A skin abscess is an infection of the dermis and deeper layers of skin that contains purulent material.
Abscesses occur when pathogens enter a break in the skin following trauma or when they spread from infected hair follicles (Figure 77–8). When a single follicle is infected and tracks down into the dermis, it is termed a furuncle (“boil”), and when multiple infected hair follicles coalesce, it is termed a carbuncle. Occasionally, an abscess may develop following hematogenous dissemination of an infection.
Abscess. Note localized area of inflammation containing a central core of yellowish pus (arrow) on medial aspect of foot. This lesion occurred at the site of a sewing needle injury. (Reproduced with permission from Wolff K, Johnson R. Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York, NY: McGraw-Hill Education; 2009.)
A furuncle consists of a central pustule usually surrounded by an area of erythema, warmth, and tenderness with underlying fluctuance. Patients may have multiple furuncles. A carbuncle is a larger, more serious lesion than a furuncle. It is composed of several adjacent furuncles that have coalesced into an inflamed, indurated lesion that typically extends deep into subcutaneous tissue. Carbuncles are often found on the nape of the neck, where a shirt collar rubs in people with poor hygiene (Figure 77–9). Patients may have signs and symptoms of systemic infection, and this should alert the provider that more severe disease exists.
Carbuncle. Note multiple furuncles that have coalesced to form a large area of inflammatory lesion. (Reproduced with permission from Kang S, Amagai M, Bruckner AL, et al. Fitzpatrick’s Dermatology in General Medicine. 9th ed. New York, NY: McGraw-Hill Education; 2019.)
S. aureus is, by far, the most common cause of skin abscesses (more than 75% of cases). Beta-hemolytic streptococci are also capable of causing these types of infections. Rarely Mycobacterium tuberculosis, nontuberculous mycobacteria, and fungi such as Coccidioides, Candida, and Cryptococcus can cause abscesses.
Gram stain and culture of purulent material obtained from the abscess allow for diagnosis. Radiographic imaging such as ultrasound or computed tomography (CT) may help further define the size and extent of an abscess.
The primary treatment for abscesses is incision and drainage and is highly effective alone. However, a short course (≤7 days) of active antibiotics increased cure rates and can help reduce the risk of recurrent disease. Antibiotics should also be added when the patient has signs and symptoms of systemic infection, a rapidly progressive or severe infection, infection in a hard-to-drain area of the body, extremes of age, immunocompromised state, or failure to resolve with previous incision and drainage.
When antibiotics are indicated, the patient should be treated with an empiric antibiotic regimen that has activity against MRSA. Such oral antibiotic regimens include clindamycin, trimethoprim-sulfamethoxazole, delafloxacin, or doxycycline (Table 77–3). Empiric intravenous regimens include vancomycin or daptomycin. If antibiotic susceptibility testing demonstrates that MSSA is the pathogen, oral regimens can include cephalexin or dicloxacillin, whereas intravenous antibiotics would include nafcillin or cefazolin.
TABLE 77–3Antibiotics Active and Inactive Against Methicillin-Resistant Staphylococcus aureus (MRSA) ||Download (.pdf) TABLE 77–3 Antibiotics Active and Inactive Against Methicillin-Resistant Staphylococcus aureus (MRSA)
Handwashing and covering draining lesions should be used to prevent the spread of bacteria.