Skin and Soft Tissue Infections

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184 Skin and Soft Tissue Infections

Ellen M. Slaven

• Uncomplicated subcutaneous abscesses in otherwise healthy patients require incision and drainage alone, without antibiotic therapy.

• Cellulitis with induration may harbor a deep purulent collection despite the lack of fluctuance on physical examination. Ultrasound or needle aspiration may be used to assist in the search for areas of pus that require incision and drainage.

• Cellulitis with purulence should be treated with antibiotics that are active against community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA), and cellulitis without purulence should be treated with antibiotics active against Streptococcus species.

• Patients with mild diabetic foot infections may be treated with oral agents that target gram-positive organisms, including CA-MRSA.

• Emergency physicians must always search for signs of necrotizing infection to exclude the deadly diagnosis early in the management of skin and soft tissue infections.

• Emergency consultation with a surgeon is mandated when a suspicion of necrotizing skin and soft tissue infection arises.

Community-Acquired Methicillin-Resistant Staphylococcus Aureus

Since the late 1990s, an epidemic of skin and soft tissue infections has been caused by community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA). Currently in the United States, CA-MRSA is the most common cause of skin and soft tissue infection in patients presenting to emergency departments. Patients who develop these infections have been previously healthy, and most have not been exposed to health care settings or prior antibiotic therapy.¹

CA-MRSA is distinguished from hospital-acquired MRSA (HA-MRSA) not only by the location of acquisition, but also by the type of infection produced. HA-MRSA causes wound infections, sepsis, endocarditis, and metastatic infections, but CA-MRSA causes predominantly purulent skin infections. HA-MRSA rarely carries the Panton-Valentine leukocidin that is believed to be a potent virulence factor found in most CA-MRSA strains. In addition, HA-MRSA is generally resistant to many antibiotics, whereas CA-MRSA tends to be resistant predominantly to β-lactam agents but remains susceptible to many other antibiotics, such as trimethoprim-sulfamethoxazole and the tetracyclines. Unfortunately, the designation of these staphylococcal strains based on the site of acquisition has led to confusion now that newly developed CA-MRSA infections have been increasingly identified within hospitals.

In 2011, the Infectious Disease Society of America released its long-anticipated first clinical guidelines for the treatment of MRSA infections, including CA-MRSA, in adults and children.¹ The panel of experts provided an evidence-based framework for the clinical evaluation and treatment of skin and soft tissue infections and more invasive infections such as bacteremia and pneumonia.

Superficial Skin Infections

Impetigo

Impetigo is most commonly encountered in preschool-age children. It is a superficial infection involving the epidermis and is highly contagious. Impetigo is readily spread from one site to another on one child and just as easily from one child to another. Two types of impetigo are recognized. The classic nonbullous impetigo begins with a vesicular lesion that becomes purulent. The vesicle then ruptures and reveals the typical, honey-colored crusted lesion. The face and extremities are commonly involved. Streptococcus pyogenes (group A Streptococcus) is the pathogen. The infection is self-limited and generally heals without scarring. Treatment is aimed at rapid resolution and prevention of transmission. The second type of impetigo, bullous impetigo, is caused by a toxin-producing strain of S. aureus. The appearance of bullous impetigo is of a flaccid, bullous lesion that may become filled with purulent fluid. Approximately 10% to 20% of all cases of impetigo are the bullous type.

Treatment with topical mupirocin is effective in patients with mild impetigo. Patients with more severe infections require systemic therapy with cephalexin for 7 days. Azithromycin (500 mg orally on day 1, then 250 mg orally on days 2 to 5) is an alternative for patients who are allergic to penicillin. Empiric therapy for CA-MRSA bullous impetigo should be reserved for patients in whom this standard treatment fails.

Erysipelas

S. pyogenes is the primary pathogen responsible for the intradermal infection known as erysipelas, or Saint Anthony’s fire. This infection also involves the lymphatic drainage of the skin and produces a bright, erythematous raised area of skin. It is very well demarcated. The diagnosis is based on clinical appearance. The rash is common in older persons and in young children. Seventy percent of infections occur on the lower leg, and approximately 20% of infections occur on the face. The onset is quite sudden, fever may be present, and the rash is frequently very painful.

The drug of choice is penicillin. Admission to the hospital for intravenous antibiotics and supportive care may be indicated for patients who suffer from serious comorbidities or for patients who appear toxic. Azithromycin is an alternative agent. Always examine the feet carefully for signs of fissuring and include topical antifungals for treatment of tenia pedis because fissures may be a portal of entry for streptococci.

Cellulitis

Cellulitis results from bacterial infection of the dermis and the subcutaneous fat. Infection may arise following the entry of bacteria into the dermis through small breaks in the skin, larger wounds, or preexisting dermatitis. The infection may be limited to a small patch, or it may spread to include extensive areas of skin. Cellulitis is manifested by erythematous, warm, tender regions of skin that frequently spread. Lymphangitis and lymphadenitis may be present. Fever, chills, and malaise are common associated symptoms.

The pathogens of cellulitis are rarely identified in any particular patient, but they are thought to be primarily S. pyogenes (less commonly, other β-hemolytic streptococci such as groups B, C and G) and S. aureus. Culture of material aspirated from the involved skin is not routinely performed because of the invasive nature of the procedure and the low diagnostic yield. Blood cultures are of little value; only approximately 2% to 5% of these cultures yield results. A newer distinction has been proposed between nonpurulent cellulitis, which is believed to be more likely caused by Streptococcus species, and cellulitis that is associated with purulence and is probably caused by S. aureus. (See the later discussion of purulent skin infections.)

Patients with mild cases of nonpurulent cellulitis may be treated on an outpatient basis with an antibiotic effective against Streptococcus species such as cephalexin or dicloxacillin. The precise length of therapy is determined by the patient’s response to therapy. A 5- to 10-day course of antibiotics is recommended. Empiric coverage for CA-MRSA should be reserved for patients who do not respond to β-lactam agents.

In patients with cellulitis who require hospital admission (e.g., those with extensive disease or underlying immunocompromises those who appear toxic), blood cultures are often obtained. Although the diagnostic yield of blood cultures is quite low, these cultures may provide useful information for patients who are significantly ill. Radiography may provide valuable information about patients with significant cellulitis, in particular to establish the presence of gas or foreign bodies. Patients with nonpurulent cellulitis who require admission may be treated with intravenous antibiotics, such as oxacillin, nafcillin, or cefazolin. For patients allergic to penicillin, azithromycin or levofloxacin may be substituted. Empiric coverage for CA-MRSA may be reserved for patients who do not improve during β-lactam therapy.

Purulent Skin Infections

Small, superficial pustular infections arising from the hair follicle are usually caused by S. aureus and are referred to as folliculitis. The lesions of folliculitis tend to be approximately 2 to 5 mm in diameter, they are isolated to the epidermis, and they generally produce pruritus rather than pain. Treatment consists of warm, moist compresses and topical antibiotic ointment, such as mupirocin. If systemic therapy is desired (because of a lack of response to topical therapy, extensive infection, or the presence of underlying immunocompromising medical condition), an agent active against CA-MRSA is recommended (Table 184.1).

Table 184.1 Antibiotics for Community-Acquired Methicillin-Resistant Staphylococcus aureus Skin Infections

DEGREE OF INFECTION	ANTIBIOTIC REGIMEN
Mild	Trimethoprim-sulfamethoxazole (double strength) 1 PO bid Clindamycin 300 mg PO qid Doxycycline 100 mg PO bid Linezolid 600 mg PO bid

Moderate to severe

Vancomycin 15-20 mg/kg/dose IV q8-12h (not to exceed 2 g/dose)

Clindamycin 600 mg IV q8h

Linezolid 600 mg IV q12h

Daptomycin 4 mg/kg IV q24h

Telavancin 10 mg/kg IV over 60 min q24h *

bid, Twice daily; IV, intravenously; PO, orally; q, every; qid, four times daily.

^* Women of childbearing potential must have a serum pregnancy test before administration of telavancin because of the possibility of fetal malformations.

Furuncles begin as simple folliculitis and extend deeper into the subcutaneous tissue and surrounding dermis. Furuncles are also called boils or subcutaneous nodules. These painful lesions tend to erupt in areas with hairy skin, in particular the face, axilla, and buttock.

When multiple adjacent furuncles coalesce, a large, purulent mass develops in the subcutaneous tissues. This is a carbuncle. These lesions tend to occur in areas of overlying thick skin, such as the nape of the neck, back, and posterior thighs. Carbuncles appear as erythematous soft tissue masses, and they may contain several orifices capable of draining purulent material. Diabetes is a risk factor for the development of carbuncles. These lesions are quite painful and are frequently associated with fever.

Incision and drainage are required for both furuncles and carbuncles. Carbuncles generally require drainage in the operating room to provide sufficient analgesia and complete drainage. Antibiotics are indicated in patients with furuncles that are complicated by surrounding cellulitis and in all patients with carbuncles, and the selected antibiotic should be active against CA-MRSA (see Table 184.1).

Subcutaneous abscesses are collections of purulent material in the subcutaneous tissues, and they may occur anywhere on the body. The overlying skin may be uninvolved and may appear intact, or it may be erythematous and indurated. A pustule may be noted, or the overlying skin may be thin and in various stages of breakdown as the purulence drains spontaneously. Cellulitis of the surrounding dermis may also be noted. Purulent material must be sought out in patients who have cellulitis with moderate induration because pus may lie deep beneath the surface, without any signs of pustules or fluctuance. Ultrasound scanning or aspiration with a large-gauge needle (following local anesthesia) may reveal occult purulent collections. Incision and drainage are required for all subcutaneous abscesses. Antibiotics are not indicated for uncomplicated subcutaneous abscesses in healthy patients. The indications for antibiotic use are listed in Table 184.2.

Table 184.2 Indications for Antibiotic Therapy for Subcutaneous Abscesses

Abscesses in Patients with Compromised Immune Systems

Acquired immunodeficiency syndrome

Organ transplantation

Medications, such as long-term corticosteroid therapy

Extremes of age

Abscesses Complicated by Other Conditions

Abscesses located on the central face

Extensive or severe disease (e.g., multiple abscesses)

Lack of response to incision and drainage alone

Recurrent subcutaneous abscesses are common in this era of CA-MRSA. All patients should be educated regarding proper wound care, including keeping the incision site covered with a clean and dry bandage, regular hand washing, cleaning of surfaces that may become contaminated in the home, and avoiding shared personal items, such as razors. Attempts at decolonization may be beneficial for patients with recurrent abscesses or when intrapersonal transmission occurs with a household. These efforts, however, may be more successful after the patient has recovered and no longer has an open moist wound that may likely continue to harbor staphylococci. Regimens for decolonizing are not clearly defined. Some experts recommend intranasal mupirocin, twice daily for 5 to 10 days, in conjunction with daily chlorhexidine body washes for 5 to 10 days. Oral antibiotic therapy is not recommended for decolonization.

For outpatient management in patients with purulent cellulitis, empiric therapy with an agent active against CA-MRSA is recommended (see Table 184.1). The high cost of linezolid makes it a less desirable choice.

Intravenous vancomycin (15 to 20 mg/kg/dose actual body weight every 8 to 12 hours, not to exceed 2 g per dose) is the drug of choice for patients admitted with purulent cellulitis. Alternatives include clindamycin (600 mg orally or intravenously [IV] three times a day), linezolid (600 mg orally or IV twice daily), daptomycin (4 mg/kg/dose IV once daily), or telavancin (10 mg/kg/dose IV once daily).

No scientific data exist to support the routine culturing of purulent skin infections. Most patients improve with antibiotic therapy and incision and drainage, and microbiologic data are unnecessary. Most experts agree that cultures should be obtained when patients present with serious infections, when a course of outpatient antibiotics has failed, or when infections are complicated by underlying immune compromise, thus requiring hospitalization.

Diabetic Foot Infections

With more than 16 million diabetic patients in the United States and a 3-year incidence of foot ulceration of 6% among them, emergency physicians are likely to treat many patients with diabetic foot infections. The ulcers may begin as small lesions, but these sites are prone to infection and are associated with great morbidity and mortality. Fifteen percent of diabetic patients with foot ulcerations require amputation, and the 3-year mortality is almost 30%.²

Diabetic foot infections are diagnosed by their clinical appearance. Laboratory testing and radiography may be helpful to support the presence of infection or to define complications. The white blood cell count and erythrocyte sedimentation rate may be elevated in the presence of infection, but these tests are neither sufficiently sensitive nor definitively specific. Blood glucose testing is important because infections often lead to hyperglycemia. Uremia is known to impair healing. Radiographs may reveal the bony changes of osteomyelitis or subcutaneous gas produced by a necrotizing infection.

Obtaining a sample for microbiologic testing is worthwhile, particularly in the setting of severe infection. Swabbing the surface of an infected ulcer, or capturing purulent material as it makes its way to the surface, is likely to result in culture of contaminating bacteria, not necessarily the offending pathogen. Aspiration of deep-lying purulent material and sampling of tissue obtained during débridement are superior methods of acquiring material for microbiologic testing. The management of diabetic foot infections is directed by the severity of infection. Emergency physicians must assess the extent and depth of infection and the presence of foreign bodies or necrosis.

Aerobic gram-positive cocci, in particular S. aureus, including CA-MRSA, are the most predominant pathogens in diabetic foot infections. Patients with mild infections, defined as superficial when the lesions have less than 2 cm of surrounding cellulitis and are lacking abscess or necrosis, may be treated with antibiotics targeting S. aureus, including CA-MRSA (see Table 184.1). Moderate to severe infections tend to be caused by gram-positive cocci in addition to gram-negative bacilli and anaerobes. Moderate infections are defined as those with cellulitis extending more than 2 cm, lymphangitis, spread to deep tissues, or the presence of abscess or necrosis. Severe infections cause systemic toxicity, including features such as fever, hypotension, confusion, acidosis, or azotemia. Empiric therapy for these polymicrobial infections requires a broad-spectrum regimen until the results of microbiologic culture are available. When a limb- or life-threatening infection is present, imipenem and vancomycin or linezolid are recommended (Table 184.3). Surgical consultation should be considered for all diabetic patients with foot infections that may be complicated by abscess or necrosis.

Table 184.3 Empiric Antimicrobial Therapy for Infections of the Diabetic Foot

DEGREE OF INFECTION	ANTIBIOTIC REGIMEN
Mild	Clindamycin 300 mg PO qid

Moderate to severe Limb- or life-threatening infections

IV, Intravenously; PO, orally; q, every; qid, four times daily.

^* Alternative regimens for patients with severe β-lactam allergy.

Necrotizing Infections

Necrotizing skin and soft tissue infections comprise a group of potentially limb- and life-threatening diseases caused by various virulent pathogens. The common theme is infection leading to ischemia and necrosis of skin, subcutaneous tissue, fat, fascia, and even muscle. These infections are also characterized by their capacity to progress rapidly. During the initial presentation of patients with necrotizing skin and soft tissue infections, emergency physicians are not able to discern the depth of the infection precisely, nor can they identify the particular pathogen or pathogens. The most important component of the emergency physician’s initial evaluation is to detect, or at least suspect, the presence of necrosis based on clinical findings alone. Be cautious when intense, localized pain is otherwise unexplainable, and consider the presence of an infection. Early use of antibiotics and consultation with surgeons may allow for lifesaving surgical débridement.³

Gas gangrene, or clostridial myonecrosis, is the classic necrotizing skin and soft tissue infection. It often develops after a wound is contaminated with soil containing Clostridium perfringens. Another virulent bacterium, Vibrio vulnificus, can also cause necrotizing soft tissue infections. Particularly during warm months, V. vulnificus may cause infections in patients with wounds that are exposed to salt or brackish waters. Additionally, S. pyogenes can produce necrotizing soft tissue infections in previously healthy patients.

Many patients with necrotizing skin and soft tissue infections are found to be infected with multiple pathogens. An example is Fournier gangrene. This highly lethal infection of the perineum is commonly diagnosed in men with diabetes. This infection tends to be “mixed” because several pathogens, such as Streptococcus species, Enterobacteriaceae, and anaerobes (e.g., Bacteroides species), are recovered from culture.

When a necrotizing skin and soft tissue infection is suspected, no diagnostic testing, in particular imaging studies, should delay consultation with a surgeon. Although many diagnostic imaging studies have been employed in attempts to confirm and exclude necrotizing skin and soft tissue infections, none is superior to surgical exploration. A plain radiograph may demonstrate the presence of gas within the tissues, but it does not exclude a diagnosis of necrosis. Ultrasound scanning and computed tomography may identify gas within the tissues, in addition to deep-seated abscesses. Computed tomography and magnetic resonance imaging may show edema in the subcutaneous tissues or muscle and enhancement of inflamed fascia. None of these studies are adequately sensitive to exclude the presence of necrosis.

The definitive diagnosis of necrotizing skin and soft tissue infection is made by surgical exploration with direct visualization of the affected tissues and histologic examination of a frozen section biopsy specimen. Early surgical débridement is the definitive therapy.

Microbiologic studies, including blood cultures and culture of purulent material aspirated from a deep source, are indicated. Simply obtaining a swab from an open wound is likely to yield only contaminating bacteria. Infected tissue and fluids obtained during surgical débridement should also be cultured.

Antimicrobial therapy is vitally important early in the management of necrotizing skin and soft tissue infections. Antimicrobial therapy alone without surgical débridement leads to a mortality rate approaching 100%.

Surgical consultation must be obtained as soon as the presence of a necrotizing soft tissue infection is suspected. Box 184.1 lists common clinical indicators that mandate surgical consultation.

Early empiric antimicrobial regimens require broad-spectrum antimicrobial activity, including coverage for gram-positive, gram-negative, and anaerobic bacteria and for pathogens with multidrug resistance, such as MRSA. No excellent options for monotherapy are available in this setting. Tigecycline is a broad-spectrum intravenous agent that is active against gram-positive, gram-negative, and anaerobic bacteria, including MRSA, but it is not as effective against Pseudomonas species, and safety concerns have limited its utility in this setting. Combination regimens include the carbapenems (imipenem or meropenem) or piperacillin-tazobactam combined with either vancomycin or linezolid (Table 184.4). Clindamycin and linezolid inhibit bacterial protein synthesis and may be beneficial by blocking toxin production in several organisms responsible for necrotizing skin and soft tissue infections, in particular streptococcal and clostridial infections. All patients with necrotizing skin and soft tissue infections, even those with suspected yet unproven infections, should be admitted to the hospital for intravenous antibiotic therapy and surgical evaluation.

Table 184.4 Antimicrobial Regimens for the Empiric Treatment of Necrotizing Skin and Soft Tissue Infections

CONVENTIONAL REGIMEN	ALTERNATE REGIMEN FOR PATIENTS WITH SEVERE β-LACTAM ALLERGY
Imipenem 500 mg IV q6h or Piperacillin/tazobactam 4.5 g IV q6h *and* Vancomycin 15-20 mg/kg/dose q8-12h (not to exceed 2 g/dose) or Linezolid 600 mg IV or PO q12h

*Clindamycin 900 mg IV q8h

Gentamicin 5 mg/kg IV q24h

Vancomycin 15-20 mg/kg/dose q8-12h (not to exceed 2 g/dose)

Linezolid 600 mg IV or PO q12h

IV, Intravenously; q, every.

1 Liu C, Bayer A, Cosgrove SE, et al. Clinical practice guidelines by the Infectious Disease Society of America for the treatment of methicillin-resistant Staphylococcus aureus infections in adults and children. Clin Infect Dis. 2011;52:285–292.

2 Lipsky BA, Berendt AR, Deery G, et al. Diagnosis and treatment of diabetic foot infections. Clin Infect Dis. 2004;39:885–910.

3 Anaya DA, Dellinger P. Necrotizing soft-tissue infection: diagnosis and management. Clin Infect Dis. 2007;44:705–710.

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