Both superficial epidermis and dermis can be evaluated and appear as a hyperechoic layer on ultrasound. Subcutaneous tissue lies just deep to the dermis and can be visualized as hypoechoic fat lobules with hyperechoic septae. The dense fibrous membrane is the fascial layer and usually appears as a linear hyperechoic layer
Indications
Contraindications
Equipment and Probe Selection
The high-frequency (12–7 MHz) linear array probe is ideal for evaluation of superficial structures. Color Doppler can aid in identifying surrounding neurovascular structures. One percent lidocaine with or without epinephrine is a commonly used anesthetic. Chlorhexidine, a needle driver for blunt dissection, and a #11 blade scalpel should be readily available prior to starting the procedure. In more delicate areas or for smaller fluid collections, an 18- or 20-gauge needle attached to a 10 mL syringe can be used as an alternative to the #11 blade scalpel.
Of note, if the subcutaneous abscess is in a location of thick subcutaneous fat, such as the buttock, thigh, or abdominal wall, use of the low frequency curved array probe for deeper imaging may be helpful.
Preparation and Pre-procedural Evaluation
The patient should be positioned such that the affected area is easily accessible to the provider. Topical anesthetic, such as lidocaine-epinephrine-tetracaine gel, can be used over the affected area to minimize discomfort.
Procedure
Complications
Color Doppler can aid in the identification of neurovascular structures, lymph nodes, solid masses, etc. In addition, ultrasound should be used after the procedure to verify complete evacuation of the abscess cavity. A partially drained abscess may result in the need for further intervention.
Pearls and Pitfalls
- 1.
The use of color Doppler during the initial evaluation and real-time ultrasound guidance during the procedure can help avoid neurovascular complications.
- 2.
Purulent material in the abscess cavity may appear isoechoic, and therefore, the use of graded compression and color Doppler can help in identification of an abscess cavity (Fig. 11.9).
- 3.
Use of a larger 18-gauge needle is recommended for needle aspiration procedures, as purulent material may be difficult to aspirate when using a smaller gauge needle.
- 4.
When purulent material is too viscous for needle aspiration or if the abscess cavity is too large to adequately evaluate using needle aspiration, a #11 blade scalpel can be used to create a small stab incision to allow drainage.
- 5.
Be aware that necrotic lymph nodes appear similar to an abscess. Use gray-scale imaging and color Doppler imaging to evaluate for necrotic lymph nodes before performing an incision and drainage, and incision and drainage of necrotic lymph nodes is discouraged.
Integration into Clinical Practice
Ultrasound-guided incision and drainage of abscess and hematoma provide definitive treatment. The features to distinguish abscess from cellulitis are easily learned and can help guide management decisions about the need for incision and drainage. Furthermore, ultrasound-guided drainage provides real-time visualization of the surrounding neurovascular structures which helps reduce the incidence of complications.
Evidence
Ultrasound-guided abscess drainages are technically uncomplicated and minimally invasive. In a study by Kjær et al., subcutaneous truncal abscesses were treated successfully in 93% of their patients [4]. This approach yielded high patient satisfaction and was well-tolerated with short healing times.
Ultrasound-guided breast abscess drainages have replaced open treatment of breast abscesses, with 97% resolution rate in puerperal abscesses and 81% resolution rate for nonpuerperal abscesses [5]. Needle drainage under ultrasound decrease pain and scar formation. The evidence for using ultrasound to directly guide abscess drainage makes it suitable for outpatient settings.
Key Points
POCUS for the diagnosis or confirmation of a subcutaneous abscess is recommended in addition to physical examination due to concern of a suspected abscess being a mass, lymph node, aneurysm, etc.
For smaller abscesses, it is reasonable to use a large gauge needle to attempt aspiration and proceed to incision if necessary.
In-plane technique is always preferred, if possible, to ensure the needle tip does not injury nearby structures.
Always use color Doppler to evaluate abscesses to identify vascular or blood flow within the fluid collection.
Drainage of Subcutaneous Hematomas
Advantages of Ultrasound Guidance
POCUS is a useful tool to differentiate soft tissue swelling from hematoma, although differentiation of hematoma from abscess can be more challenging. Differentiation between a hematoma and an abscess will depend more on the clinical picture. Patients can also present with an infected hematoma, which may present with signs and symptoms of an abscess over an area of previous trauma or if the patient is prone to bleeding.
Anatomy
Indications
Patients with an area of fluctuance, induration, erythema, increased warmth, swelling, and/or discomfort should undergo ultrasound imaging to evaluate for underlying pathology. The differentiation of abscess and hematoma is largely clinical, with only very subtle differences by ultrasound. Hematoma should be clinically suspected in patient who present with history of trauma, easy bruising, thrombocytopenia, coagulopathy, anticoagulation, or recent surgery, although abscess remains on the differential diagnosis. When the pressure in the hematoma cavity exceeds that of the dermal and subdermal capillaries, there is increasing potential for overlying skin necrosis.
Contraindications
The contraindications for this procedure are very similar to those for draining abscesses. POCUS should be used to ensure that no neurovascular structures lie in the region of interest, which would warrant an alternative method for incision and drainage [2]. Use color Doppler to ensure the area in question is not a necrotic lymph node, aneurysm, or pseudoaneurysm. Discretion should be used when performing bedside incision and drainage of a hematoma in patients on anticoagulation or with known coagulopathic states, as bleeding is a concerning potential risk; patients should be counseled regarding the potential risk of bleeding prior to starting the procedure. Laboratory testing such as complete blood count to check platelet count and a coagulation panel (PTT and PT/INR) may be helpful prior to starting the procedure to further assess for bleeding risk.
Equipment and Probe Selection
Preparation and Pre-procedural Evaluation
The patient should be positioned such that the affected area is easily accessible to the provider. Topical anesthetic, such as lidocaine-epinephrine-tetracaine gel, can be used over the affected area to minimize discomfort. As mentioned previously, evaluation of the patient’s platelet count and coagulation studies may be indicated. Additionally, for larger hematomas, consider a type and screen and ensure that appropriate blood products are readily available, should a bleeding emergency result. Gauze impregnated with hemostatic agents or tranexamic acid can also be at bedside to assist with bleeding control.
Procedure
Evaluate the patient’s normal anatomy by initially scanning away from the affected area. Scan the entire length and width of involved area in two planes—both sagittal and transverse—using the linear array probe. Applying graded compression helps ensure a loculated fluid collection, which can be easily overlooked in the case of hematoma, is not missed. Application of color Doppler flow confirms lack of vascularity. If a fluid collection concerning for hematoma is located, note the boundaries, depth from the tissue surface, and estimated size of the cavity. The sonographic appearance of a hematoma is non-specific and is challenging to differentiate from an abscess [3].
Complications
Bleeding is a potential complication associated with this procedure. Caution is advised in patients with a known bleeding diathesis or on anticoagulation. There may be persistent oozing from the incision site. It is advised to apply a compression dressing after the incision and drainage.
Pearls/Pitfalls
- 1.
Use of color Doppler pre-procedure and real-time sonographic evaluation during the procedure can help avoid potential neurovascular complications.
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