FOAM Party, Part I: Abscess Diagnosis, at Faster than the Speed of Sound

Last month, as part of our regularly-scheduled Tuesday didactic conference, our residents participated in a Team-Based Learning exercise. This was not your typical TBL, as the purpose was to generate original FOAMed content. The residents, divided into teams, pored over a curated list of available FOAMed resources regarding a very bread-and-butter but still somewhat controversial topic in Emergency Medicine – abscess management. They critically appraised their most high-yield resource (utilizing the ALiEM AIR assessment tool as a blueprint), and delved into the primary literature as well. The teams worked together to generate their lists of “pearls” from each resource – the bits of information they found the most impactful. These pearls have been collated and edited to form the FOAM Party post below. This is Part 1 of a three-part series on diagnosis & management of abscesses. 

We hope this exemplifies the power of crowdsourcing and “swarm medicine” in generating FOAMed content. As always, your comments and feedback are appreciated! Enjoy!

Part 1, Diagnosis:

We all know what an abscess is – an accumulation of pus beneath the epidermal layers as a result of bacterial infection, usually with Staph or Strep spp [1]. It presents as a red, swollen, fluctuant, tender mass, possibly with active drainage of purulent material. Varying degrees of surrounding cellulitis and induration can be appreciated. Providers must determine the presence of an abscess – requiring incision & drainage (I&D) for treatment, versus cellulitis – treated with antibiotics alone.

Unfortunately, studies of abscess diagnosis indicate the issue is not always straightforward...

- A prospective, cross-sectional study of interrater reliability of exams of patients presenting to a pediatric ED with suspected skin or soft-tissue infection (SSTI) found only “fair” or “moderate” agreement (weighted k statistics ≈ 0.4) for diagnoses of lesions as abscesses and for determination of whether lesions required I&D [2]. Experience of the examining physicians did not impact this result.

- A similar study found “fair” or “moderate” agreement for assessment of purulent drainage, tenderness, and fluctuance, but no agreement was seen in assessment of induration (k = -0.08) [3]. For diagnosis of a lesion as an abscess, k = 0.39. For determination of whether a lesion required I&D, k = 0.44. Again, physician experience did not affect the results.

Our residents noted that they commonly use bedside ultrasound to help determine if there is a drainable abscess in indeterminate cases. Thus far, we feel the preponderance of published data favors the diagnostic utility of POCUS for SSTI:

- A study group led by Chao et al correlated ultrasound features of SSTI with clinical symptoms and lab values in 86 children believed to have SSTI [4]. They described four different stages of infection: 1) subcutaneous tissue thickening without disarray or pus accumulation, 2) disarray of subcutaneous tissue without pus accumulation, 3) disarray of subcutaneous tissues with pus accumulation, 4) disarray of subcutaneous tissues with frank abscess formation. Sonographic tissue disarray +/- pus accumulation was statistically significantly associated with longer duration of symptoms, high-grade fever, leukocytosis, and higher CRP levels.

Image credit: Sonoguide.com

- A prospective observational study by Squire et al compared the accuracy of clinical examination to ultrasound in differentiating cellulitis from abscess [5]. The composite reference standard used in this study was either the presence of pus on I&D or the resolution of symptoms on antibiotics alone if no I&D was performed. They showed that ultrasound was superior to clinical examination and physician judgment in sensitivity (98% vs 86%), specificity (88% vs 70%), PPV (93% vs 81%), and NPV (97% vs 77%). Of note, almost half of the patients who did not have I&D performed were lost to follow-up (25/51) – if even a small number of these ended up having an abscess, the diagnostic utility of POCUS would be significantly lower than the reported values.

- A prospective study of the impact of POCUS on management of SSTI without obvious signs of abscess was published in 2006 [6]. Examining physicians were asked to provide their pretest probability of need for an I&D and for presence of a visible fluid pocket. The results of the ultrasound examination changed the ED management plan in 56% of patients (95% CI 47% - 64%). Results of the ultrasound led to I&D being performed in 33 of 82 patients believed not to need drainage on initial exam (all of these I&D are reported as being positive for abscess). The ultrasound led to I&D not being performed in 16/44 patients initially believed to have needed it. These 16 patients did not return to the study hospital in the 72hr follow-up period of the study. Several other patients had further changes to their management plan including adjustment of incisional approach, need for specialist/surgical consult, and further imaging studies.

Image credit: Sonoguide.com

- The utility of POCUS for diagnosis of abscess in the pediatric population was examined by Sivitz et al in 2010 [7]. In this study, POCUS changed management plan after initial clinical assessment in 11/50 cases. Diagnostic utility was also assessed, with gold standard being positive I&D for pus or return visit within 1 week. POCUS demonstrated greater sensitivity than clinical exam (90% vs 75%) and similar specificity (83% vs 80%) in detecting abscess.

- Another prospective observational cohort study of pediatric patients with SSTI again tried to determine if results of POCUS had an effect on the treating physician’s management of the patient [8]. Compared with clinical exam, POCUS had increased sensitivity (97.5% vs 79%) and similar specificity (69% vs 67%). Unlike some earlier studies, these results affected clinical management in only a few cases.  Unfortunately there was no clearly-defined “gold standard” for abscess diagnosis or outcome measures, and there was no control group.

(See St. Emlyn’s excellent Journal Club assessment of this paper here.)

- The latest study to investigate the diagnostic utility of POCUS for SSTI in pediatric patients was published by Marin et al in 2013 [9]. In this study, the treating physician conducted a clinical exam and based his/her management accordingly. Completely separate from this, a study physician performed his/her own clinical exam, and then performed POCUS. These results were not known to the treating physician and did not affect management. The reference standard was positive I&D and follow-up at 2 days. This study did have a much larger sample size than prior studies, with over 700 lesions analyzed. Once again, there was only fair agreement in physicians’ clinical exams (k = 0.39). If both treating and study physicians’ exams agreed that the lesion in question was likely an abscess or likely not an abscess, then POCUS added nothing to the diagnostic utility of exam alone in terms of sensitivity, specificity, or accuracy. However, for lesions in which the clinical exam was uncertain, POCUS significantly improved sensitivity (78% vs 44%), along with PPV and NPV. The trend toward increased specificity for POCUS did not reach clinical significance.

Take-home:

- As a group, we feel our use of ultrasound in SSTI reflects the conclusions of the Marin study – probably unnecessary if the diagnosis is obvious, but potentially very helpful if you are uncertain.

- We feel there are additional benefits to ultrasound of these lesions, including improvement in planning I&D approach, identifying sensitive anatomic structures in proximity to the lesion, or potentially uncovering alternative diagnoses. The clinical applicability of these benefits have been described in the literature [10-12].

- We recognize the potential for overdiagnosis – of performing I&D on a lesion with an identifiable fluid pocket on ultrasound, but that may have otherwise improved on its own with conservative management +/- antibiotics had the POCUS imaging not been available. However, standard care at this point for a pus collection of any size is drainage, and until further study is done to delineate those lesions that may improve with medical management alone, we feel the identification of fluid collections indicating abscess is a useful application of POCUS.

Take this FOAM party on the road and check out these other excellent resources for ultrasound diagnosis of SSTI:

- Sonoguide on how to perform SSTI POCUS, with reference images.

- A video guide to SSTI POCUS can be found at EmergencyUltrasound Teaching.

- Summary and reference images from the PEM Fellows Blog.

- Further reference images at Sinai EM Ultrasound.

- Sonoguide on other soft-tissue POCUS findings.

- Test your knowledge of soft-tissue POCUS on EM Sono.

Keep on FOAMing,
C. Sam Smith, PGY-3 (@CSamSmithMD)

References:

[1] Singer AJ, Talan DA. Management of Skin Abscesses in the Era of Methicillin-Resistant Staphylococcus aureus. N Engl J Med 2014;370:1039-47.

[2] Marin JR, Bilker W, Lautenbach E, Alpern ER. Reliability of clinical examinations for pediatric skin and soft-tissue infections. Pediatrics. 2010 Nov;126(5):925-30.

[3] Giovanni JE, Dowd MD, Kennedy C, Michael JG. Interexaminer agreement in physical examination for children with suspected soft tissue abscesses. Pediatr Emerg Care. 2011 Jun;27(6):475-8.

[4] Chao HC, Lin SJ, Huang YC, et al. Sonographic evaluation of cellulitis in children. J Ultrasound Med. 2000;19:743-749.

[5] Squire BT, Fox JC, Anderson C. ABSCESS: applied bedside sonography for convenient evaluation of superficial soft tissue infections. Acad Emerg Med. 2005;12:601-606.

[6] 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;13:384-8.

[7] Sivitz AB, Lam SH, Ramirez-Schrempp D, Valente JH, Nagdev AD. Effect of bedside ultrasound on management of pediatric soft-tissue infection.  J Emerg Med. 2010 Nov;39(5):637-43.

[8] Iverson K, Haritos D, Thomas R, Kannikeswaran N. The effect of bedside ultrasound on diagnosis and management of soft tissue infections in a pediatric ED. Am J Emerg Med 2012;30:1347-51.

[9] Marin JR, Dean AJ, Bilker WB, Panebianco NL, Brown NJ, Alpern ER. Emergency ultrasound-assisted examination of skin and soft tissue infections in the pediatric emergency department. Acad Emerg Med. 2013 Jun;20(6):545-53. doi: 10.1111/acem.12148.

[10] Cardinal E, Bureau NJ, Aubin B, et al. Role of ultrasound in musculoskeletal infections. Radiol Clin North Am. 2001;39:191-201.

[11] Craig JG. Infection: ultrasound-guided procedures. Radiol Clin North Am. 1999;37:669-678.

[12] 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.