What is the differential diagnosis (Dd) and management for a patient presenting with fever for 8 days and a rash that starts on the face and then involves the trunk?

Differential Diagnosis of 8-Day Fever with Rash Starting on Face Then Trunk

The most critical diagnoses to consider are Still's disease (systemic juvenile idiopathic arthritis/adult-onset Still's disease), Kawasaki disease in children, measles, and scarlet fever, with the pattern of rash progression and fever duration being key distinguishing features.

Primary Differential Diagnoses

Still's Disease (sJIA/AOSD)

• Fever ≥39°C (102.2°F) for at least 7 days is the hallmark presentation, making this highly consistent with your 8-day fever duration 1
• The rash is typically salmon-pink, erythematous, transient, and coincides with fever spikes, preferentially involving the trunk 1
• Musculoskeletal involvement with arthralgia/myalgia is usually present, though overt arthritis may appear later and is not required for diagnosis 1
• Laboratory findings show neutrophilic leukocytosis, markedly elevated CRP, ESR, and ferritin 1
• Marked elevation of serum IL-18 and/or S100 proteins (calprotectin) strongly supports the diagnosis if available 1
• This diagnosis should be considered in both children and adults, as they represent the same disease entity 1

Kawasaki Disease (Primarily Pediatric)

• Fever persisting at least 5 days with high spiking temperatures (39-40°C) is characteristic 1
• The rash is polymorphous, typically nonspecific diffuse maculopapular eruption appearing within 5 days of fever onset, with trunk and extremity involvement and perineal accentuation 1
• Bilateral bulbar conjunctival injection without exudate is a key distinguishing feature 1
• Changes in lips and oral cavity (erythema, cracking, strawberry tongue) and changes in extremities (erythema of palms/soles, edema) are critical diagnostic features 1
• Cervical lymphadenopathy (≥1.5 cm diameter) may be present 1
• This is a medical emergency requiring prompt echocardiography to assess for coronary artery involvement 1

Measles

• Fever for several days followed by a maculopapular rash that begins on the face and spreads cephalocaudally to the trunk and extremities is the classic pattern 1, 2
• Consider recent travel history or exposure to unvaccinated individuals 3
• Cough, coryza, and conjunctivitis (the "3 Cs") typically precede the rash 2

Scarlet Fever

• Rash typically develops on the upper trunk then spreads throughout the body, sparing palms and soles 4
• The rash has a sandpaper-like texture and is associated with streptococcal pharyngitis 5, 4
• Desquamation occurs in periungual regions and may extend to palms and soles in the recovery phase 5
• Fever duration is typically shorter than 8 days unless complications develop 4

Secondary Considerations

Rat Bite Fever

• Rash can involve palms and soles, appearing 2-10 days after exposure 6
• Requires history of rodent exposure 6
• The rash may be nonspecific and mimic other conditions 6

Rocky Mountain Spotted Fever

• Rash begins as small blanching pink macules evolving to maculopapular with central petechiae by day 5-6 7
• Rash can involve palms and soles, which is a critical diagnostic clue 6, 7
• Requires tick exposure history in endemic areas 7

Drug Hypersensitivity Reactions

• Must exclude recent medication exposures 1, 2
• Timing of drug initiation relative to symptom onset is crucial 2

Critical Diagnostic Approach

Immediate Assessment Priorities

• Measure temperature pattern: Document if fever is spiking (characteristic of Still's disease) versus continuous 1
• Characterize rash morphology: Salmon-pink and transient (Still's), polymorphous (Kawasaki), maculopapular spreading cephalocaudally (measles), sandpaper-like (scarlet fever) 1, 4
• Assess for conjunctival injection, oral changes, and extremity changes to identify Kawasaki disease 1
• Evaluate for arthralgia/myalgia suggesting Still's disease 1

Essential Laboratory Workup

• Complete blood count with differential: Look for neutrophilic leukocytosis (Still's), thrombocytopenia (RMSF, viral infections) 1, 7
• Inflammatory markers: CRP, ESR, ferritin (markedly elevated in Still's disease) 1
• IL-18 and calprotectin if available to support Still's disease diagnosis 1
• Blood cultures to exclude bacterial infections 2
• Throat culture or rapid strep test if scarlet fever suspected 5

Life-Threatening Complications to Monitor

• Macrophage activation syndrome (MAS) in Still's disease: Monitor for persistent fever, splenomegaly, rising ferritin, falling cell counts, abnormal liver function 1
• Coronary artery involvement in Kawasaki disease: Requires urgent echocardiography 1
• Severe vasculitis in RMSF: Requires immediate doxycycline 7

Critical Pitfalls to Avoid

• Do not wait for arthritis to develop before diagnosing Still's disease—arthralgia alone with appropriate fever and rash pattern is sufficient 1
• Do not dismiss Kawasaki disease in older children or adults—while rare, it can occur beyond typical age ranges 1
• Do not overlook travel history—many infectious causes have geographic distributions 3, 8
• Do not assume viral etiology without excluding treatable bacterial and inflammatory conditions—the 8-day fever duration suggests a more serious process 1, 2