Organisms

The vast majority of skin infections are caused by gram-positive organisms that are resident flora on the skin of human beings. Gram-negative organisms may infect the soft tissues but usually in unique circumstances such as bite wounds, patients with extended hospital stays (and thus increased exposure to gram negative bacteria), and immunocompromised hosts. Staphylococci and streptococci make up the majority of gram-positive infections. Whereas staphylococci are more predominant as causes of furuncles and carbuncles and impetigo, streptococci are more common in cellulitis and erysipelas. The predominant staphylococci species in skin and soft tissue infections is Staphylococcus aureus, with methicillin-resistant S. aureus (MRSA) of particular concern because it is increasingly being detected in the nonhospitalized population. In addition, children with atopic dermatitis are more commonly colonized with S. aureus. The predominant streptococcus group for skin infections is group A (Streptococcus pyogenes). Although S. pyogenes’ primary site of colonization is the oropharynx, it frequently makes transient appearances on the skin via droplets or the fingers of the host. Other organisms that are less frequently the cause of infection include other Streptococcus spp., gram-negative organisms such as Escherichia coli and Pseudomonas spp., and anaerobic bacteria.

Folliculitis

Folliculitis is a superficial pustule or local area of inflammation surrounding a hair follicle (Figure 90-2). It can be solitary, but it can also occur in clusters. The most commonly affected areas include those of high moisture and friction, such as the axillae and inguinal creases, but the scalp, extremities, and perioral and paranasal areas are also commonly affected. Poor hygiene and a humid environment are risk factors, as are active drainage from more severe nearby wounds. S. aureus is the predominant organism, with the exception of folliculitis that occurs shortly after immersion in a poorly maintained pool or hot tub, in which case Pseudomonas aeruginosa is the likely organism. Folliculitis is not usually painful, but if progression to more significant infections takes place, pain can become significant.

Figure 90-2 Follicular infection.

Furuncles and Carbuncles

Furuncles (boils) and carbuncles are uncommon in childhood, with the notable exception of children with atopic dermatitis (Figure 90-3). This population, perhaps because of its higher rates of S. aureus (the primary causative organism) colonization, is at risk for these infections. Both of these infections can be sequelae of poorly managed folliculitis. A furuncle is an acute infection of the hair follicle, often accompanied by necrosis, that begins as a nodule and then progresses to a pustule. Common locations are the neck, face, axillae, groin, and buttocks, and risk factors are similar to those of folliculitis, with the addition of hyperhidrosis, anemia, and obesity. A carbuncle is a collection of confluent furuncles, often with multiple drainage points. They can be single or multiple, frequently appearing in crops in areas similar to furuncles. Both lesions are erythematous and can be painful, and occasionally, carbuncles can progress to the point where the patient develops constitutional symptoms and laboratory evidence of more severe infection.

Figure 90-3 Child with atopic dermatitis, frontal view.

Impetigo

Impetigo is a superficial bacterial infection that is confined to the outermost layer of skin, the epidermis, and can present in a bullous form, with lesions >1 cm or the more common nonbullous forms. S. aureus is the sole agent for bullous impetigo, but it can also cause nonbullous infection. Many species of group A streptococci can lead to nonbullous impetigo. Typically affected are young children, who present with vesicles or pustules, which rupture and form a yellowish-brown crust. The bullous form presents with larger, flaccid lesions that are easily ruptured. Surrounding erythema is minimal and is usually painless or occasionally pruritic, and only in the most severe of cases does the patient have any signs of systemic illness such as fever. In cases in which infection is predisposed by underlying atopic dermatitis, early infection can be difficult to differentiate from the baseline lichenification and excoriation related trauma.

Erysipelas

Erysipelas and cellulitis are skin infections that are both characterized by erythema, warmth, and pain (Figure 90-4). Erysipelas is the more superficial of the two infections, with invasion confined to the dermis and frequently the superficial lymphatics. S. pyogenes is the most common pathogen, but other Streptococcus spp. have been isolated. The bacteria are usually established as colonizers in the host’s nasopharynx and autoinoculated into a break in the patient’s skin. The skin lesions are often preceded by prodromal symptoms of fever, malaise, and chills up to 48 hours before the onset of lesions. Skin lesions begin as brightly erythematous, raised areas with sharply demarcated, potentially rapidly advancing borders. Warmth, local edema, and tenderness are nearly universally present, and less frequently, there are signs of lymphatic spread such as streaking and regional lymph node inflammation. Severe infection can lead to the formation of vesicles and skin necrosis.

Figure 90-4 Appearance of erysipelas and cellulitis.

Cellulitis

Cellulitis is another frequently encountered skin infection, but unlike erysipelas, it extends deeper into the soft tissues below the dermis. It is also frequently seeded by relatively minor wounds or skin breakdown, such as insect bites, atopic dermatitis, tattoos, or blisters. It presents with erythema, tenderness, warmth, and induration of the affected tissues. Unlike erysipelas, the erythema is less clearly demarcated, although the two can frequently coexist, making differentiation more difficult. Lymphatic spread and systemic signs of illness do occur with cellulitis but are less ubiquitous than in erysipelas. Findings that warrant concern for a clinician are evidence of rapid progression, lymphatic streaking or abscess formation, pain out of proportion to the remainder of examination, and signs of systemic illness. Risk factors for infection include recent trauma or local infections such as folliculitis or carbuncles or furuncles and underlying skin conditions such as atopic dermatitis. Risk factors for more severe or recurrent infections include chronic liver or kidney disease, immune compromise, and poorly controlled diabetes mellitus.

As with erysipelas, S. pyogenes is a common pathogen, but other gram-positive organisms such as S. aureus and other groups of streptococci play a more prominent role. S. aureus may be of particular concern when significant purulence is identified on examination. Gram-negative and polymicrobial cellulitis can also occur, but these infections are usually preceded by a more invasive injury such as bite wounds or other forms of penetrating trauma.

Necrotizing Fasciitis

Necrotizing fasciitis, a severe soft tissue infection that can rapidly progress to severe morbidity or death, is fortunately very rare in the pediatric population. Infection usually begins in the superficial tissues and rapidly spreads along fascial planes into the deeper tissues. Pathogen-produced toxins lead to rapid local necrosis, vascular compromise, peripheral nervous system damage, and potentially profound systemic illness. Cases are most commonly associated with recent trauma, retained foreign bodies, or recent surgical procedures, although spontaneous infections have been reported. The patient initially presents with significant pain, often near the site of recent trauma or surgery. As the infection progresses to involve the peripheral nerves, the pain may progress to anesthesia. Infection can begin with an area of erythema, but other classic signs of local superficial infection such as induration and warmth may be absent. As the infection rapidly spreads to the deeper tissues, there may be evidence of cell necrosis, gas production (crepitus), bullae formation, and discharge, although none of these findings may be apparent in the superficial tissues. Patients may initially seem well-appearing, but because of tremendous toxin production and inflammation, they more frequently show signs of severe illness such as toxic appearance, fever, and cardiovascular instability. Infections can be either mono- or polymicrobial, involving gram-positive, gram-negative, or anaerobic bacteria. Whereas cases with invasive trauma are more likely to involve anaerobes and mixed flora, those caused by minimally apparent trauma are more likely secondary to monomicrobial group A β-hemolytic strep infections.

Mild Infections

Mild infections are those for which minimal treatment is required and include folliculitis and solitary furuncles. Obtaining a bacterial sample in this cases is of minimal yield because bacterial load in the lesions themselves is minimal and would not be worth the invasiveness of procedures required to obtain a sample. Skin swabs would most likely yield the host’s colonizing flora, not the pathogen at the source of infection. Decreasing moisture, the use of antibacterial soap, and improved hand hygiene may be all that is needed, with topical or oral antibiotics reserved for refractory cases.

Moderate Infections

Moderate infections include furunculosis, carbuncles, impetigo, erysipelas, and all but the most severe cases of cellulitis. Although more important in severe infections, initial evaluation and management should focus on the need for any resuscitation. Although there may not be direct toxin-mediated cardiovascular compromise, associated fever (either prodromal or subsequent) and malaise may contribute to a varying degree of dehydration from poor intake and insensible fluid losses, possibly requiring parenteral fluid administration.

Obtaining a specimen, when possible, is very important because it has implications for both management and treatment. Not only will a culture of the material be useful for appropriate antimicrobial selection, but removal of fluid within a collection or abscess is an essential therapeutic step because the likelihood that such lesions would improve with antimicrobials alone is minimal. For many lesions (impetigo, carbuncles), drainage may be spontaneous or may be achieved with soaks and compresses. In other cases, incision and drainage may be required (Figure 90-5). If an abscess or collection is identified on examination, the procedure is straightforward. If a collection is not apparent but the clinician has a high index of suspicion based on history (abscesses in past, duration, fevers) or examination (location, size), imaging may be indicated. Optimal modalities vary largely with the location of the lesions: ultrasonography is most useful when the infection is confined to the superficial tissues of an extremity, computed tomography is frequently used when evaluating the head or neck, and magnetic resonance imaging is used when there is the question of bony involvement. When the infection is causing systemic signs and symptoms that require fluid resuscitation and more extensive incision and drainage, the clinician should consider the need for further evaluation, including laboratory testing (blood culture and complete blood count), intravenous antimicrobials, and hospital admission. Inflammatory markers such as C-reactive protein may be used to monitor response to therapy in more severe infections. Blood cultures clearly do not have as high a yield as wound or lesion cultures, but their yield increases when signs of systemic illness are present.

Figure 90-5 Cellulitis and epidermal abscess.

Severe Infections

Severe infections of the skin and soft tissues include any of the moderate infections that have significant cardiovascular instability and necrotizing fasciitis. Evaluation and management of severe infections are similar to those of moderate infections, with more attention to resuscitation as needed. Along with the blood tests mentioned for moderate infections, clinicians should also obtain a complete metabolic panel and coagulation parameters to evaluate for possible end-organ damage and disseminated intravascular coagulation. In suspected cases of necrotizing fasciitis, prompt surgical evaluation with possible exploration and subsequent debridement are essential to minimize morbidity and mortality. If cardiovascular compromise is significant, vasopressors and admission to an intensive care unit may be indicated.

Antimicrobials

For the vast majority of cases, therapy is directed at gram-positive organisms, namely S. pyogenes and S. aureus, with consideration given to MRSA. For mild infections such as folliculitis, solitary furuncles and mild cases of nonbullous impetigo, topical treatment may be sufficient. Mupirocin is the topical agent of choice, and the infection should be followed closely by the outpatient provider for signs of progression and the need for more aggressive treatment. If the infection requires systemic antimicrobials, there are several factors for the provider to consider, including the severity of infection and presence of systemic illness, appearance of the infection on examination, medical history, risk factors for MRSA (prior MRSA infections, hemodialysis, recent antibiotic usage or hospitalization), and the epidemiology of local bacterial flora. If it is the patient’s first infection, there are no MRSA risk factors, and the patient is without any systemic signs of illness, the clinician may choose to provide coverage for β-hemolytic streptococci and methicillin-susceptible S. aureus (MSSA) with oral cephalexin or dicloxacillin. If the patient has any risk factors but the severity of the infection is such that outpatient management is preferred, choices for empiric coverage include clindamycin, trimethoprim–sulfamethoxazole, or linezolid. The choice between these agents depends largely on the local patterns of resistance for S. aureus, with some areas possible requiring a combination of the two. For parenteral coverage, choices are the same, although the parenteral form of trimethoprim–sulfamethoxazole is caustic and should be avoided. In addition, for those with severe illness or for whom there is concern for hospital-acquired strains of MRSA, vancomycin should be considered as well.