Abscess Evaluation With Bedside Ultrasonography

Abscesses and other superficial soft tissue infections are common presentations in the emergency department (ED). ^{[1, 2, 3, 4, 5, 6]}  The number of ambulatory and ED visits for skin and soft tissue infections (SSTIs) have doubled over the past 2 decades, with over 14 million estimated annually in the United States. This rise in SSTIs is largely attributable to the increase in incidence of cellulitis and abscess, as well as the rising prevalence of community-associated methicillin resistant Staphylococcus aureus (MRSA). ^[7]  Patients who present with localized signs of swelling, pain, and erythema, and in whom cellulitis and abscess might be present, are excellent candidates for bedside ultrasonography. ^{[8, 9, 10, 11, 12]}

In a subset of patients, the physical examination reveals a grossly fluctuant subcutaneous collection that is indicative of an abscess. In many cases, however, the clinical presentation is not that clear. Physical examination alone is often inadequate when differentiating between simple cellulitis and deeper SSTIs. ^[1] These disease entities are managed differently, and misdiagnosis can lead to unnecessary painful procedures or an increase in morbidity, time lost, or cost to the patient. ^{[2, 13]}

Studies have shown that bedside ultrasonography significantly improves clinicians’ ability to differentiate between cellulitis and abscess and, thus, to initiate the most appropriate treatment from the outset. ^{[3, 14]} The advantages of bedside ultrasonography include low cost, portability, patient comfort, speed of detection (usually < 1 min), and wide availability. Additionally, if a fluid collection is detected, ultrasonography can better localize the fluid for subsequent aspiration or incision and drainage. ^[12]

In a systematic review performed to determine the accuracy of point-of-care ultrasonography (POCUS) in ED patients with SSTIs, sensitivity was 96.2% (95% CI 91.1% to 98.4%); specificity, 82.9% (95% CI 60.4% to 93.9%); positive likelihood ratio, 5.63 (95% CI 2.2 to 14.6); and negative likelihood ratio, 0.05 (95% CI 0.01 to 0.11). In pediatric patients, sensitivity was 94.9% (95% CI 88.0 to 97.8) and specificity 83.1% (95% CI 46.6 to 96.5). ^[15]

Setup

Set up the needed equipment at the bedside. Start with the linear transducer set at a frequency between 5.0 and 7.5 MHz. If a deeper collection is suspected, consider switching to a curvilinear probe set at 3.5 MHz.

Position the patient as described above and clear the area of examination.

Procedure

Place the probe at the border of the region of interest and begin scanning the length of the affected area. This should allow for transition from normal to affected tissue for comparison. Extend the area scanned to completely visualize the edges of the infection in one plane.

Repeat the scan in an orthogonal plane to form a mental 3-dimensional image of the area of interest.

Identify normal subcutaneous tissue, fascial planes, and muscle as landmarks for comparison. Also be sure to identify any adjacent structures of interest, such as blood vessels or peripheral nerves (which can have a honeycomb appearance; see below).

Continue scanning the region of interest to characterize the presence of fluid collection, the extent of collection, the depth of collection, echogenicity, and heterogeneity.

Consider using a skin marker while scanning to mark sites for subsequent aspiration or incision and drainage.

The video below depicts demonstration of ultrasonographic abscess evaluation.

Image interpretation

Normal subcutaneous tissue is hypoechoic with strands of hyperechoic connective tissue. Fascial planes are highly hyperechoic, and muscle tissue demonstrates a characteristic striated appearance. Vascular structures are anechoic and usually compressible with the transducer.

Cellulitis results in hyperechoic subcutaneous fat lobules floating in edematous fluid. This is classically described as cobblestoning (see below).

Abscesses have a wide range of sonographic appearances. Typically, they appear as anechoic or hypoechoic spherical collections of echogenic fluid with poorly defined borders. Additionally, septae, sediment, or even gas may be present within the fluid collection. Compression with the transducer may induce movement or swirling of the contained pus. (See the images below.) If an abscess is identified, thorough characterization of its location, size, and extent helps dictate further management.

 

A standoff pad may be useful to elevate the transducer, as this improves image resolution by moving the region of interest into the focal zone. This is particularly helpful when evaluating superficial structures such as the hands and feet. Commercial pads can be purchased and customized to a given situation. Alternatively, bags of saline, water-filled gloves, or water baths may be substituted.

Color flow Doppler sonography (see below) is useful in identifying adjacent vascular structures prior to performing any percutaneous interventions. This modality can also help identify lymph nodes, which can easily be mistaken for fluid collections. Lymph nodes are highly vascular and demonstrate strong color flow signals, while abscesses do not. In addition, compressing lymph nodes does not result in the typical swirling flow of fluid that is often seen with abscesses.

Computed tomography (CT) remains the criterion standard for abscess evaluation. CT may still be necessary if the ultrasound is indeterminate or unable to adequately delineate the full extent of the abscess. ^[8]

Herniated bowel (see below) may be confused for an abscess but can be differentiated by the presence of peristalsis.

Scanning the contralateral side of the body for comparison is always helpful when trying to differentiate normal tissue from abnormal tissue.

If complicated, percutaneous interventions should be ultrasound-guided to ensure proper positioning and complete drainage.

Probe covers can help reduce the risk of spreading infectious agents and should be considered. ^{[16, 17]}