Clinical Presentation
There are two main types of impetigo—bullous (characterized by bullae) and nonbullous. Bullae are fluid-filled lesions >0.5 cm. Impetigo may also be classified as a primary or secondary infection. The former refers to a direct invasion of bacteria to intact skin, and the latter refers to an underlying condition that disrupts the skin.
Bullous impetigo almost exclusively results from an S aureus dermatologic infection that manifests when the stratum corneum layer of the skin is raised. This form presents as an erythroderma that is toxin mediated, resulting in the sloughing of the epidermal layer of the skin, ultimately leading to large areas of skin loss. Vesicles turn into bullae and fill with fluid, which later rupture to form a light-brown crust. The bullous form is the least common type of impetigo.
Nonbullous impetigo is the most common form, occurring in about 70% of cases. It indicates a host response to the infection. Methicillin-resistant S aureus (MRSA) impetigo is most commonly associated with the nonbullous form. Although the lesions are not considered to be bullae, the fluid-filled vesicles may appear transiently and are <0.5 cm in diameter. The infection initially appears as red maculae/papulae on the surface of the skin. Impetigo typically presents in the face, but may be transmitted wherever skin is exposed. After 4 to 6 days, the maculae turn into painful pustules or blisters that rupture and dry into a thick, golden-yellow crust that is often pruritic. The impetigo affects areas of the skin (usually on the face and extremities) that have been associated with trauma such as mosquito bites and scratches.
The skin infection resolves within 2 weeks when left untreated, and often heals without scarring. Diagnosis is usually made through visual observation; however, a culture may be indicated when a patient has not responded to therapy, which may be suggestive of a MRSA infection. Systemic manifestations of impetigo are minimal and include weakness, fever, and diarrhea.