Ultrasound for the Evaluation of Skin and Soft Tissue Infections

Abstract

Skin and soft tissue infections, including abscesses and cellulitides, are common problems seen by physicians. The treatment of soft tissue infections varies depending on the depth of infection or the presence of a fluid collection requiring incision and drainage. Ultrasound is a valuable tool in the evaluation of skin and soft tissue infections, enhancing our ability to diagnose an abscess cavity or deeper infection and has been shown to be more reliable than clinical exam alone. The judicious use of ultrasound allows for more appropriate patient care and management of their underlying infection. It can prevent an unnecessary procedure or identify occult abscesses that may go on to develop a more severe infection requiring hospitalization. In this article, we discuss the utility of ultrasound as a diagnostic tool for skin and soft tissue infections, techniques to optimize scanning, and the potential drawbacks of its use.

Introduction

Skin and soft tissue infections (SSTIs) such as abscess, wound infection, and cellulitis are encountered frequently in the emergency department (ED) and primary care setting. Greater than three million ED visits per year in the US are due to SSTIs, accounting for approximately three percent of all ED visits.¹ Staphylococcus aureus and Streptococcus species are thought to cause the majority of these infections, though various other pathogens are occasionally cultured. The vast majority of SSTIs with purulence are caused by S. aureus. Community acquired methicillin-resistant S. aureus (CA-MRSA) has become the dominant strain in many locations.² Though the causes of cellulitis are often more difficult to elucidate due to lack of culture evidence, S. aureus and B-hemolytic streptococci are thought to be the culprit pathogen in the majority of cellulitis cases, as well.

The majority of SSTIs can be managed on an outpatient basis with treatments such as aspiration, suture removal (for infected wounds), or incision and drainage (I & D) in the case of SSTI with fluid collection, with or without oral antibiotics. Current recommendations for uncomplicated abscess are for I & D only, with empiric antibiotics to cover for CA-MRSA only in cases with complicating factors such as signs of systemic illness or immunocompromise. For full current SSTI management recommendations please see the Infectious Diseases Society of America’s 2014 guidelines on this topic.³ Determining the presence of a drainable fluid collection at the site of an apparent SSTI is important to clarify the appropriate treatment, as many SSTIs with fluid collections may be cured with a simple I & D. Occasionally patients will also require hospitalization for more advanced surgical procedures and/or intravenous antibiotics. Ascertaining the presence of a drainable fluid collection or deeper infection is essential to initiate the appropriate treatment from the onset and may help prevent unnecessary return visits, hospitalization, and morbidity and mortality.

There is a growing body of evidence that ultrasound (US) increases diagnostic accuracy in SSTIs and can influence treatment decisions.⁴ Using US the physician can demonstrate the presence of fluid collections, edema, or soft tissue stranding indicating inflammation. Signs of a deeper infection, such as peri-fascial fluid and subcutaneous emphysema may also be apparent via US imaging.⁵ One prospective ED study showed that US changed management in approximately fifty percent of adult patients clinically thought to have cellulitis, either by identifying an occult abscess or confirming the lack of one and eliminating the need for I & D.⁶ Other studies have shown that US is more sensitive than physical exam alone in the diagnosis of SSTIs in pediatric populations.^7,8 US may be especially helpful in cases of cellulitis with large areas of induration in which the presence of an abscess cannot be ruled out through physical exam alone or in cases of failed treatment where an occult abscess may be the culprit.⁹ Ascertaining the presence of any of the above can be invaluable to the treating physician in initiating the appropriate therapy.

US is fast, cost-effective, portable, readily available in many practice settings, and does not expose patients to ionizing radiation. Studies also indicate that US for soft tissue is an easily learned skill and that ED physicians can accurately interpret them.¹⁰ Though CT and MRI are both sensitive and specific for soft tissue infections, they are more costly, often more difficult to obtain quickly, and, in the case of CT, expose the patient to the risks of IV contrast and ionizing radiation. In the case of ill, unstable, or hospitalized patients, US has the advantage of bedside capabilities.

US can locate a fluid collection thus determining the need for an I & D and it may help determine the need for consultation or further imaging. If no fluid collection is found, it may help avoid unnecessary procedures.⁶ A foreign body contributing to an infection may also be located.¹¹ There is evidence that US can indicate the responsible pathogen for SSTIs as abscesses due to CA-MRSA have been shown to have different characteristics such as an irregular shape and indistinct borders.¹² As noted above, US may be useful for demonstrating the presence of subcutaneous emphysema and/or deep tissue stranding or edema which are characteristic of necrotizing fasciitis.⁵ Early recognition of deep or necrotizing infections is key to their diagnosis and treatment since early surgical intervention is vital to prevent further morbidity and mortality.

As discussed, skin and soft tissue ultrasound is a skill that is easily learned and interpreted. Ideally, a high frequency probe should be selected to scan the area of interest. A high-frequency probe (greater than 8 MHz, often referred to as a vascular probe), allows for greater image resolution and clarity than low-frequency probes when scanning superficial structures. The linear footprint of a high frequency probe has a flat face which allows maintenance of good contact with the skin. Using ample amounts of ultrasound gel will decrease the amount of air between the transducer and the patient’s skin, producing a cleaner image.

Obtaining high quality US images in SSTI may occasionally prove difficult because of patient discomfort or because of a lesion on a small anatomical area, such as digits. In these instances an acoustic standoff pad is useful. An acoustic standoff pad is a commercially available gel-filled pad to assist the clinician with imaging extremely superficial structures by increasing the distance between transducer and the area of interest. These products may be cost prohibitive and a water bath, a reliable and free alternative, has been shown to provide the same benefits.¹³ Simply place the patient’s hand or foot into a basin of water and obtain images by hovering the transducer in the water close to the anatomic region of interest. Scanning the area of interest in two perpendicular planes is another important practice that will allow the sonographer to better delineate the lesion.

Familiarity with the normal skin and soft tissue anatomy and their ultrasonographic appearance is necessary to recognize the appearance of pathological findings. Skin (the epidermis and dermis) appears as a thin, homogenous, and hyperechoic line immediately underneath the transducer. Skin resides superior to the superficial fascia, which is a fibrous connective tissue layer that appears hyperechoic on ultrasound. Beneath the superficial fascia lies the subcutaneous tissue, mainly composed of adipose tissue and superficial nerves, arteries, and veins. Adipose tissue appears hypoechoic with a thin line separating lobules. Arteries and veins have an anechoic appearance and will either be circular or tubular depending on the transducer’s orientation. Deep to the subcutaneous tissue is the deep fascia, another hyperechoic layer, which surrounds muscles and bones. Muscles are hypoechoic with a striated pattern, surrounded by the hyperechoic fascia, while bones are hyperechoic on the anterior cortex, which casts an acoustic shadow inferiorly. (See Figure 1.)

Figure 1.

Normal skin. At the top of the image is normal epidermis/dermis (1) – a thin hyperechoic (bright white) line. Below the skin is the subcutaneous tissue (2) - a hypoechoic (gray) area. Anechoic (black) blood vessels (3) are shown in cross-section. The hyperechoic deep fascia surrounds muscles (4) and bone (5).

While the ultrasonographic findings associated with cellulitis are nonspecific, when used in conjunction with the clinical context (redness, warmth, and pain), they can be used to confirm the diagnosis and rule out an abscess. The edema and inflammation in the skin and subcutaneous tissue that results from cellulitis is responsible for the changes visible by ultrasound. The most common finding seen with cellulitis is described as “cobblestoning.” (See Figure 2.) Cobblestoning refers to areas of hypoechoic fluid that separate the subcutaneous tissue and fat in a reticular pattern similar to the spaces along a cobblestone street. Other findings may include thickened, hyperechoic skin and loss of detail in the subcutaneous tissue with increased echogenicity. Comparison to normal skin on the unaffected side can help the sonographer recognize subtle abnormalities.

Figure 2.

Cellulitis. This is an example of skin and soft tissue with cobblestoning. Note the hypoechoic fluid (edema (1)) separating the subcutaneous tissue and fat. The subcutaneous tissue (double arrow) is thickened and there is loss of the normal hyperechoic epidermis/dermis at the top of the image. Also, note the loss of normal landmarks deep to the subcutaneous tissue.

An abscess can cause many findings on ultrasound. Most abscesses are surrounded by some degree of cellulitis or soft tissue edema, therefore cobblestoning is often present adjacent to or surrounding the fluid collection. Abscess cavities are usually round, though they can be irregularly shaped, and have an anechoic or hypoechoic center from the combination of purulent material and blood. (See Figure 3.) Abscess cavities will also have posterior acoustic enhancement, an artifact that occurs because the acoustic energy of the ultrasound beam is less attenuated through the liquid center of the abscess, creating a hyperechoic shadow along the posterior wall of the abscess. Very rarely, the contents of an abscess may appear isoechoic or hyperechoic due to loculations or very dense fluid. Ultrasonic fluctuance is the term used when the sonographer applies gentle pressure over an abscess cavity and is able to see the abscess contents swirling around inside. This technique may be especially helpful when the abscess cavity is isoechoic or hyperechoic.

Figure 3.

Abscess. The abscess (A) is shown deep to the skin and located within the subcutaneous tissue. The abscess is a round, anechoic (black) cavity with hyperechoic posterior acoustic enhancement (P).

Significant pitfalls do exist in the use of ultrasound when evaluating patients with an SSTI. As noted above, not all abscesses are hypoechoic. Occasionally they can be sonographically subtle and may result in a false-negative ultrasound examination. Abscesses that are in or around muscle tissue or deep and obscured by overlying hyperechoic soft tissues, can appear isoechoic and, thus, be difficult to identify. The operator’s skill and training can also play a role. The ability to appropriately optimize the image via use of appropriate frequency, focus, depth settings, and transducer pressure may help avoid this type of hazard.¹⁴

Operator skill is also required to differentiate an abscess from some other causes of skin swelling such as a pseudoaneursym, hematoma, herniated bowel, lymph node, etc. Prudent use of ultrasound can assist in the identification of these other etiologies. Color flow Doppler sonography will aid in recognition of adjacent vascular structures and may also detect blood flow through a pseudoaneurysm. Lymph nodes are highly vascular and should demonstrate strong color flow throughout while abscesses should not. Compression of the lymph node will not result in the swirling flow of fluid seen in abscesses described above. Herniated bowel may be differentiated from an abscess by the presence of peristalsis.¹⁵ Distinguishing an abscess from a hematoma can be quite difficult, as both typically appear to be a hypoechoic fluid collection. The stasis time of the hematoma will affect the appearance, with established clots appearing more hyperechoic. As a rule, pus is heterogeneously echogenic while clotted blood appears uniformly echogenic. ¹⁶ The ability to differentiate an abscess from some of these other abnormalities can depend on the skill of the sonographer and clinical correlation may often be the deciding factor.

US can be used to identify structures near a planned incision site for the I & D of an abscess. Vasculature, nerve bundles, and tendons can be localized and, thus, avoided by the operator. If these structures are located too close to the planned incision site, the physician should consider consultation with a surgeon for performance of the procedure.¹⁴

Although there is some evidence that US may be useful in the diagnosis of necrotizing fasciitis, the studies are limited and if clinical concern exists for a deeper or more extensive infection, appropriate consultation and/or imaging should still be obtained.^5,14

Despite the above drawbacks, US has been shown in multiple studies to be useful in the diagnosis and treatment of SSTIs. In some instances, management may be changed, ultimately benefitting patients. Though clinical judgment should always be the deciding factor, US is an easy, fast, and proven way to augment decision making in SSTIs.

Biography

Kevin O’Rourke, MD, (top center) Nicole Kibbee, MD, (bottom left) and Amy Stubbs, MD (bottom right) are all Assistant Professors of Emergency Medicine at the University of Missouri-Kansas City School of Medicine and in the Department of Emgerncy Medicine at Truman Medical Center in Kansas City, Missouri.

Contact: kevin.orourke@tmcmed.org