculin syringe.
  • Syringe preparation. While the local anesthetic is taking effect, fill a syringe with 10 cc of 1% lidocaine without epinephrine. Bupivacaine should not be used by novice providers in case of inadvertent vascular puncture. In our practice, we use a standard 25-gauge, 1.5-inch needle to perform the block, and our providers often use longer-acting anesthetics once they've become familiar with performing these procedures.
  • Ultrasound-guided injection. With the transducer held in a transverse orientation at the predetermined site, introduce the needle approximately 0.5 cm laterally to the base of the transducer. For blocking the median nerve, the needle can be introduced on either side of the probe, with the location of other neurovascular structures and examiner's hand dominance guiding the decision. For the radial nerve, the needle should be introduced on the radial side of the probe (photo 4), while for the ulnar nerve the needle should be introduced on the ulnar side of the probe (photo 5). Note that this is a description of the in-plane technique. Although we also perform these blocks using an out-of-plane technique (similar to a short-axis approach to vascular access), we have found that novice users are more successful using an in-plane technique.

    The needle should be visualized from the moment it passes under the ultrasound transducer (photo 6). If the needle is not identified, the operator should not advance the needle. The needle should be removed or the probe moved to allow clear visualization of the needle. The needle should be directed from the skin entry site to the most distal border (either superficial or deep) of the nerve (photo 7). The needle should not be allowed to penetrate the actual nerve structure at any point.

    Prior to injecting any anesthetic agent, draw back on the syringe to confirm that the needle is not in a vascular structure. Gently inject 0.5-1 cc of anesthetic and observe a spread of anechoic (black) fluid around the superior border of the nerve (photo 8). If anechoic fluid is not seen exiting the needle tip, the operator should not deposit more anesthetic. The probe should be adjusted to confirm that the tip of the needle is clearly visualized before more anesthetic agent is deposited. The goal is to have the anechoic fluid surround the nerve completely. Withdraw the needle towards the skin and redirect it to the area where the fluid has not reached. Repeat the injection at this site (photo 9). When complete, the nerve should be circumferentially surrounded by a layer of anechoic fluid, resembling a donut (photo 10). With the transducer, trace the nerve both proximally and distally to confirm this "donut" appearance.

    Inject only as much anesthetic as is needed to create this "donut" appearance. With experience, we have found that the three forearm nerves can be adequately blocked with a total of 10 mL of anesthetic agent (much lower than the maximum dosing guidelines).

    Withdraw the needle and apply a dressing to the skin site.

  • Duration and toxicity of anesthetic agents. Table 2 shows the duration and maximum dosages of the most commonly used anesthetic agents.Common Anesthetic Agents - Table 2 

Complications 
Ultrasound-guided nerve blocks are a useful technique to supply anesthesia to the hand, but like all procedures, may have associated complications. Inadvertent vascular puncture and intraneural injections (two feared but uncommon complications) can be avoided by selecting an injection site where the nerve and artery are sufficiently separate and by visualizing the needle tip during the procedure. Inadvertent vascular puncture can also be avoided by drawing back on the syringe before injection, and ensuring that anechoic fluid is visualized on the ultrasound screen when anesthetic is deposited. To avoid intraneural injections, infuse anesthetic under low pressures. If the patient experiences any pain or paresthesias during the injection, stop injecting and pull back the needle until it is clear that the needle tip is not within the nerve bundle.

References 

  1. Ferrera PC, Chandler R. Anesthesia in the emergency setting: Part I. Hand and foot injuries. Am. Fam. Physician 1994;50:569-73.
  2. Salam GA. Regional anesthesia for office procedures: Part II. Extremity and inguinal area surgeries. Am. Fam. Physician 2004;69:896-9.
  3. Herring AA, Stone MB, Nagdev AD. Ultrasound-guided abdominal wall nerve blocks in the ED. Am. J. Emerg. Med. 2011.
  4. Herring AA, Stone MB, Fischer J, et al. Ultrasound-guided distal popliteal sciatic nerve block for ED anesthesia. Am. J. Emerg. Med. 2011;29:697, e3-5.
  5. Redborg KE, Antonakakis JG, Beach ML, Chinn CD, Sites BD. Ultrasound improves the success rate of a tibial nerve block at the ankle. Reg. Anesth. Pain Med. 2009;34:256-60.
  6. Redborg KE, Sites BD, Chinn CD, et al. Ultrasound improves the success rate of a sural nerve block at the ankle. Reg. Anesth. Pain Med. 2009;34:24-8.
  7. Stone MB, Carnell J, Fischer JW, Herring AA, Nagdev A. Ultrasound-guided intercostal nerve block for traumatic pneumothorax requiring tube thoracostomy. Am. J. Emerg. Med. 2011;29:697, e1-2.
  8. Stone MB, Wang R, Price DD. Ultrasound-guided supraclavicular brachial plexus nerve block vs. procedural sedation for the treatment of upper extremity emergencies. Am. J. Emerg. Med. 2008;26:706-10.
  9. Liebmann O, Price D, Mills C, et al. Feasibility of forearm ultrasonography-guided nerve blocks of the radial, ulnar, and median nerves for hand procedures in the emergency department. Ann. Emerg. Med. 2006;48:558-62.
  10. Ganesh A, Gurnaney HG. Ultrasound guidance for pediatric peripheral nerve blockade. Anesthesiol. Clin. 2009;27:197-212.
  11. Wilhelmi BJ. Hand Anatomy. Emedicine 2011.
  12. Williams BR. Peripheral Nerves of the Upper Extremity. Orthopaedia Clerkship. In: Orthopaedia - Collaborative Orthopaedic Knowledgebase 2010.
  13. Tintinalli JE, Kelen GD, Stapczynski JS. Emergency Medicine: A Comprehensive Study Guide. The McGraw Hill Companies, Inc.; 2004.
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