Infectious Disease · Skin and Soft Tissue Infections
The facts most likely to be tested
Erysipelas is a superficial form of cellulitis involving the upper dermis and superficial lymphatics.
The most common causative pathogen is Group A Streptococcus (Streptococcus pyogenes).
Physical examination reveals a sharply demarcated, raised, erythematous plaque with a clear border.
The classic clinical presentation involves the face or lower extremities accompanied by acute onset of fever and chills.
Erysipelas is distinguished from cellulitis by its well-defined margins and superficial involvement compared to the deeper dermis and subcutaneous fat involvement of cellulitis.
First-line treatment for non-purulent, mild cases is penicillin V, amoxicillin, or cephalexin.
Diagnosis is primarily clinical based on the characteristic bright red appearance and palpable border.
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A 55-year-old woman presents to the urgent care clinic with a 2-day history of a painful, spreading rash on her left cheek. She reports associated malaise, fever, and chills. On physical examination, there is a bright red, indurated plaque with a sharply demarcated border that is warm to the touch. The patient has no history of trauma or recent surgery in the area. There is no evidence of purulence or fluctuance.
What is the most likely diagnosis?
Erysipelas
The diagnosis is based on the classic presentation of a sharply demarcated, raised, erythematous plaque on the face, which is pathognomonic for erysipelas caused by Group A Streptococcus.
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Etiology / Epidemiology
Caused by Group A Streptococcus (*S. pyogenes*). Primary risk factors include lymphatic obstruction, venous insufficiency, and skin barrier disruption.
Clinical Manifestations
Presents as a well-demarcated, raised, erythematous plaque. Classic St. Anthony's fire appearance with sharp borders.
Diagnosis
Primarily a clinical diagnosis. Blood cultures are low yield and reserved for systemic toxicity.
Treatment
First-line is Penicillin V or Amoxicillin. Anaphylaxis requires alternative agents like clindamycin.
Prognosis
Rapid response to antibiotics expected. Recurrence is common without treating underlying tinea pedis or edema.
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Epidemiology & Etiology
Erysipelas is a superficial form of cellulitis caused almost exclusively by Group A Beta-hemolytic Streptococcus. It most frequently affects the lower extremities and face. Patients with lymphedema or chronic venous stasis are at significantly higher risk for recurrent infection.
Pertinent Anatomy
The infection is confined to the upper dermis and superficial lymphatics. This superficial localization explains the characteristic raised, sharp borders that distinguish it from deeper cellulitis.
Pathophysiology
Entry occurs via minor skin trauma or fissures, often secondary to tinea pedis. The bacteria proliferate in the superficial lymphatic vessels, triggering an intense inflammatory response. This rapid spread leads to the classic St. Anthony's fire presentation of intense, burning erythema.
Clinical Manifestations
The hallmark is a well-demarcated, indurated, erythematous plaque with a raised border. Patients often present with acute onset of fever, chills, and malaise. Red flags include bullae formation, skin necrosis, or rapid progression, which may suggest necrotizing fasciitis.
Diagnosis
Diagnosis is clinical. Laboratory studies are generally unnecessary in uncomplicated cases. If systemic toxicity is present, blood cultures may be obtained, though they are positive in less than 5% of cases.
Treatment
For mild cases, oral Penicillin V or Amoxicillin is the treatment of choice. In patients with penicillin allergy, use clindamycin or a macrolide. Severe or systemic cases require IV Penicillin G or ceftriaxone.
Prognosis
Most patients show improvement within 24-48 hours of antibiotic initiation. Recurrence is a major clinical challenge; long-term management of edema and treatment of fungal skin infections are essential to prevent future episodes.
Differential Diagnosis
Cellulitis: Indistinct, non-palpable borders
Necrotizing fasciitis: Pain out of proportion to exam, crepitus
Contact dermatitis: Pruritic, history of exposure
Stasis dermatitis: Bilateral, chronic, associated with venous insufficiency
Erythema migrans: Targetoid lesion, history of tick bite