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Porrigo impetigo contagiosa

Image: “Porrigo (impetigo contagiosa).” by George Henry Fox – Fox, George Henry (1886) Photographic illustrations of skin diseases (2nd ed.), E.B. Treat Retrieved on 25 September 2010. License: Public Domain

Definition of Impetigo and Related Disorders

Impetigo is a contagious, bacterial infection of the skin, characterized by bullous or non-bullous lesions. The crusts in non-bullous impetigo are honey-colored and located on the face and upper and lower extremities. Bullous impetigo, as the name implies, involves the formation of bullous skin eruptions.

Staphylococcal scalded skin syndrome (SSSS) is a variant of impetigo in newborns. It manifests as acute exfoliation of the skin that is associated with acute cellulitis. The condition can be mild, which presents with a few blisters localized to the site of infection, or severe, which involves the whole body.

Epidemiology of Impetigo

Impetigo is one of the most common skin infections diagnosed by family physicians. Every year, more than 11 million skin infections are caused in the United States by Staphylococcus aureus alone. Children aged 2-5 years old are most commonly affected. The most common cause of impetigo is a secondary bacterial infection arising from a mosquito bite.

On the other hand, SSSS is often encountered in neonates and young children. Adults can develop SSSS too; however, it is usually limited to those who are critically ill and have co-morbidities such as renal failure and immunosuppression. The mortality of localized SSSS is estimated to be 1%, while that of SSSS associated with sepsis can be as high as 60%.

Pathophysiology of Impetigo

Staphylococcus pyogenes and S. aureus can colonize the nasal cavity, axilla, and perineal areas. If the integrity of the skin is compromised, i.e. by an insect bite, these bacteria, considered as possible pathogens, can multiply and cause infections.

Impetigo elbow

Image: “Impetigo.” by Evanherk at Dutch Wikipedia. License: CC BY-SA 3.0

Therefore, skin injury and trauma are key risk factors for this condition. Poor hygiene, hot weather, diabetesand malnutrition are also possible risk factors. Additionally, anaerobic bacterial infections can be implicated in the etiology of impetigo.

SSSS results from a reaction to the exfoliative toxins produced by S. aureus in approximately 5% of infections. These toxins are responsible for the separation of the epidermis from the granular cell layer, which results in the formation of bullae.

The condition can be localized, or generalized to the whole body. The toxins produced in SSSS are believed to spread through the hematogenous route in contrast to simple bullous impetigo, where toxins remain localized to the skin.

Clinical Presentation of Impetigo

Impetigo can be classified into bullous and non-bullous variants. The non-bullous type is most common and responsible for 70% of cases. Impetigo can be preceded by trauma, insect bites, scabies, or herpetic skin lesions. A history of diabetes or other systemic conditions predispose patients to impetigo.

The non-bullous form presents with a maculopapular skin rash that includes small vesicles. These vesicles rupture and form pruritic skin erosions and honey-colored crusts. The condition is characterized by spontaneous healing without scarring.

On the other hand, patients with bullous impetigo present with large and fragile bullae that can rupture. These bullae contain a yellow-colored fluid. These bullae result from the local reaction to exfoliative toxins produced by S. aureus. Simple bullous impetigo differs from SSSS in that, toxins do not spread by the hematogenous route in the former.

Patients with severe impetigo can develop a fever; diarrhea and fatigue can complicate the severity of the condition.

Patients with SSSS develop fever, irritability, and skin tenderness. The skin is edematous and warm to the touch and shows the formation of multiple bullae. Despite the severity of SSSS, most patients neither develop sepsis nor are they critically ill.

Diagnostic Work-up for Impetigo

Both types of impetigo are diagnosed clinically. Diagnoses between impetigo and other blistering diseases of the skin may be challenging in some cases. Laboratory investigations are usually reserved for patients who do not respond to antibacterial therapy.

A culture of the pus samples or bullous fluid can reveal the responsible pathogen in several cases. Serologic testing for streptococcal antibodies is not useful in impetigo.

Patients might develop septicemia and leukocytosis. Impetigo near the joints can spread to the joint space and cause septic arthritis. Osteomyelitis, caused by the hematogenous or direct spread of bacteria from the infected skin site to the bone can also complicate the picture.

Bullous Impetigo biopsy

Image: “H&E Staining of Bullous Impetigo.” by Romanoanthony28 – Own work. License: CC0 1.0

X-rays can help confirm the diagnosis of osteomyelitis. Magnetic resonance imaging is indicated in complicated cases to assess signs of osteomyelitis that involve soft tissues in addition to the bone.

Erythrocyte sedimentation rate and white blood cell counts may be elevated in SSSS. Patients with sepsis and those severely dehydrated should undergo renal function tests to exclude possible acute renal injury or electrolyte disturbances. A biopsy of the infected area usually shows a separation of the epidermis at the granular cell layer.

SSSS can evolve into toxic epidermal necrolysis, which carries a significant risk of sepsis and mortality. A biopsy can help differentiate between SSSS and toxic epidermal necrolysis.

In SSSS, there are no significant inflammatory cellular infiltrates in the skin. On the other hand, patients with toxic epidermal necrolysis usually have lymphocytic infiltrates in addition to the deeper separation of the epidermis at the basement membrane rather than the granular cell layer.

Treatment of Impetigo

Patients with simple non-bullous impetigo benefit from topical antibiotics. Fusidic acid, mupirocin, and retapamulin are often considered for the treatment of impetigo.

Patients with large bullae, or those who do not respond well to treatment with topical antibiotics, should receive oral antibiotics. Amoxicillin/clavulanate, cephalexin, and clindamycin have been used as systemic (oral) antibiotics for the treatment of bullous impetigo. The choice of the antibiotic should be based on the physician’s judgment and factoring in any history of allergies, considering that these different classes of antibiotics are equally efficacious.

Therefore, the current recommendation is to use topical antibiotics for impetigo. Oral antibiotics should be used when patients do not respond to topical treatment or when this modality is not practical. Patients should not be indicated erythromycin because drug resistance among the causative organisms is high. Topical disinfectants and antibacterial soaps are not useful in impetigo.

Patients with SSSS are often dehydrated and develop high fever; therefore, antipyretic therapy in addition to fluid replacement therapy is indicated. Patients who are confirmed to have SSSS should undergo an extensive physical examination to identify the primary focus of the infection.

Since SSSS is caused by S. aureus, antibiotics against staphylococcal infections are indicated. Patients should receive nafcillin or oxacillin because these antibiotics are resistant to penicillinase, which is an enzyme produced by S. aureus and responsible for penicillin resistance.

Patients who are suspected to have methicillin-resistant S. aureus SSSS should receive either vancomycin or linezolid.

Patients with SSSS have severe skin eruptions; therefore, wound care should be provided in a specialized burn unit, if necessary.

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