What is the differential diagnosis for a 7-year-old boy with migrating erythematous and painful macules, initially retroauricular then on the trunk, without fever or other symptoms?

Differential Diagnosis for Migrating Erythematous Painful Macules in a 7-Year-Old Boy

The most likely diagnosis is a viral exanthem, particularly human herpesvirus 6 (HHV-6) or HHV-7, given the retroauricular onset, migration to trunk, absence of fever, and age of the patient. 1

Primary Diagnostic Considerations

Viral Exanthems (Most Likely)

• HHV-6 and HHV-7 are the most common causes of fever and maculopapular rash in children, with HHV-6 accounting for 24% and HHV-7 for 9% of cases presenting with fever and rash. 1
• HHV-6 typically affects younger children (mean age 1.6 years), while HHV-7 affects slightly older children (mean age 4.5 years), making HHV-7 more likely in this 7-year-old. 1
• The retroauricular onset is characteristic of viral exanthems, particularly rubella-like presentations, though the absence of fever is atypical. 1
• Co-infection with multiple herpesviruses occurs in approximately 4% of cases. 1

Unilateral Laterothoracic Exanthem (Asymmetric Periflexural Exanthem)

• This condition presents with sudden onset of unilateral erythematous macules with scaling affecting trunk, axillary, and inguinal regions, often following a recent upper respiratory infection. 2
• The migration pattern from retroauricular to trunk with only 3-4 lesions could represent an atypical or early presentation. 2
• Typically self-limited and requires no specific treatment. 2

Important Exclusions Required

Kawasaki Disease (Must Rule Out)

• Kawasaki disease requires fever persisting at least 5 days plus 4 of 5 principal features: extremity changes, polymorphous exanthem, bilateral conjunctival injection, oral/lip changes, and cervical lymphadenopathy ≥1.5 cm. 3
• The absence of fever makes Kawasaki disease extremely unlikely, as fever is an absolute requirement for diagnosis. 3
• However, the retroauricular location could represent cervical lymphadenopathy, and the trunk rash could be the polymorphous exanthem. 3
• Critical pitfall: Incomplete Kawasaki disease can present with fewer than 4 principal features if coronary artery disease is detected on echocardiography. 3

Rocky Mountain Spotted Fever (Critical to Exclude)

• RMSF presents with maculopapular rash involving palms and soles, but up to 40% of patients report no tick bite history. 3
• The absence of fever is highly atypical for RMSF, as fever is nearly universal. 3
• RMSF can have rapid progression with 50% of deaths occurring within 9 days of illness onset, making early recognition critical. 3
• The painful nature of the macules and migration pattern are not typical for RMSF. 3

Drug Reaction or Hypersensitivity

• Medication history within the past 8 weeks is crucial, as Stevens-Johnson syndrome/toxic epidermal necrolysis typically occurs 1-3 weeks after drug exposure. 4
• The absence of mucosal involvement (eyes, mouth, genitals), skin detachment, or systemic symptoms makes SJS/TEN unlikely. 4
• Simple drug eruptions typically do not present with painful lesions or specific migration patterns. 4

Diagnostic Algorithm

Immediate Assessment

• Obtain detailed history of recent viral illness, particularly upper respiratory infection or fever 2-4 weeks prior. 2, 1
• Document exact progression pattern: timing of retroauricular lesions, interval to trunk involvement, and whether lesions are truly migrating or new lesions appearing. 3
• Examine for all Kawasaki disease criteria even without fever: conjunctival injection, oral changes, extremity changes (erythema, edema, desquamation), and lymphadenopathy. 3
• Assess for mucosal involvement, skin pain beyond the lesions themselves, or positive Nikolsky sign to exclude SJS/TEN. 4

Laboratory Evaluation

• If Kawasaki disease cannot be excluded clinically: obtain ESR, CRP, CBC (looking for thrombocytosis in subacute phase), albumin, and echocardiography. 3
• If viral exanthem suspected: multiplex PCR for HHV-6, HHV-7, EBV, and parvovirus can confirm diagnosis. 1
• If RMSF considered based on geographic location or tick exposure: do not delay empiric doxycycline while awaiting serology, as early serology is often negative. 3

Physical Examination Specifics

• Measure any retroauricular lymph nodes; nodes ≥1.5 cm diameter meet Kawasaki criteria for cervical lymphadenopathy. 3
• Examine palms and soles for erythema or edema (Kawasaki disease) or petechial rash (RMSF). 3
• Assess lesion characteristics: true macules versus papules, presence of scale, central pallor or necrosis. 3, 2
• Document pain characteristics: whether pain is in the lesions themselves or surrounding tissue. 3

Management Approach

If Viral Exanthem Most Likely

• Supportive care only with observation for development of fever or additional symptoms. 1
• Reassurance that viral exanthems are self-limited, typically resolving within 5-14 days. 3
• Return precautions for development of fever, mucosal involvement, or worsening rash. 4

If Kawasaki Disease Cannot Be Excluded

• Urgent pediatric cardiology consultation and echocardiography, as coronary artery disease can occur even with incomplete presentations. 3
• Consider empiric IVIG and aspirin if clinical suspicion is high, as delayed treatment increases risk of coronary complications. 3

If Diagnosis Remains Unclear

• Dermatology consultation within 24-48 hours for uncertain cases. 4
• Document rash progression with photography to assess evolution. 4
• Close follow-up in 24-48 hours to reassess for development of fever or additional diagnostic features. 3, 4

Critical Pitfalls to Avoid

• Do not dismiss the absence of fever as excluding Kawasaki disease entirely; some patients present with incomplete criteria and develop coronary aneurysms. 3
• Never assume a benign viral exanthem without excluding Kawasaki disease in any child with rash and lymphadenopathy. 3
• Do not delay empiric antibiotic therapy if RMSF is considered based on geography or season, as mortality is high with delayed treatment. 3
• Avoid empirical topical corticosteroids without establishing diagnosis, as they can worsen HSV infections. 4
• Do not ignore the "painful" nature of the lesions; this is atypical for simple viral exanthems and warrants closer evaluation. 3