Dermatology

Impetigo

Impetigo (Impetigo, ICD-9 code 684)

Are You Confident of the Diagnosis?

Impetigo is a superficial bacterial infection of the skin, which is caused by Staphylococcus aureus and beta-hemolytic streptococci. It is the most common bacterial skin infection in children. It is highly infectious, and is spread with direct contact with the skin or fomite.

Characteristic findings on physical examination

Impetigo is classified as bullous or nonbullous. In the latter form, which is seen most often, clinical findings include suprficial lesions with yellow-honey crusts (Figure 1). In the bullous type, bullae are evident on examination. Bacterial culture and sensitivity can be performed to determine the pathogenic agent, as well as used to direct appropriate therapy.

Figure 1.

Honey-colored moist crusted patchs on the cheek and ear. (Courtesy of Bryan Anderson, MD)

While impetigo can occur as a primary infection of the skin, it can also occur secondary to preexisting dermatoses and traumatized skin.

Expected results of diagnostic studies

Impetigo is usually a clinical diagnosis. Bacterial culture and sensitivity can be used to confirm the diagnosis and identify the causative pathogen.

A bacterial culture of the nares can be performed to ascertain if the individual carries S aureus. A skin biopsy is rarly done, but will show a superfical subcorneal collection of neutrophils and bacteria (Figure 2).

Figure 2.

Superficial neutrophilic subcorneal collection in impetigo. (Courtesy of Bryan Anderson, MD)

Diagnosis confirmation

The differential diagnosis includes candidiasis, cellulitis, herpes simplex, and burns of the skin Candidiasis can be differentiated by its presentation with “beefy” erythema, satellite pustules, and a positve KOH examination for pseudohyphae and buds. Cellulitis usually has more erythema and edema, and involves a larger area. Herpes simplex has grouped vesicles on an erythematous base, and can demonstrate a positive Tzanck preparation. Burns can usually be differentiated on the basis of clinical history and the presence of pain.

Who is at Risk for Developing this Disease?

Impetigo occurs more commonly in children. Neonates are a particularly high-risk group. Impetigo occurs equally in males and females.

What is the Cause of the Disease?

Etiology

S aureus is the causal agent in all cases of bullous impetigo, and most cases of nonbullous impetigo. Streptococcus pyogenes causes the remaining cases of nonbullous impetigo.

Pathophysiology

In terms of pathophysiology, in nonbullous impetigo, infection usually occurs at sites of trauma, such as bites or abrasions. The exposed skin allows the organisms to invade and establish cutaneous infection. In bullous impetigo, the bullae are formed as a result of staphylococcal toxins, which cause acantholysis The target molecule of the exotoxins is is desmoglein 1, and therefore the appearance can be similar to that of pemphigus foliaceus. In contrast to nonbullous impetigo, these lesions can manifest on intact skin.

Systemic Implications and Complications

Impetigo very rarely progresses to systemic infection. Glomerulonephritis can occur rarely as a sequela of streptococcal infection.

Treatment Options

Treatment options for impetigo are summarized in Table I.

Table I.

Treatment options for impetigo
Topical Oral Physical
Mupirocin Ointment cephalosporin/semisynthetic penicllin gentle debridement of crusts
Retapamulin Ointment tetracyclines, clindamycin, trimethoprim-sulfamethoxasole, linezolid  

Optimal Therapeutic Approach for this Disease

For localized infection, topical therapy is appropriate. For mupurocin ointment, the medication should be applied to the affected area three times daily. The area treated may be covered with a gauze dressing if desired. Patients not showing a clinical response within 3 to 5 days should be reevaluated. The drug is usually appplied for a 7-day course.

Retapamulin ointment, a novel agent, is indicated for use in adults and pediatric patients aged 9 months and older for the topical treatment of impetigo. It is applied twice daily for 5 days. Systemic antibiotics are indicated for more extensive disease and for cases of bullous impetigo. Cephalexin 500mg orally twice daily for 7 days or dicloxicillin 500 mg orally twice daily for 7 days in adults. Weight based dosing in children.

Patient Management

Patients should be closely followed for response to therapy. If there is suboptimal response to topical therapy, a switch to oral antibiotics would be warranted.

Unusual Clinical Scenarios to Consider in Patient Management

Household spread is common. Skin care should be reviewed with the family. If this is not done, the chance of re-infection or persistent infection is high. Bleach baths are an option for decolonization.

What is the Evidence?

Hirschmann, JV. "Impetigo: etiology and therapy". Curr Clin Top Infect Dis. vol. 22. 2002. pp. 42-51.

Cole, C, Gazewood, J. "Diagnosis and treatment of impetigo". Am Fam Physician. vol. 75. 2007. pp. 859-64.

(These articles review the diagnosis and treatment of impetigo.)

Champney, W, Rodgers, W. "Retapamulin inhibition of translation and 50S ribosomal subunit formation in Staphylococcus aureus cells". Antimicrob Agents Chemother. vol. 51. 2007. pp. 3385-7.

Davidovich, C, Bashan, A, Auerbach-Nevo, T. "Induced-fit tightens pleuromutilins binding to ribosomes and remote interactions enable their selectivity". Proc Natl Acad Sci USA. vol. 104. 2007. pp. 4291-6.

(These two articles review the mechanism of action of retapamulin.)
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