Brian Euerle, MD, RDMS
I. Introduction and Indications
Abscess and cellulitis are two of the most common soft-tissue infections seen in patients treated in emergency departments. Although they sometimes occur together, they are different disease processes requiring different treatments. An abscess is treated with incision and drainage and may not require antibiotics; cellulitis is treated with antibiotics alone.
It can be difficult to differentiate cellulitis from abscess based only on history and physical examination findings. Both processes may generally be characterized by warmth, erythema, tenderness, swelling, and induration.(1) In some patients, an abscess is clearly evident because of obvious fluctuance and copious purulent drainage; however, this distinct presentation is not seen in the majority of cases. Because of the difficulty in diagnosis, the emergency physician might decide upon an inappropriate treatment, resulting in one of two possible errors. Incision and drainage might not be done in a patient with an abscess, or incision and drainage could be performed on a patient who has cellulitis but no abscess. Increased pain and poor patient outcome can result from either one of these errors.
Many researchers, including emergency physicians, have reported on the utility of ultrasound in the evaluation of abscesses and cellulitis.(1–8) One group of emergency physicians (6) found that soft-tissue ultrasound changed the management strategy for approximately half of their patients and concluded that ultrasound was useful because it could detect occult abscesses and avoid invasive procedures.
Ultrasound also allows many procedures to be done with greater safety. In the case of abscess drainage, ultrasound can locate adjacent structures such as large blood vessels and nerves that need to be avoided during the drainage procedure.(4)
The use of bedside ultrasound can also help determine the treatment of a specific abscess based on its size and depth. If an abscess is very small (<1 cm diameter), the physician might choose treatment with antibiotics and warm compresses rather than incision and drainage.(6)Although the majority of abscesses are treated with incision and drainage, in certain cases, usually because of cosmesis, treatment with needle aspiration and antibiotics may be an option. (3,7) Ozseker and colleagues (7) found that ultrasound-guided aspiration and irrigation of breast abscesses was preferred to surgical drainage for abscesses with a diameter less than 3 cm. (7) Ultrasound provides dynamic real-time guidance for needle aspiration, resulting in increased success.
The structures that are imaged in bedside ultrasound for abscess evaluation are primarily the skin, subcutaneous tissue, and fascia. The skin consists of two layers: the superficial epidermis and the deeper, thicker dermis. Subcutaneous tissue, located beneath the dermis, consists of connective tissue septa and fat lobules.8 Fascia, a deeper structure, is a dense, fibrous membrane.9
III. Scanning Technique, Normal Findings and Common Variants
It is important to select an appropriate transducer when using bedside ultrasound for abscess evaluation. Because most subcutaneous abscesses are relatively superficial, a high-frequency (7-12 MHz) linear array transducer is most useful. At times a deeper abscess may be beyond the range of the linear transducer and a lower frequency transducer must be used. In this situation, a lower frequency linear array transducer may be helpful. If this type of transducer is not available, a curvilinear transducer may be used.
Evaluation for peritonsillar abscess is a specialized application in which a high frequency intracavitary probe can be used.
Many emergency practitioners are familiar with the use of color Doppler ultrasound and this can be useful in abscess evaluation. One way in which Doppler can be helpful is by identifying large blood vessels that are adjacent to an abscess. Color Doppler can also be helpful in the evaluation of groin masses because it can help differentiate an abscess from a pseudoaneurysm.4 Power Doppler is a more advanced technique that is able to detect low velocity blood flow and movement. This can be used in abscess evaluation because it can identify hyperemia in the walls of abscesses and the surrounding tissues.5,10
It is helpful to begin scanning a short distance away from the area of interest to gain an appreciation of the appearance of the normal, uninvolved anatomy. It may also be helpful to view the contralateral side of the patient’s body to obtain information about the normal appearance of structures. (11)
Next, slide the probe over the extent of the abscess or cellulitis, maintaining the same orientation. Once the area has been visualized appropriately, rotate the transducer 90 degrees and repeat the process. If an abscess is present, place a gloved finger of the nondominant hand on the point of maximal fluctuance or “point” of the abscess. Then slide the transducer so that its mid-point is located against the finger. This can help you correlate the image on the screen with the anatomy and help plan the site of the incision. Body markings can also be used to indicate the extent of the abscess or location of the incision.
Once the presence of an abscess is confirmed, set the ultrasound probe aside and proceed with the incision and drainage. Another option is to incise the area while observing in real time under ultrasound. This technique may be helpful for deep or small collections, but it generally is not necessary.
It may be helpful to repeat the ultrasound examination after incision and drainage to assess the success of the procedure and locate undrained collections of purulence.
With the equipment that is typically used for bedside ultrasonography, the epidermis and dermis cannot be differentiated. They appear together as a thin, hyperechoic layer .(8) The subcutaneous layer appears hypoechoic on ultrasound, with two components: hypoechoic fat interspersed with hyperechoic linear echoes running mostly parallel to the skin, which represent connective tissue septa (Figure 1 ) (8) Veins and nerves may be visualized within the subcutaneous layer.
Fascia appears as a linear hyperechoic layer. Its thickness may vary depending on the location.
Figure 1: Normal skin, subcutaneous tissue, and fascia.
On ultrasound an abscess is a spherical or oblong shaped structure that is largely anechoic or hypoechoic (Figure 2). However, as opposed to a simple cyst that will be uniformly anechoic throughout, an abscess will contain hyperechoic debris. This feature can be used to differentiate an abscess from a cyst. The walls of the abscess cavity may be distinct and hyperechoic, or may have a ragged appearance and intermix with the adjacent tissue. Because of the anechoic nature of the abscess, posterior acoustic enhancement may be seen. Dynamic scanning, achieved with gentle compression of the probe, may cause the contents of the abscess to swirl, which can be diagnostic of an abscess. Hyperechoic foci of air may be seen in necrotizing fasciitis.
Figure 2: Abscess containing hyperechoic debris.
The ultrasound appearance of cellulitis may vary depending on the stage and severity. The initial appearance may be generalized swelling and increased echogenicity of the skin and subcutaneous tissues.(2,5) As cellulitis progresses and the amount of subcutaneous fluid increases, hyperechoic fat lobules become separated by hypechoic fluid-filled areas. This later stage of cellulitis is most typical and has been described as having a cobblestone appearance (Figure 3) (5)
Figure 3: Cobblestone appearance of advanced cellulitis.
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