What does a polymorphous rash in a child with suspected Kawasaki disease look like?

Polymorphous Rash in Kawasaki Disease

The polymorphous rash in Kawasaki disease appears as a maculopapular, diffuse erythroderma, or erythema multiforme-like eruption that can vary significantly in appearance between patients, often with perineal accentuation and early desquamation in that region. 1

Primary Morphologic Patterns

The rash can present in three distinct forms, all considered diagnostic:

• Maculopapular pattern: The most common presentation, appearing as discrete red macules and papules distributed across the trunk and extremities 1
• Diffuse erythroderma: Presents as widespread redness of the skin, resembling a sunburn-like appearance 1
• Erythema multiforme-like: Target-like lesions that can mimic erythema multiforme, though true vesicles or bullae should prompt consideration of alternative diagnoses 1

Key Distinguishing Features

The term "polymorphous" refers to the variability in appearance between different patients, not necessarily multiple morphologies in the same patient. 1

Critical characteristics that help identify this rash:

• Perineal involvement: The rash frequently shows accentuation in the perineal region with early desquamation, which is a particularly helpful diagnostic clue 1, 2
• Timing: The rash typically appears within the first 5 days of fever onset 1, 2
• Distribution: Generally involves the trunk and may extend to extremities, though distribution patterns vary 1
• Non-vesicular: The absence of vesicles is important—vesicular or bullous lesions should raise concern for alternative diagnoses like varicella 1

What the Rash Does NOT Look Like

Important exclusionary features that argue against Kawasaki disease:

• No exudative features: Vesiculobullous or petechial rashes should prompt consideration of other diagnoses 1
• No purpura: Petechial or purpuric elements are not typical and suggest alternative conditions like meningococcemia 1
• No exudate: Any exudative component should raise suspicion for bacterial or other infectious etiologies 1

Clinical Context and Diagnostic Pitfalls

The rash must be interpreted within the complete clinical picture:

• Diagnostic criteria: The rash is one of five principal features; diagnosis requires fever ≥5 days plus ≥4 of the 5 features (including the polymorphous exanthem, bilateral conjunctival injection, oral/lip changes, extremity changes, and cervical lymphadenopathy) 1
• Incomplete presentation: The rash may be subtle, fleeting, or absent in incomplete Kawasaki disease, which still carries significant risk for coronary complications 1
• Temporal evolution: A careful history may reveal that the rash was present earlier in the illness but resolved by the time of presentation 1

Critical pitfall: The polymorphous nature means the rash can be easily mistaken for viral exanthems, drug reactions, or other conditions. The key is recognizing the constellation of features rather than relying on the rash appearance alone. 1, 3

Associated Skin Findings

Beyond the primary rash, look for:

• Perineal desquamation: Often occurs in the subacute phase and can be an early sign 1
• Beau's lines: Transverse furrows of fingernails appear during convalescence 1
• BCG site reactivation: Erythema and induration at previous BCG vaccination sites can occur 1, 4

The diagnosis of Kawasaki disease should never be delayed waiting for the "perfect" rash appearance, as early treatment (within 10 days of fever onset) is critical to prevent coronary artery aneurysms. 1, 2