Ektima

01Pathophysiology

Ecthyma is best understood as a deeper extension of non-bullous impetigo. While impetigo is confined to the epidermis, ecthyma penetrates through the full thickness of the epidermis and into the superficial dermis, producing true ulceration. This distinction in depth is the single most important concept for the exam.

The infection begins when Group A Streptococcus (Streptococcus pyogenes) gains entry through a break in the skin barrier, most commonly from insect bites, minor trauma, or pre-existing dermatitis. The organism produces streptokinase, hyaluronidase, and streptococcal pyrogenic exotoxins that facilitate tissue invasion beyond the superficial layers. Secondary colonization by Staphylococcus aureus is common and contributes to purulence, but the primary driver of tissue destruction is streptococcal.

Once the infection reaches the dermis, the host inflammatory response creates a necrotic ulcer base surrounded by a dense crust composed of dried serum, fibrin, neutrophilic debris, and bacteria. This crust becomes firmly adherent to the ulcer bed, which is why ecthyma lesions are characteristically described as having a "stuck-on," dirty-gray crust that is difficult to remove. Beneath this crust lies a shallow, well-demarcated, punched-out ulcer with purulent material at the base.

Because ecthyma involves the dermis, it heals with scarring, unlike impetigo which resolves without permanent marks. This is a testable distinction. The predilection for the lower extremities reflects the fact that these areas are most prone to insect bites, minor trauma, and relative neglect of wound care, particularly in children living in resource-limited or tropical settings.

The streptococcal etiology also means ecthyma carries a risk of post-streptococcal glomerulonephritis (PSGN) as a non-suppurative complication. However, unlike streptococcal pharyngitis, cutaneous streptococcal infections do not cause acute rheumatic fever. This is a frequently tested pearl.

02Classification and Clinical Manifestation

It is critical to recognize that classic ecthyma and ecthyma gangrenosum are entirely different diseases. Exam writers frequently place both in the same question stem or answer set to test whether you can distinguish them. Classic ecthyma is a localized streptococcal skin infection in an otherwise well patient; ecthyma gangrenosum is a sign of systemic Pseudomonas infection in a severely immunocompromised host with high mortality.

Lesions are most commonly found on the shins, dorsal feet, and thighs. They may be single or multiple, and regional lymphadenopathy can be present. Systemic symptoms such as fever are uncommon unless secondary cellulitis or bacteremia develops.

03Diagnostic Workup

Ecthyma is primarily a clinical diagnosis. The combination of a punched-out ulcer with a thick, adherent crust on the lower extremities of a child in a tropical setting or with poor hygiene is often sufficient.

When the diagnosis is uncertain or the lesion fails to respond to empiric therapy, a wound culture should be obtained. The correct technique is to remove the crust first and culture the base of the ulcer, as surface cultures may yield only colonizing organisms rather than the true pathogen.

Skin biopsy is reserved for atypical cases where the differential includes other ulcerative conditions such as cutaneous leishmaniasis, vasculitis, or pyoderma gangrenosum. Histologically, ecthyma shows full-thickness necrosis of the epidermis with extension into the dermis, a dense neutrophilic infiltrate, and fibrinopurulent debris.

An important serologic point: for cutaneous streptococcal infections, the anti-DNase B antibody is the preferred marker of recent infection, as the ASO titer is often negative in skin infections. This is a testable detail, particularly in questions linking ecthyma to subsequent PSGN. If a patient develops edema, hypertension, or dark urine weeks after ecthyma, order a urinalysis and anti-DNase B titer.

04Management and Treatment

The key management principle that distinguishes ecthyma from simple impetigo is that ecthyma requires systemic antibiotics. Topical therapy alone is insufficient because the infection extends into the dermis, beyond the reach of topical agents. This is a frequent exam question: a student may select mupirocin as the answer for ecthyma, which would be appropriate for impetigo but not for ecthyma.

Step 1: Wound care. Begin by softening the adherent crust with warm saline or antiseptic soaks, then gently remove it. This allows both better drug penetration and facilitates wound healing. Leaving the crust intact delays resolution and increases the risk of deeper extension.

Step 2: Start systemic antibiotics. The chosen agent must cover both S. pyogenes and S. aureus, since co-infection is common. A first-generation cephalosporin such as cephalexin is the standard first-line choice. In penicillin-allergic patients, a macrolide such as azithromycin or clindamycin can be used, although local resistance patterns should guide selection.

Step 3: Consider MRSA coverage. If the patient has risk factors for methicillin-resistant S. aureus (recurrent skin infections, prior MRSA, healthcare exposure), switch to TMP-SMX, doxycycline, or clindamycin. Note that TMP-SMX covers MRSA well but has unreliable activity against Group A Streptococcus, so some clinicians combine it with a beta-lactam or use clindamycin instead when streptococcal coverage is also needed.

Step 4: Monitor for complications. Patients should be monitored for surrounding cellulitis, lymphangitis, or systemic signs that would warrant IV antibiotics. Urinalysis should be performed 1 to 3 weeks after resolution if PSGN is a concern.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get questions about ecthyma wrong is by confusing it with simple impetigo and selecting topical mupirocin as treatment. Because ecthyma extends into the dermis, topical therapy alone will not resolve the infection. When a vignette describes an ulcer beneath a thick, adherent crust on the leg, the answer is systemic antibiotics, not topical.

A second frequent trap is confusing classic ecthyma with ecthyma gangrenosum. If the patient is immunocompromised and the lesion has a black necrotic eschar, think Pseudomonas bacteremia and antipseudomonal antibiotics. If the patient is an otherwise healthy child with a gray-crusted ulcer on the shin, think streptococcal ecthyma and oral cephalexin.

Third, remember the streptococcal complication asymmetry: skin infections with Group A Streptococcus can trigger post-streptococcal glomerulonephritis but NOT acute rheumatic fever. If a vignette shows a child who had ecthyma two weeks ago and now presents with periorbital edema, hypertension, and cola-colored urine, the answer is PSGN. Also recall that anti-DNase B is the better serologic marker for preceding skin infection, not ASO.

Finally, the depth of infection is the core competency being tested. Exam writers want you to know the hierarchy: impetigo (epidermis only, honey crust, no scar, topical treatment sufficient) versus ecthyma (extends into dermis, thick adherent crust, punched-out ulcer, scars, systemic treatment required). If you can place the infection at the correct tissue depth based on clinical description, you will answer the question correctly.