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Cellulitis

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Authors

Christopher Guyer, MD, FACEP, CAQ-SM
Assistant Professor, Wayne State University
Adjunct Physician Instructor, University of Michigan
Senior Staff Physician, Henry Ford Health System

Gisele Papo, MD
Ultrasound Fellow
Henry Ford Health System

Summary

  • Point of care ultrasound is a key diagnostic modality to aid clinicians in the diagnosis of cellulitis.
  • Physical examination alone can be inadequate in differentiating cellulitis from other diagnoses. 
  • Ultrasound can be used to discern cellulitis from abscess and lymphadenopathy and identify nearby structures.
Skin US classic example of cellulitis cobblestoning
Typical sonogram of cellulitis with hyperechoic fat lobules separated by hypoechoic fluid-filled areas, classically described as “cobblestone” appearance.[1]Case courtesy of Dr Hadiel Kaiyasah, Radiopaedia.org, rID: 46177
Cellulitis of the pretibial skin in short axis, scanning from the top of the leg down towards the ankle.
5 Minute Sono video with Dr Avilla reviewing cellulitis.

Introduction

  • Soft tissue infections are commonly encountered in the Emergency Department.
  • The most common infections seen are cellulitis and abscesses. Even experienced clinicians may have difficulty differentiating cellulitis and abscess as they share many common clinical features.[2]Hersh AL, Chambers HF, Maselli JH, Gonzales R. National trends in ambulatory visits and antibiotic prescribing for skin and soft-tissue infections. Arch Intern Med. Jul 28 2008;168(14):1585-91. … Continue reading
  • Point of care ultrasound is an excellent modality to aid in the diagnosis of cellulitis and to help differentiate it from abscess.[3]Knaysi G, Ringelberg J, Stadlberger N, Soucy Z. Point of Care Ultrasound Use by Associate Providers for Differentiating Abscess Versus Cellulitis Skin and Soft Tissue Infection in the Emergency … Continue reading
  • The high resolution of ultrasound and the ability to perform dynamic testing like compression of structures are two big advantages to using this imaging modality when evaluating skin infections.
  • Physical examination alone can be inadequate and lead to unnecessary incision and drainage or, conversely, a missed opportunity for a needed procedure.
  • Minimal training is required to become proficient in sonographically distinguishing cellulitis from abscess and other soft tissue pathology.[4]Knaysi G, Ringelberg J, Stadlberger N, Soucy Z. Point of Care Ultrasound Use by Associate Providers for Differentiating Abscess Versus Cellulitis Skin and Soft Tissue Infection in the Emergency … Continue reading[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. Nov 2016;23(11):1298-1306. doi:10.1111/acem.13049

Anatomy

  • The skin is a protective barrier covering the entire outside of the body and represents the body’s largest organ. 
    • Cellulitis and abscesses can occur anywhere on the skin.
    • Cellulitis is most commonly seen on the feet and the legs.
    • Abscesses most commonly occur on the face, back, chest, buttocks, underarms, and groin.[6]Bystritsky R, Chambers H. Cellulitis and Soft Tissue Infections. Ann Intern Med. Feb 6 2018;168(3):ITC17-ITC32. doi:10.7326/AITC201802060
Normal appearance of skin under ultrasound with labels.[7]Image courtesy of www.acep.org, “Soft Tissue Ultrasound”
  • On ultrasound an examiner can visualize the epidermis/dermis, subcutaneous layer (comprised of fat and connective tissue), fascia, muscle, and cortical bone.
    • The epidermis and dermis are identified as a single thin superficial hyperechoic layer.
    • The thicker deeper subcutaneous layer appears as hypoechoic fatty tissue with hyperechoic fibrous septa interspersed between fat globules.
    • Fascia has a thin linear hyperechoic appearance.
    • The muscular layer is viewed as bright fibrillar striations. Finally, the bony cortex is seen as a dense hyperechoic line with acoustic shadowing.
  • The visibility and size of the structures can vary based on a patient’s body habitus (i.e., thin, obese, muscular, etc.).[8]Baston CM, Moore C, Krebs EA, Dean AJ, Panebianco N. eds. Pocket Guide to POCUS: Point-of-Care Tips for Point-of-Care Ultrasound. McGraw Hill; 2018.

Ultrasound Transducer and Settings

  • These studies are usually performed with a high frequency (5-12 MHz or greater) linear array transducer.
  • To improve resolution an acoustic standoff pad can be utilized.
    • Scanning using a water bath technique can also be considered for imaging of the distal extremities.[9]O’Rourke K, Kibbee N, Stubbs A. Ultrasound for the Evaluation of Skin and Soft Tissue Infections. Mo Med. May-Jun 2015;112(3):202-5.
  • We recommend using a barrier dressing (e.g. Tegaderm) or a probe cover to cover the end of the probe
    • This is especially true if the skin is not intact or if lesions are draining.
  • Technique
    • A generous amount of ultrasound coupling gel should be applied to the area of concern to allow for imaging of the superficial layers of the skin and to minimize the pressure applied to the patient.
    • Scanning the adjacent area or contralateral side can be beneficial for comparison and to establish baseline anatomy.
    • The area of concern should be scanned in both long and short axis.
    • Tissues should be lightly compressed to look for motion of the contents of fluid collections.
    • If a fluid collection is visualized it should be closely assessed for internal echoes, septations, and acoustic enhancement.
  • A complete study should also include evaluation for subcutaneous air or foreign bodies, and depth from the skin to any areas of abnormality or vital structures.
  • Color doppler is also useful and can demonstrate hypervascularity as well as define adjacent structures such as neurovascular bundles.[10]Creditt AB, Joyce M, Tozer J. Skin and Soft Tissue Ultrasound. Clinical Ultrasound: A Pocket Manual. Springer International Publishing; 2018:267-277.
    • Elastography and compound imaging may also be of diagnostic benefit.

Pathology

  • Cellulitis is commonly described as having a “cobblestone” appearance on ultrasound. This is a result of anechoic fluid tracking between fat lobules.
  • Similar changes can be seen with soft tissue edema from noninfectious causes like heart failure, lymphedema, and venous insufficiency.
  • In a patient with cellulitis no discrete fluid collection will be visualized.
  • Acoustic enhancement can be seen if heavy cobblestoning is present but this will be more diffuse than that seen with an abscess.[11]Creditt AB, Joyce M, Tozer J. Skin and Soft Tissue Ultrasound. Clinical Ultrasound: A Pocket Manual. Springer International Publishing; 2018:267-277. [12]Comer AB. Point-of-Care Ultrasound for Skin and Soft Tissue Infections. Adv Emerg Nurs J. Oct/Dec 2018;40(4):296-303. doi:10.1097/TME.0000000000000208
Hand US Cellulitis of the hand with cobblestoning
Ultrasound image demonstrating cellulitis of the dorsal hand. The dermis is thickened with anechoic fluid tracking between subcutaneous tissues creating a cobblestone appearance. Note the fluid does not track into the tendon sheath.
  • Cellulitis may show hypervascularity or hyperemia 
    • Color doppler can differentiate abscess from lymph nodes and vascular structures.
Ultrasound of the thigh showing cellulitis in long axis.
Ultrasound of the thigh showing cellulitis in short axis.
  • Abscesses demonstrate many unique ultrasound characteristics:
    • They will appear as a fluid collection with an irregular border.
    • The material can be of mixed echogenicity because of pus, tissue liquefaction, and cellular debris.
    • Chronic abscesses can be septated.
Skin US Abscess example
Abscess containing hyperechoic debris.[13]Image courtesy of acep.org, “Abscess Evaluation”
US normal cervical lymph node
Normal cervical lymph node.[14]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.
Skin US Soft Tissue Gas
Necrotizing fasciitis as demonstrated bymultiple hyperechoic foci with reverberation artifact and ”dirty” shadowing (arrowheads)[15]Kim DJ, Kendall JL. Fournier’s gangrene and its characteristic ultrasound findings. J Emerg Med. 2013;44(1):e99-101.
ying yang sign
Pseudoaneurysm and the so-called Yin-Yang Sign[16]Thomassen, Irene, et al. “Treatment of temporal artery pseudoaneurysms.” Vascular 22.4 (2014): 274-279.

Additional Considerations

  • Benefits
    • Point of care ultrasound is an excellent diagnostic modality to aid clinicians in the diagnosis of abscess and cellulitis.
    • Ultrasound can also be used to discern these diagnoses from lymphadenopathy and identify nearby vascular structures.
    • Ultrasound is also beneficial in determining the need for incision and drainage and accurate localization.
  • Challenges
    • Air in an abscess and foreign bodies may be difficult to tell from one another.
    • Perirectal abscesses can present as an infection on the medial area of the buttocks and additional evaluation may be required for accurate diagnosis.
    • Body habitus can also be a limiting step as soft tissue evaluation becomes more challenging in larger patients.

Pearls & Pitfalls

  • Color doppler should be used to evaluate for hypervascularity and nearby neurovascular bundles.
  • The depth to any lesions as well as the length, width, and height should all be recorded.
  • Always be certain to visualize the far wall of an abscess. Additional physical exam or other imaging modalities like CT may be needed for correct diagnosis.
  • An abscess with echogenic pus may be difficult to recognize. Optimize gain, and look for posterior acoustic enhancement and pustalisis.
  • Cellulitis with extensive cobblestoning can be difficult to distinguish from an abscess.

References

References[+]