Etiology and Pathogenesis

In the majority of cases, infection occurs after there has been breakdown of the skin, allowing bacteria that are normal colonizing flora of the host to invade into the subcutaneous tissues and beyond. Sources of the breakdown include direct trauma to the area, excoriation of an insect bite, or underlying conditions such as atopic dermatitis, which disrupt the integrity of the skin and can be intensely pruritic. The seeding point for the infection may be caused by micro trauma and not clear to the naked eye. After bacteria are beyond the skin barrier, they can invade to varying depths, determining the severity of the infection. Hair follicles and their surrounding glands are other sources of cutaneous infections, as seen in folliculitis, carbuncles, and furuncles. In addition to the level of introduction of the bacteria, host factors play a role in the severity and progression of illness. Children with underlying illness, particularly atopic dermatitis, diabetes mellitus, and renal failure requiring hemodialysis or those who are immunocompromised, are at a higher risk for colonization with pathogenic bacteria and for invasive disease.

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.

Clinical Presentation

Nearly all skin and soft tissue infections are characterized by a varying degree of erythema, pain or tenderness, and warmth. For clinicians, after it has been established that there is a likely bacterial infection, the next steps are to determine the depth and degree of the infection and its rate of spread (Figure 90-1).

Figure 90-1 Cross-section of the skin showing layers and types of infections.

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.