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An abscess is a localized collection of pus caused by suppuration buried in a tissue, organ, or confined space.8 Intact skin is very resistant to bacterial invasion. Localized pyogenic infections are usually initiated by a breakdown in the normal epithelial defense mechanisms in the normal host. Plugging of the ducts of a superficial exocrine gland, such as apocrine and sebaceous glands or a congenital cyst or sinus, may initiate the process. Occlusion prevents desquamation and provides a moist environment for organisms to proliferate. The combination of a high concentration of organisms, the presence of nutrients, and sufficient damage to the corneal skin layer to allow organisms to penetrate the skin defenses results in abscess formation.1,9
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Subcutaneous abscesses typically begin as a cellulitis with organisms that cause necrosis, liquefaction, and accumulation of leukocytes and debris. Early stages appear as an area of hyperemia and tender inflammation that later becomes fluctuant as an exudate of leukocytes, necrotic material, and cellular debris accumulates. This is followed by loculation and walling off of the pus. This progresses and the area of liquefaction increases until it “points” and eventually ruptures through the area of least resistance.5
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The body area involved depends upon host factors such as drug use, employment-related exposures, or minor trauma.9,10 Areas with a compromised blood supply are more prone to infection as normal host cell-mediated immunity is not as available.9 The frequency of abscess occurrence in different body areas includes the buttocks and perirectal area in 25% of cases, the head and neck in 20%, the extremities in 18%, the axilla in 16%, and the inguinal area in 15%.1
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The majority of abscesses are polymicrobial with the isolated organisms usually representing the normal resident flora associated with the body area on which the abscess is found.1,11 Nonresident bacteria are found in abscesses that occur as a result of direct inoculation of extraneous organisms such as those following human bite wounds, intravenous drug use, or bacterial seeding of embedded foreign bodies.12 In normal hosts, aerobic Staphylococcus and group A Streptococcus are the most common organisms isolated from abscesses of the head, neck, extremities, and trunk.11,13 Anaerobes are found in all areas of the body but predominate in abscesses of the buttocks and perirectal regions.11,13
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Staphylococcus aureus occurs in 24% to 60% of abscesses and in pure cultures is the only organism in 21% to 72% of cases.1,13–16 Community-acquired MRSA (CA-MRSA) is presently considered to be the most common identifiable pathogen causing abscess at major centers.17 CA-MRSA is defined as infection with MRSA acquired in the community lacking the traditional risk factors of hospital-acquired MRSA including a recent stay in a long-term care facility or in a day care setting, recent healthcare contacts or surgery, an indwelling device, dialysis, immunosuppression, chronic illness, or recent antibiotic use.18,19 Many cohorts of patients who are at a higher risk of acquiring MRSA infections include those with recent household or daycare contacts, children, men who have sex with men, those in the military, incarcerated patients, athletes (especially those in contact sports or who share equipment), Native Americans, Pacific Islanders, patients with previous MRSA infections, and intravenous drug users.18–21 While it is important to consider MRSA in patients with aforementioned risk factors it is essential not to exclude MRSA in the absence of them.17 Patients presenting with a complaint of a “spider bite” should be investigated for MRSA.18,19,22 In a recent observational study of urban patients, MRSA was isolated from abscesses in 51% of patients.23 An American study of 11 academic centers found MRSA incidence rates as high as 74% in patients presenting with abscesses.17 As much as 77% of MRSA-related illness, especially presenting in the Emergency Department, are skin and soft tissue infections (SSTI).24
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CA-MRSA carries the mecA gene, which is thought to impart the antimicrobial resistance. This is carried on the Staphylococcal Cassette Chromosome Type-IV, which is distinct from other forms of MRSA.18,19 CA-MRSA also produces an exotoxin called Panton-Valentine Leukocidin (PVL) that predisposes patients to cutaneous infections.18,19 PVL has been identified in up to 98% of CA-MRSA cases.17
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Up to 17% of abscesses are sterile.1,7,13 Nearly 40% of these are secondary to intravenous drug use and most likely result from injection of necrotizing chemical irritants.3 Viruses (e.g., herpes), autoimmune mechanisms, or systemic illnesses including metastatic tumors, benign tumors, and granulomatous disease may also cause sterile abscesses.4,25 These atypical etiologies may present with the absence of local inflammatory signs and symptoms and only with an exacerbation of the underlying disease process.
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Specific Clinical Entities
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Furuncles, or boils, are acute circumscribed abscesses of the skin and subcutaneous tissue that most commonly occur on the face, neck, buttocks, thigh, perineum, breast, or axilla. Carbuncles are aggregates of interconnected furuncles that frequently occur on the back of the neck where the thick skin causes lateral extension of the infection rather than pointing toward the skin surface. These occur with a higher frequency in diabetics. They can be large and cause systemic signs, symptoms, and complications. Carbuncles often require surgical consultation and treatment in the Operating Room.
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Hidradenitis suppurativa is a chronic relapsing inflammatory disease process affecting the apocrine glands primarily in the axilla, the inguinal region, or both.26 Initially, this process will appear like a typical abscess and is only identifiable in its chronic scarring phase when there are multiple lesions with tender areas of induration and inflammation in various stages of healing. The chronic process leads to draining fistulous tracts that require ongoing surgical management. Emergency Department management involves the usual incision and drainage procedure of any area of fluctuance. Patients should be informed that the intervention is not curative and that the problem is chronic. Arrange a referral to a General Surgeon, Dermatologist, or Plastic Surgeon for long-term follow-up.
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Up to 80% of breast abscesses occur in nonlactating women.4 Peripheral and superficial lesions are similar to abscesses elsewhere on the body and respond to conservative incision and drainage with an incision that radiates out (centripetally) from the nipple.27 Deeper and periareolar abscesses are often complex and require surgical referral and general anesthesia to properly treat. Postpartum mastitis is common and precipitated by milk stasis and bacterial invasion through a cracked nipple. The offending organism is commonly Staphylococcus aureus or Streptococcus species. Treatment includes the application of heat, oral antibiotics, and continued breast emptying with a breast pump or feeding of the baby. The mastitis may evolve into an abscess and is often associated with systemic symptoms. Appropriate antibiotic therapy and follow-up in 24 to 48 hours is required.
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Sebaceous cysts are a common cause of a subcutaneous abscess. They can persist for long periods as nontender subcutaneous swellings before becoming infected. They appear like most other abscesses. Sebaceous cysts can be identified by a small punctate sinus tract near the center of the fluctuant area. The initial treatment is incision and drainage. The contents are usually thick cheesy material that needs to be manually expressed. A sebaceous cyst has a definite shiny white capsule that must be excised, preferably at the time of incision and drainage or at the first follow-up visit, to prevent recurrence. The area is then treated as any other healing abscess cavity.
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The recurrence of an abscess that has been previously drained should suggest the possibility of underlying osteomyelitis, a retained foreign body, or the presence of unusual organisms such as mycobacteria or fungi. Recurrent abscesses should prompt further investigation including an assessment of the patient's immune status.
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Special Considerations
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The precise risk for endocarditis associated with subcutaneous abscesses is unknown. Up to 5% of patients with abscesses have bacteremia at the time of presentation.4,16 Incision and drainage of cutaneous abscesses can result in transient bacteremia with the organism causing the abscess.7,14,28 More recently, the clinical relevance of this bacteremia has become controversial.16 At this time, only patients considered to be at high risk for endocarditis are recommended to receive antimicrobial prophylaxis before incision and drainage (Table 106-1).29 Bacterial endocarditis prophylaxis should be directed at the most likely pathogen causing the infection. An antistaphylococcal penicillin or a first-generation cephalosporin is an appropriate choice for most soft tissue infections (Table 106-2). Clindamycin is an acceptable alternative for patients allergic to penicillin. Patients with immunodeficiency and localized soft tissue abscesses may be at higher risk for developing septicemia secondary to bacteremia induced by incision and drainage, but it is unclear if they are at higher risk of complications and death.30,31 These patients may benefit from prophylactic antibiotics prior to incision and drainage, but only indirect evidence and no controlled studies are available.32 High-risk patients with known or suspected MRSA infection should receive IV Vancomycin or Clindamycin when undergoing incision and drainage.29
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