Summary

Bedside ultrasound is useful for diagnosing and managing many skin soft tissue conditions. It can help distinguish cellulitis from abscess more reliably than clinical exam alone as well as guide drainage. It also helps to identify and locate foreign bodies. In addition to adding diagnostic value, bedside ultrasound has also been shown to decrease length of stay in the emergency department, increase patient perception of emergency physician skill, and improve patient satisfaction with ED care. Mastering skin and soft tissue sonography will improve your clinical skills and enhance the care you provide to you patients.

 

References

1. Thamburaj R, Sivitz A. Does the use of bedside pelvic ultrasound decrease length of stay in the emergency department? Pediatr Emerg Care. 2013 Jan;29(1):67-70. doi: 10.1097/PEC.0b013e31827b53f9. PubMed PMID: 23283267.
2. Howard ZD, Noble VE, Marill KA, Sajed D, Rodrigues M, Bertuzzi B, Liteplo AS. Bedside ultrasound maximizes patient satisfaction. J Emerg Med. 2014 Jan;46(1):46-53. doi: 10.1016/j.jemermed.2013.05.044. Epub 2013 Aug 12. PubMedPMID: 23942153.
3. Ramirez-Schrempp D, Dorfman DH, Baker WE, Liteplo AS. Ultrasound soft-tissue applications in the pediatric emergency department: to drain or not to drain? Pediatr Emerg Care. 2009 Jan;25(1):44-8. doi: 10.1097/PEC.0b013e318191d963. Review. PubMed PMID: 19148015.
4. Squire BT, Fox JC, Anderson C. ABSCESS: applied bedside sonography for convenient evaluation of superficial soft tissue infections. Acad Emerg Med. 2005 Jul;12(7):601-6. PubMed PMID: 15995090.
5. Subramaniam S, Bober J, Chao J, Zehtabchi S. Point-of-care Ultrasound for Diagnosis of Abscess in Skin and Soft Tissue Infections. Acad Emerg Med. 2016 Nov;23(11):1298-1306. doi: 10.1111/acem.13049. Epub 2016 Nov 1. Review. PubMed PMID: 27770490.
6. Marin et al. Emergency Ultrasound-assisted Examination of Skin and Soft Tissue Infections in the Pediatric Emergency Department. Acad Emerg Med. 2006; 20:545-53. doi: 10.1111/acem.12148
7. Iverson et al. The effect of bedside ultrasound on diagnosis and management of soft tissue infections in a pediatric ED. Am J of Emerg Med. 2012; 30: 1347-51. doi:10.1016/j.ajem.2011.09.020.
8. Tayal VS, Hasan N, Norton HJ, Tomaszewski CA. The effect of soft-tissue ultrasound on the management of cellulitis in the emergency department. Acad Emerg Med. 2006 Apr;13(4):384-8. Epub 2006 Mar 10. PubMed PMID: 16531602.
9. Sivitz et al. Effect of Bedside Ultrasound on Management of Pediatric Soft Tissue Infection. J of Emerg Med. 2010; 39(5): 637-43. doi:10.1016/j.jemermed.2009.05.013.
10. Yen ZS, Wang HP, Ma HM, Chen SC, Chen WJ. Ultrasonographic screening of clinically-suspected necrotizing fasciitis. Acad Emerg Med. 2002 Dec;9(12):1448-51. PubMed PMID: 12460854.
11. Jacobson JA, Powell A, Craig JG, Bouffard JA, van Holsbeeck MT. Wooden foreign bodies in soft tissue: detection at US. Radiology. 1998 Jan;206(1):45-8. PubMed PMID: 9423650.9.     Davis J, Czerniski B, Au A, Adhikari S, Farrell I, Fields JM. Diagnostic Accuracy of Ultrasonography in Retained Soft Tissue Foreign Bodies: A Systematic Review and Meta-analysis. Acad Emerg Med. 2015 Jul;22(7):777-87. doi: 10.1111/acem.12714. Epub 2015 Jun 25. Review. PubMed PMID: 26111545.
12. Fabiszewska, E., Pasicz, K., Grabska, I., Skrzyński, W., Ślusarczyk-Kacprzyk, W., & Bulski, W. (2017). Evaluation of Imaging Parameters of Ultrasound Scanners: Baseline for Future Testing. Polish journal of radiology, 82, 773-782. doi:10.12659/PJR.904135
13. Lee MC, Rios AM, Aten MF, Mejias A, Cavuoti D, McCracken GH Jr, Hardy RD. Management and outcome of children with skin and soft tissue abscesses caused by community-acquired methicillin-resistant Staphylococcus aureus. Pediatr Infect Dis J. 2004 Feb;23(2):123-7. PubMed PMID: 14872177.
14. Tayal VS, Hasan N, Norton HJ, Tomaszewski CA. The effect of soft-tissue ultrasound on the management of cellulitis in the emergency department. Acad Emerg Med. 2006 Apr;13(4):384-8. Epub 2006 Mar 10. PubMed PMID: 16531602.
15. Barbaric D et al. In patients presenting to the emergency department with skin and soft tissue infections what is the diagnostic accuracy of point-of-care ultrasonography for the diagnosis of abscess compared to the current standard of care? A systematic review and meta-analysis. 2017. BMJ Open. Jan 10;7(1):e013688.
16. Dean AJ et al. Techniquefor emergency medicine bedside ultrasound identification of a radiolucent foreign body. 2003. J Emerg Med. Apr;24(3):303-8.
17. Gaspari RJ and Sanseverino A. Ultrasound-Guided Drainage for Pediatric Soft Tissue. 2018. J Ultrasound Med. Jan;37(1):131-136.
18. Lin MJ et al. Point-of-Care Ultrasound is Associated With Decreased Length of Stay in Children Presenting to the Emergency Department With Soft Tissue Infection. 2018. J Emerg Med. Jan;54(1):96-101
19. Mallin M and Dawson M. “Introduction to Bedside Ultrasound: Volume 2.” Emergency Ultrasound Solutions, 2013. iBooks. https://itunes.apple.com/us/book/introduction-to-bedside-ultrasound-volume-2/id647356692?mt=11
20. Nienaber A et al. Accuracyof bedside ultrasound for the detection of soft tissue foreign bodies by emergency doctors. Emerg Med Australas. 2010 Feb;22(1):30-4.

 

Note: Videos and images thanks to Amanda Toney at Denver Health, Jon Orsborn at Children’s Hospital Colorado and personal images of the author Julia Brant from Children’s Hospital Co.

Pearls & Pitfalls

 

• Ultrasound is meant as a diagnostic aid. Always rely on clinical information to guide management decisions. If it is unclear what you are seeing, obtain formal imaging.

 

• Vascular structures can mimic abscess. Be sure to accurately identify common abscess mimics. Always use color doppler before making an incision: abscess should never have central blood flow.

 

• Small foreign bodies may be difficult to see and can be time consuming to identify. The location of foreign bodies can also limit imaging if there is a hematoma or large wound around the object.

What is NOT normal?

Cellulitis

Cellulitis can have a highly variable appearance. In general, the subcutaneous tissue appears thickened and will have a “cobblestone” appearance. This is caused by subcutaneous edema and inflammation generated by the surrounding infection. Cobblestoning is not a specific finding to cellulitis but a finding in tissue edema, care must be taken to correlate to the clinical scenario. If uncertain, compare the area of interest to an unaffected area of skin.

 

Figure 3: Cobblestoning

Cobblestone appearance of cellulitis	Cobblestone sidewalk

 

Video 4: Soft tissue cobblestoning

 

Pitfalls

Edema from any source including:

• Insect bites
• Trauma
• Generalized edema due to systemic causes

Abscess

Abscesses appear as heterogeneous anechoic, isoechoic (gray) or hypoechoic fluid collections. Their appearance is often not uniform. Abscesses can be well-circumscribed or be irregularly shaped areas of fluid which may demonstrate posterior acoustic enhancement (bright white in the far field). As with cellulitis, measure the size of the abscess in both the transverse and longitudinal plane. There is some evidence from an immunocompetent pediatric emergency department population that abscesses larger than 5 cm in diameter predict need for hospitalization [14]. In addition, measure the depth from the skin and the width from the base in order to guide future attempts at incision and drainage. As part of the evaluation, apply firm and graded pressure with the probe over the abscess to look for fluctuance (often over a point of maximal tenderness) and the “swirl sign.” Swirl sign refers to the movement of fluid and debris within the abscess and is generated by compression over the area. Finally, always evaluate the abscess with color Doppler to look for internal blood flow and surrounding vasculature prior to making an incision.

 

Video set 5: Examples of soft tissue abscesses

Courtesy of EDSONOshare library

 

Video 6: Video illustrating a positive “Swirl Sign”

 

Practice Pearl: Prior to performing any incision and drainage of an abscess, use color Doppler to look for evidence of blood flow. In addition, color Doppler can be used to identify surrounding blood vessels in order to decrease the rate of procedural complications.

 

Figure 4: Color Doppler over a suspected abscess

This mass was a clinically suspected abscess but bedside PoCUS identified significant blood flow. Planned incision was aborted and the patient was referred to surgery for further management.

Pitfalls:

• Hematoma
• AV malformation
• Lymph nodes
• Cysts

Necrotizing Fasciitis

Due to the low incidence and high morbidity of necrotizing soft-tissue infections, it remains a diagnosis that requires physicians to maintain a high clinical index of suspicion. The use of bedside ultrasound should never delay surgical consultation for these patients but should instead be used to expedite diagnosis. There are three main sonographic findings in necrotizing fasciitis. These include subcutaneous air (often tracking along a fascial plane) with accompanying “comet-tail” artifact, diffuse subcutaneous tissue thickening (which often requires comparison to a contralateral side), and >5mm of fluid accumulation along a facial plane [10].

 

Figure 5: Sonographic findings suggestive of necrotizing infection

 

Video 7: PoCUS suggesting necrotizing infection

Courtesy of EDSONOshare Library

 

Foreign bodies

Research has shown that over one-third of hand foreign bodies are missed at the index visit. Bedside ultrasound can be used to both diagnose foreign bodies and plan removal by accurately identifying the depth of the target and surrounding structures which may need to be avoided (i.e., vasculature). Ultrasound is superior to plain films for radiolucent objects such as plastic, wood or plant matter. When using ultrasound to guide removal of a foreign body, be sure to measure the distance from the skin surface with the calipers and scan the object in 2 planes to fully characterize its length, depth and width.

Practice Pearl: When using ultrasound to guide removal of a foreign body, consider placing multiple high gauge needles to mark the location of the object prior to dissecting down to find and then remove the foreign body. This can save a lot of time and avoid unnecessarily damaging tissue.

 

Figure 6: Foreign bodies on bedside ultrasound

Wood	Glass	Metal
Wood often demonstrates attenuation artifact or acoustic shadow.	Glass often demonstrates reverberation or comet tail artifact.	Metal often demonstrates reverberation artifact on ultrasound.

 

Video set 8: Soft tissue foreign bodies

Wood

Glass

Metal

Courtesy of EDSONOshare library

 

Pitfalls:

• Fascia
• Bones
• Subcutaneous air

What is normal?

Normal skin has a highly organized structure. The most superficial structures are the epidermis and dermis, which are often difficult to distinguish from each other. They will appear as a hyperechoic line immediately superficial to the globules of relatively hypoechoic subcutaneous fat. Within the hypodermis you may visualize blood vessels, nerves, and lymph nodes of varying echogenicity. Deep to the subcutaneous fat, the fascial layer appears as a hyperechoic layer immediately superficial to the more organized hypoechoic and striated muscle fibers. Lastly, depending of the depth setting, a hyperechoic and smooth bone cortex may be visualized.

 

Figure 1: Anatomy of Normal Skin

Courtesy of Matt Tabbut @ mtabbut

 

Video 2: Video illustrating normal skin and soft tissue

 

Lymph nodes can often mimic abscesses as they can be well circumscribed with hypoechoic central areas. In children, in particular reactive lymph nodes are often clinically challenging to distinguish from abscess. However, sonographically lymph nodes, are more oval and organized in shape than abscesses. Classically, lymph nodes will display a central stalk or hilum that is hyperechoic (bright white). At the base of the stalk, there is accompanying vasculature which will demonstrate flow when color Doppler is applied. Lymph nodes should not have surrounding signs of cellulitis or cobblestoning. Lastly, there is less posterior acoustic enhancement in lymph nodes than in abscesses.

 

Figure 2: Sonography of a normal lymph node

 

Video 3: Lymph node video

Skin and Soft Tissue Ultrasound: A Stepwise Approach

Indications

• Evaluate soft tissue skin infection
• Differentiate cellulitis from abscess
• Evaluate for features of necrotizing fasciitis
• Identify foreign bodies

Equipment

• Ultrasound machine
• High-frequency linear probe (8-12 MHz)
• Water bath or step off pad – if needed
• Sterile probe sheath –if needed

Patient Positioning

1. Place the patient in a position of comfort and expose the area of interest.
2. Palpate the area of interest ± mark any areas of maximal tenderness or fluctuance.
3. Consider allowing access to the opposite side as a “normal” comparison

Ultrasound Machine Preparation

1. Ensure the ultrasound screen is positioned in order to be clearly seen.
2. Select the high-frequency linear ultrasound probe.
3. Consider whether use of a sterile sheath is clinically indicated.
4. Choose a vascular or soft-tissue pre-set on the ultrasound machine.
5. Select depth of field to between 4-6 cm.
6. Focus gain to clearly visualize the soft tissue structures of interest.

Scanning Principles

1. Place the linear probe gently over the area in question.
2. Adjust the depth and optimize the gain to ensure that the fascial plane is visualized.
3. Slowly and systematically scan across the area of interest.
4. Scan in both the transverse and longitudinal planes in order to not miss any findings.
5. Use color doppler to help identify and differentiate vascular structures

 

Clinical Pearl: The Dead Zone 

The “dead zone” prevents visualization of superficial structures in the near field. This is caused by the ultrasound machine’s inability to send and receive sound waves simultaneously. The depth of the dead zone decreases as frequency increases, and with higher quality send/receive function of the machine and probe. As technology improves the dead zone becomes less clinically significant.

 

Tips and Tricks: “What if my area of interest is very superficial?”
A water bath or standoff pad (such as an IV fluid bag or glove filled with water) can be used to alleviate patient discomfort and improve image quality of very superficial objects. Ensure that the probe is perpendicular to the skin at all times as an oblique angle can cause false hypoechogenicity.   Note: A recent study showed that although most (89%) new ultrasound probes have a “dead zone” of 0 mm, older probes have a “dead zone” of up to 3 mm [13].
Water Bath	Standoff Pad

 

Video 1: Video of normal finger in a Waterbath

From EDSonoshare Library

Introduction

Point-of-care ultrasound is a useful tool for evaluating patients with skin or soft tissue complaints. It can accurately differentiate between cellulitis and abscess, aid in the diagnosis of necrotizing soft tissue infections and assists in foreign body localization. In addition, the use of point-of-care ultrasound has been shown to decrease wait times and improve patient satisfaction with emergency department care [1,2].

Why ultrasound?

Patient presentations for soft tissue infections are common. Over the last 40 years, the number of children presenting to emergency departments with skin and soft tissue infections has doubled [3]. Ultrasound is invaluable when examining soft tissue and has much better test characteristics than physician judgement and physical exam combined for differentiating between abscess and cellulitis [4]. With the growing prevalence of MRSA and the need for different management plans for cellulitis versus abscess, being able to differentiate between the two entities is important for frontline providers.

PoCUS has been shown to have a sensitivity of 98%, specificity of 88%, positive predictive value of 93% and negative predictive value of 97% for the diagnosis of cellulitis this is significantly better than clinical exam alone which has a sensitivity of 86%, specificity of 70%, positive predictive value of 81% and negative predictive value of 77% [4]. Additional studies have shown that ultrasound can be used to identify abscesses with a positive likelihood ratio of 5.5 and negative likelihood ratio of 0.04 [5]. Test characteristics are similar among both adult and pediatric patients [6,7]. A prospective cohort study of adult patients presenting to an urban ED with clinical soft-tissue infection (but no obvious abscess) found that 48% of patients who were clinically thought to be cellulitis alone, were found to have abscesses requiring incision and drainage when examined with ultrasound [8]. A similar study in pediatric patients found the use of PoCUS changed management in 22% of patients [9]. Point-of-care ultrasound also helps differentiate abscess from other pathology, thereby decreasing the chance of complications arising from incision and drainage. In complicated cases, the completeness of abscess drainage can be assessed, reducing the rate of treatment failure.

PoCUS can also aid in differentiating necrotizing fasciitis from other infections. When compared to biopsy findings, PoCUS demonstrating diffuse subcutaneous tissue thickening and fluid accumulation along the fascial plane has a sensitivity of 88% and specificity of 93% for necrotizing fasciitis [10].

Table 1:

Diagnostic Accuracy of Ultrasound in Soft Tissue Infection
	Cellulitis	Abscess	Necrotizing Infection
Sensitivity	98%	97% (CI: 94-98%)	88.2% (CI: 63.6-98.5%)
Specificity	88%	83% (CI: 75-88%)	93.3% (CI: 81.7-98.6)
Positive LR	—	5.5 (CI: 3.7-8.2)	—
Negative LR	—	0.04 (CI: 0.02-0.08)	—
Positive PV	93%	—	83.3% (CI: 84.5-99.4%)
Negative PV	97%	—	95.4% (CI: 82.2-97.3%)

 

Ultrasound is also a useful tool for identification of soft tissue foreign bodies. Plain film radiography can only identify radiopaque objects, while ultrasound is able to identify both radiopaque and non-radiopaque foreign bodies. In addition, ultrasound can be used procedurally to guide foreign body removal and to confirm the success of the procedure. Ultrasound is superior to x-ray for radiolucent foreign bodies, with a sensitivity of 87% and specificity of 97% for wooden foreign bodies larger than 2.5 mm in length [11] However, sensitivity and specificity decreases compared to x-ray for radiopaque foreign bodies. A good general practice when faced with a patient concerned about a soft tissue foreign body is to start with bedside ultrasound and then move to further imaging if the result of the ultrasound is negative.

Table 2:

Diagnostic Accuracy of Ultrasound in Retained Soft Tissue Foreign Bodies
	Foreign Body	Radiolucent Foreign Body
Sensitivity	72% (95% CI: 57-83%)	96.7% (95% CI: 90.0-99.1%)
Specificity	92% (95% CI: 88-95%)	84.2% (95% CI: 72.6-92.1%)
Positive LR	3.2 (95% CI: 2.1-5.1)	5.5 (CI: 3.7-8.2)
Negative LR	0.11 (95% CI: 0.08-0.16)	0.04 (CI: 0.02-0.08)

[12]

 

 

1. Grossbard GD, Love BR. Femoral nerve block: a simple and safe method of instant analgesia for femoral shaft fractures in children. Aust N Z J Surg. 1979 Oct;49(5):592-4. PubMed PMID: 292412.

2. Ting PL, Sivagnanaratnam V. Ultrasonographic study of the spread of local anaesthetic during axillary brachial plexus block. Br J Anaesth. 1989 Sep;63(3):326-9. PubMed PMID: 2679832.

3. Mariano ER, Marshall ZJ, Urman RD, Kaye AD. Ultrasound and its evolution in perioperative regional anesthesia and analgesia. Best Pract Res Clin Anaesthesiol. 2014 Mar;28(1):29-39. PubMed PMID: 24815965.

4. Marhofer P, Greher M, Kapral S. Ultrasound guidance in regional anaesthesia. Br J Anaesth. 2005 Jan;94(1):7-17. Epub 2004 Jul 26. Review. PubMed PMID: 15277302.

5. Turner AL, Stevenson MD, Cross KP. Impact of ultrasound-guided femoral nerve blocks in the pediatric emergency department. Pediatr Emerg Care. 2014 Apr;30(4):227-9. PubMed PMID: 24651214.

6. Tondare AS, Nadkarni AV. Femoral nerve block for fractured shaft of femur. Can Anaesth Soc J. 1982 May;29(3):270-1. PubMed PMID: 7074406.

7. Baker MD, Gullett JP. Ultrasound-Guided Femoral Nerve Blocks. Pediatr Emerg Care. 2015 Dec;31(12):864-8; Review. PubMed PMID: 26626896.

8. Szucs S, Morau D, Iohom G. Femoral nerve blockade. Med Ultrason. 2010 Jun;12(2):139-44. Review. PubMed PMID: 21173942.

9. Ilfeld BM, Loland VJ, Sandhu NS, Suresh PJ, Bishop MJ, Donohue MC, Ferguson EJ, Madison SJ. Continuous femoral nerve blocks: the impact of catheter tip location relative to the femoral nerve (anterior versus posterior) on quadriceps weakness and cutaneous sensory block. Anesth Analg. 2012 Sep;115(3):721-7. PubMed PMID: 22745116.

10. Szűcs S, Morau D, Sultan SF, Iohom G, Shorten G. A comparison of three techniques (local anesthetic deposited circumferential to vs. above vs. below the nerve) for ultrasound guided femoral nerve block. BMC Anesthesiol. 2014 Jan 25;14:6. PubMed PMID: 24460975.

 

• The ultrasound-guided femoral nerve block provides safe and effective analgesia for femur fracture, patellar injuries and anterior thigh wound care.
• To successfully perform an ultrasound-guided femoral nerve block one must identify the femoral nerve within the femoral crease and surround the nerve with local anesthetic.
• Care must be taken to place the needle tip correctly both to improve success of the procedure and minimize complications.

Patient Positioning

1. Place the patient in supine position and expose the groin.
2. Externally rotate and slightly abduct the hip (if the patient will tolerate).
3. Palpate mark the anterior superior iliac spine (ASIS) and pubic symphysis (PS).
4. Palpate mark the inguinal crease (the line between the ASIS and PS).
5. Palpate mark the femoral artery (it should lie 1-2 cm beneath the inguinal ligament).
6. If desired, identify sonographic landmarks prior to preparing sterile field

 

Ultrasound Machine Preparation

1. Ensure the ultrasound screen is positioned in order to be clearly seen.
2. Select the high-frequency linear ultrasound probe.
3. Choose a nerve pre-set (e.g., compound or tissue harmonic imaging enabled) and needle enhancement pre-set if available
4. Select depth of field and place the femoral nerve in the center of the screen.
5. Focus gain to clearly visualize the femoral nerve and its surrounding structures.

 

Create a Sterile Field

1. Prep the skin and prepare a sterile field
2. Prepare the US with a sterile transducer cover

 

Stepwise Approach to Sonographic Identification

1. Place the probe on the patient’s groin in the transverse plane (figure 7).
2. Ensure the probe marker is oriented towards the patient’s right.
3. Identify the circular structure of the femoral artery in transverse axis.
4. Identify the femoral vein which will lie medial to the femoral artery. Note: If you are seeing three vascular structures in your frame of view, simply slide your probe more proximally (cephalad) towards the inguinal crease as you have likely placed your probe too low and are below the level where the femoral artery trifurcates into the profunda femoris, the medial curcumflex femoris and the superficial femoral arteries
5. Identify the femoral nerve. It will appear as an echogenic structure (often circular, flat or triangular) which will lie 1-2 cm lateral to the femoral artery. It will appear immediately superior to the hypoechoic iliopsoas muscle.
6. Identify the fascia iliaca. It will appear as a faint linear hyperechoic structure which will be superficial to the femoral nerve and will be deep to the fascia lata. In order to provide a satisfactory femoral nerve block, the needle tip will need to pierce the fascia iliaca and inject the local anesthetic in a circumferential manner around the femoral nerve.

 

Needling Approach

There are two approaches that you may use for needle insertion. For the purpose of this module we will describe the in-plane approach given that without the use of an echogenic needle, it is difficult to accurately visualize the needle tip using the out-of-plane approach (figure 7).

 

Figure 7: In-plane technique

1. Ensure the probe is in the transverse plane with the probe marker to patient’s right.
2. Ensure the femoral nerve is directly in the center of the screen (as outlined above).
3. Raise a superficial weal (1-2 cc) of local anesthetic at the site of needle insertion.
4. Insert the needle into the skin with the needle tip centered on the probe and with the direction of needle insertion parallel to the probe. Note that the more acute the angle of approach in relation to the skin, the more easily the needle will be visualized on ultrasound (figure 11).
5. As you advance the needle, note the angle and depth of approach of the screen.
6. Maintain the needle shaft directly in plane with the ultrasound beam.
7. Advance the needle towards the femoral nerve (the goal is to direct the needle tip to be immediately lateral or superior to the nerve and deep to the fascia iliaca) (figure 8).

 

Figure 8: Needle placement

 

With the in-plane technique both the needle shaft and tip should be directly visualized along their full length as the needle is advanced towards the femoral nerve. It is particularly important to identify the needle tip as it can easily fall out of the field of view of the thin ultrasound beam leading to incorrect placement of local anesthetic.

 

Techniques to aid needle tip visualization:

• If the needle is not seen in the US field, try to sweep or fan the probe to identify both the needle and the area of interest into view
• If only a portion of the needle is seen, try rotating the probe to bring the full course of the needle into view
• If probe manipulation is unsuccessful you can use hydro-localization to help identify the needle, and its tip more clearly (figure 9).

 

Figure 9: Hydro-localization

If probe manipulation has been unsuccessful in locating the needle tip, hydro-localization can be used instead. To do this, you simply inject a small (1-3 cc) amount of D5W into the field and the hypoechoic fluid should help locate your needle tip.

Do NOT use local anesthetic for hydro-localization without first attempting to withdraw on the syringe to prevent intravascular injection.

 

Ultrasound Confirmation of Injection: What is normal?

1. Prior to injection of any local anesthetic agent, ensure that you are confident that the needle tip is positioned just lateral to the femoral nerve with the tip directed so that the injected anesthetic will be directed underneath the femoral nerve (the injection of local anesthetic beneath the femoral nerve will improve the echogenicity of the nerve and aid with further visualization).
2. Once you are satisfied with your needle tip location and have aspirated to ensure that the needle tip is not in a vessel, inject a small volume of the local anesthetic.
3. As you are injecting, you may see distension of the femoral nerve sheath (figure 10).
4. You may need to re-position the needle tip in order to direct the local anesthetic circumferentially around the femoral nerve. This circumferential injection of local anesthetic hastens block onset.

Note: If you encounter resistance to injection of local anesthetic, stop injecting immediately and withdraw your needle due to the possibility of intra-neuronal injection.

Figure 10: Femoral nerve sheath distension following local anesthetic injection.