What is the differential diagnosis and management for a 9-month-old infant with an eruptive rash on the right lower leg?

Differential Diagnosis for Eruptive Rash on Right Lower Leg in a 9-Month-Old

The most critical diagnosis to rule out immediately is Rocky Mountain Spotted Fever (RMSF), which can be fatal within 9 days if untreated and requires immediate doxycycline without waiting for laboratory confirmation. 1, 2

Life-Threatening Diagnoses That Cannot Be Missed

Rocky Mountain Spotted Fever

• RMSF classically begins as small (1-5 mm) blanching pink macules on ankles, wrists, or forearms that spread centrally and evolve to maculopapular with central petechiae. 1, 2
• Up to 40% of RMSF patients report no tick bite history, so absence of tick exposure should NOT exclude this diagnosis. 1, 2
• 50% of RMSF deaths occur within 9 days of illness onset, and children under 15 years develop rash earlier and more frequently than adults. 1, 2
• Peak season is April-September, but can occur year-round. 2
• Associated symptoms include fever, headache, chills, malaise, myalgia, nausea, vomiting. 1

Meningococcemia

• Cannot be reliably distinguished from tickborne diseases on clinical grounds alone. 2
• Consider intramuscular ceftriaxone pending blood cultures because of this diagnostic overlap. 2

Eczema Herpeticum

• Presents with multiple uniform "punched-out" erosions or vesiculopustular eruptions that are very similar in shape and size. 2, 3
• May progress rapidly to systemic infection without antiviral therapy. 2
• Requires immediate systemic acyclovir and empirical antibiotics for secondary bacterial infection. 2, 4

Common Infectious Causes

Viral Exanthemas

• Enteroviral infections can cause fever and rash in children, including palms and soles involvement. 2
• Roseola (human herpesvirus 6) presents with rash after fever resolution. 2, 5
• Erythema infectiosum (fifth disease) characterized by viral prodrome followed by "slapped cheek" facial rash. 5

Bacterial Infections

• Impetigo is a superficial bacterial infection that most commonly affects the face and extremities of children. 5
• Severe bacterial superinfection suggested by extensive crusting, weeping, or honey-colored discharge. 4

Fungal Infections

• Tinea is a common fungal skin infection in children that affects the scalp, body, groin, feet, hands, or nails. 5

Non-Infectious Causes

Atopic Dermatitis

• Requires pruritus plus at least three of: involvement of flexural areas, personal or family history of atopy, dry skin, and visible eczema. 1, 2, 3
• In children under 4 years, eczema commonly affects cheeks or forehead and outer limbs rather than just lower extremities. 1
• Deterioration in previously stable eczema may indicate secondary bacterial infection or contact dermatitis. 1, 3

Drug Hypersensitivity

• Beta-lactams and NSAIDs are most commonly implicated drugs. 6
• Viral exanthema can mimic drug exanthema when children take medication during viral infection. 6

Critical History Elements to Obtain

• Fever, headache, chills, myalgias, nausea/vomiting. 1, 2
• Recent outdoor activities, camping, hiking, or playing in brushy areas. 2
• Tick exposure or pet exposure. 2
• Geographic location and season. 2
• Medication history. 2, 6
• Age of onset of lesions (before 6 months suggests mastocytosis or congenital atopic dermatitis). 3
• Pruritus and scratching (mandatory criterion for atopic eczema). 1, 3
• Family history of atopy (asthma, hay fever). 1, 3

Essential Physical Examination Findings

• Check for rash on palms and soles. 1, 2
• Assess for petechiae. 1, 2
• Look for uniform "punched-out" lesions suggesting eczema herpeticum. 2, 3
• Examine scalp, axillae, groin for attached ticks. 2
• Evaluate for conjunctival injection and altered mental status. 2
• Distribution of lesions (flexural areas for atopic eczema, trunk and extremities for mastocytosis). 3
• Sign of Darier (urtication on rubbing, positive in 89-94% of mastocytoses). 3

Immediate Laboratory Testing

For suspected RMSF or tickborne disease: 2

• CBC with differential
• Comprehensive metabolic panel
• Blood cultures
• Acute serum for IgG and IgM antibodies to R. rickettsii, E. chaffeensis, A. phagocytophilum

For suspected eczema herpeticum: 2, 3

• Tzanck smear
• Viral cultures or PCR

For suspected mastocytosis: 3

• Serum tryptase
• Complete blood count
• Skin biopsy with immunostaining for tryptase and KIT

Management Algorithm

If ANY suspicion for RMSF or tickborne disease:

• Start doxycycline immediately without waiting for laboratory confirmation. 1, 2
• Consider intramuscular ceftriaxone to cover meningococcemia pending cultures. 2
• Never wait for serologic confirmation—serology is negative in first week. 1, 2

If eczema herpeticum suspected:

• Start systemic acyclovir immediately. 2, 4
• Add empirical antibiotics (cephalexin or flucloxacillin) for secondary bacterial infection. 4, 3

If atopic dermatitis diagnosed:

• Emollients at least twice daily. 1, 4
• Mild potency topical corticosteroid if significant inflammation present. 4
• Adequate amounts of emollients should be prescribed and used liberally. 1

If bacterial superinfection present:

• Flucloxacillin is most appropriate antibiotic for treating Staphylococcus aureus infection. 4

Critical Pitfalls to Avoid

• Never exclude RMSF based on absence of tick bite history or geographic location. 1, 2
• Never wait for serologic confirmation before starting treatment for suspected RMSF—serology is negative in first week. 1, 2
• Never dismiss a rapidly evolving rash as "just eczema" without ruling out eczema herpeticum. 2, 3
• Never underestimate neonatal pustules, which always require investigation to exclude infectious disease. 3
• Ticks are small and bites frequently go unnoticed because they attach in difficult-to-observe places like scalp, axillae, and inguinal regions. 1