Abscess
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Authors
Matthew Negaard, MD, CAQ-SM
Assistant Clinical Professor of Emergency Medicine
University of Iowa Hospitals and Clinics
Sports Medicine Physician
Methodist Sports Medicine (Indianapolis, Indiana)
Summary
- Ultrasound is an accurate way to help identify soft tissue infections and especially abscesses
- A high-frequency linear transducer is optimal for identifying abscesses in the soft tissues less than 4 cm from the surface
- An abscess appears as a well-circumscribed hypoechoic or anechoic collection
- Ultrasound can help aid in treatment decision of skin and soft tissue infections (i.e. antibiotics vs aspiration vs incision and drainage)
Sonographic evolution of an abscess in a porcine model demonstrating various phases of the presentation.[1]Leotta DF, Bruce M, Wang Y-N, et al. Sonographic analysis of abscess maturation in a porcine model. bioRxiv. Published online 2020:2020.09.28.317321.
Clinical example of abscess.[2]Medina Velasco AA, Gemio Del Rey I, de la Plaza Llamas R, Arteaga Peralta V, Ramia JM. Inguinal abscess as presentation of a right colon cancer. A systematic review. Rev Esp Enferm Dig. … Continue reading
Introduction
- The use of ultrasound has been shown to help identify the presence of a skin and soft tissue abscess in the Emergency Department (ED) in adults as well as pediatric patients [1-3].[3]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;23(11):1298-1306.[4]Gottlieb M, Avila J, Chottiner M, Peksa GD. Point-of-care ultrasonography for the diagnosis of skin and soft tissue abscesses: A systematic review and meta-analysis. Ann Emerg Med. 2020;76(1):67-77.[5]Mower WR, Crisp JG, Krishnadasan A, et al. Effect of initial bedside ultrasonography on emergency department skin and soft tissue infection management. Ann Emerg Med. 2019;74(3):372-380.
- While the clinical sensitivity for diagnosing an abscess is relatively high (90.3%), ultrasound has been shown to appropriately change management in 84% of patients in which it was unclear whether an abscess was present on clinical exam.[6]Mower WR, Crisp JG, Krishnadasan A, et al. Effect of initial bedside ultrasonography on emergency department skin and soft tissue infection management. Ann Emerg Med. 2019;74(3):372-380.
- A systematic review and meta-analysis performed by Gottlieb, et al in 2020 showed in all patients, ultrasound had a positive likelihood ratio of 6.5 and negative likelihood ratio of 0.06 for the presence or absence of abscess.[7]Gottlieb M, Avila J, Chottiner M, Peksa GD. Point-of-care ultrasonography for the diagnosis of skin and soft tissue abscesses: A systematic review and meta-analysis. Ann Emerg Med. 2020;76(1):67-77.
Ultrasound guided aspiration of an abscess. Note the fluid is being pulled towards the needle.
- The same analysis showed in clinical cases where it was unclear whether an abscess was present, ultrasound had a sensitivity of 91.9% and specificity of 76.9% for the presence of an abscess.[8]Gottlieb M, Avila J, Chottiner M, Peksa GD. Point-of-care ultrasonography for the diagnosis of skin and soft tissue abscesses: A systematic review and meta-analysis. Ann Emerg Med. 2020;76(1):67-77.
- Furthermore, ultrasound can identify the size and depth of an abscess which may predict which patients may fail treatment with antibiotics alone. In a retrospective review of 162 patients discharged from the ED without drainage of ultrasound confirmed abscess, it was found those with abscess greater than 0.4cm from the surface were at greater risk of antibiotic alone treatment failure.[9]Russell FM, Rutz M, Rood LK, McGee J, Sarmiento EJ. Abscess size and depth on ultrasound and association with treatment failure without drainage. West J Emerg Med. 2020;21(2):336-342.
- Overall, the use of point-of-care ultrasound for identifying abscesses in the ED is a skill set that has the potential to add to the diagnostic accuracy of skin and soft tissues abscesses and may help identify those abscesses that may require drainage compared to antibiotic treatment alone.[10]Gottlieb M, Avila J, Chottiner M, Peksa GD. Point-of-care ultrasonography for the diagnosis of skin and soft tissue abscesses: A systematic review and meta-analysis. Ann Emerg Med. 2020;76(1):67-77.[11]Russell FM, Rutz M, Rood LK, McGee J, Sarmiento EJ. Abscess size and depth on ultrasound and association with treatment failure without drainage. West J Emerg Med. 2020;21(2):336-342.
Ultrasound Transducer and Settings
- First optimize the patient’s position that gives you the best ability to reach the area of concern while still in arms length of the ultrasound machine to adjust its settings as needed.
- When an area is identified as concerning for an abscess, you must first consider the amount of adipose tissue overlying the area of concern in addition to how deep the suspected abscess is from the surface.
- For more areas of superficial concern (<4cm from the surface) with no more than a moderate layer of adipose tissue, a high-frequency linear transducer (10MHz or greater) is the optimal transducer to identify skin and soft tissue abscesses.
- For areas of concern that are >4cm from the surface or there is a large amount of adipose tissue, a low frequency curvilinear transducer should be considered. In these situations, other diagnostic modalities such as MRI or CT scan may be clinically indicated over ultrasound.
- When an area of concern is identified, center that area in your screen and optimize the depth to visualize approximately 0.5cm deep to the area of concern. This allows for identification of any acoustic shadowing in addition to identifying the entirety of the area of concern.
- Next, turn on your Doppler setting to help identify the structure as an abscess versus a vascular structure.
- If there is no hyperemia or flow present, you may increase the gain until flow is identified.
- You can then measure the abscess in long axis and in short axis. This is accomplished by centering the abscess in your screen, freezing the imaging and using the measuring tool on your ultrasound unit. You will then rotate the transducer 90 degrees, keep the same center point and measuring once more.
- Additionally, it is important to measure the distance from the most superficial portion of the abscess to the skin surface as this has significant management implications.
Ultrasound of the dorsal wrist showing abscess with surrounding cellulitis in short axis
Ultrasound of the dorsal wrist showing abscess with surrounding cellulitis in long axis
Pathology
- Sonographic findings of an abscess include a well-circumscribed hypoechoic or anechoic collection with posterior acoustic enhancement and no posterior shadowing.[12]Mower WR, Crisp JG, Krishnadasan A, et al. Effect of initial bedside ultrasonography on emergency department skin and soft tissue infection management. Ann Emerg Med. 2019;74(3):372-380.[13]Russell FM, Rutz M, Rood LK, McGee J, Sarmiento EJ. Abscess size and depth on ultrasound and association with treatment failure without drainage. West J Emerg Med. 2020;21(2):336-342.
- There is often hyperemia surrounding an abscess as well as hyperemia scattered within the abscess that can be visualized with Doppler imaging.
- Purulent material in the abscess may be visualized as hyperechoic material that can be seen swirling in the abscess
This ultrasound demonstrates a very subtle appearing abscess. Note that the purulent material moves around when compression is applied, giving it the so-called ‘Swirl Sign’.
Ultrasound demonstrating with posterior acoustic enhancement and posterior acoustic shadowing.
Ultrasound demonstrating hyperemia with color doppler.
- Often there can be sonographic evidence of surrounding cellulitis with the characteristic “cobblestone” sonographic appearance.
- “Cobblestoning” is characterised by hypoechoic or anechoic branches throughout the soft tissue. There is often hyperemia throughout this area. These findings are suggestive of cellulitis rather than a discrete abscess.
- It is not uncommon for there to be “cobblestoning” surrounding an abscess which is often formed from a central meeting point of the branches.
- Ultrasound can help identify the optimal location of incision in drainage (i.e. location of the abscess) in a large area of superficial erythema concerning infection.
Ultrasound demonstrating “cobblestoning” without an abscess (i.e. cellulitis).[14]Case courtesy of Dr Hadiel Kaiyasah, Radiopaedia.org, rID: 46177
Ultrasound of dorsal wrist showing abscess formation with surrounding cobblestoning and hyperemia on color doppler.[15]Case courtesy of Dr Subhan Iqbal, Radiopaedia.org, rID: 79387
- Depending on the location, vascular structures can be mistaken for abscesses given the well circumscribed characteristics they both have.
- The use of Doppler can help differential abscess from vascular structures. Vascular structures will have constant or pulsatile flow throughout the entire lumen of the center of the vessel (Image 3). In contrast, an abscess will often have surrounding hyperemia adjacent to the abscess and scattered hyperemia within the abscess (Image 2).
Pseudoaneurysm and the so-called Yin-Yang Sign[16]Thomassen, Irene, et al. “Treatment of temporal artery pseudoaneurysms.” Vascular 22.4 (2014): 274-279.
Normal cervical lymphd node.[17]Prativadi R, Dahiya N, Kamaya A, Bhatt S. Chapter 5 ultrasound characteristics of benign vs malignant cervical lymph nodes. Semin Ultrasound CT MR. 2017;38(5):506-515.
Pearls & Pitfalls
- Be aware of sonographic mimics of abscess, namely cysts, lymph nodes, vascular structures including aneurysm and pseudoaneurysm. The use of Doppler can help differentiate the different pathologies.
- In suspected abscess that are greater than 5cm deep or in patient with a large amount of adipose tissue over the site of suspected abscess, a curvilinear transducer can be attempted though other imaging modalities may be more appropriate (i.e. MRI or CT scan).
- Abscesses less than 0.4cm deep to the skin may respond to antibiotic therapy alone (controversial).
- Isolated “cobblestoning” is suggestive of cellulitis and not an abscess.
- Ultrasound can help identify the optimal location for incision and drainage by identifying the location of the abscess within a large area of superficial cellulitis.
References
References