What could be the diagnosis for a patient with a scattered rash, urticaria (hives) on their trunk, and erythematous (red) cheeks?

Differential Diagnosis: Urticarial Rash on Trunk with Erythematous Cheeks

The most likely diagnosis is Still's disease (systemic juvenile idiopathic arthritis or adult-onset Still's disease), which characteristically presents with urticarial or salmon-pink rash on the trunk coinciding with fever spikes, along with erythematous facial involvement. 1

Primary Diagnostic Considerations

Still's Disease (Most Likely)

• The rash in Still's disease is typically erythematous (salmon pink) and transient, often coinciding with fever spikes, and preferentially involves the trunk. 1
• Urticarial rashes are explicitly consistent with Still's disease diagnosis, not just the classic salmon-pink appearance. 1
• Very red cheeks (facial erythema) combined with truncal involvement fits the distribution pattern described in the 2024 EULAR/PRES guidelines. 1
• Fever is typically spiking with temperature ≥39°C (102.2°F) for at least 7 days—document if present. 1
• High levels of inflammation are typically identified by neutrophilic leukocytosis, increased ESR, serum CRP, and markedly elevated ferritin. 1

Acute Urticaria (Secondary Consideration)

• Generalized acute urticaria presents with clumps of wheals (hives) but typically lacks the distinctive facial erythema pattern. 1
• If there is known allergen exposure (foods, medications, insect stings), acute urticaria becomes more likely, though the red cheeks are atypical. 1
• Acute urticaria alone does not explain the erythematous cheeks unless there is concurrent angioedema. 1

Critical Diagnostic Workup

Immediate Assessment Required

• Document fever pattern meticulously—spiking fevers with temperature ≥39°C strongly support Still's disease. 1
• Assess for musculoskeletal involvement: arthralgia or myalgia is usually present in Still's disease, though overt arthritis may appear later (median delay of 1 month). 1
• Order inflammatory markers immediately: CBC with differential (looking for neutrophilic leukocytosis), ESR, CRP, and ferritin (markedly elevated in Still's disease). 1

Rule Out Life-Threatening Complications

• Screen for macrophage activation syndrome (MAS), the main life-threatening complication of Still's disease, which can occur at onset or during treatment. 1
• MAS presents with persistent fever, hepatosplenomegaly, cytopenias, elevated liver enzymes, hyperferritinemia, and coagulopathy. 1
• If MAS is suspected, treatment must be initiated rapidly as it carries significant mortality risk. 1

Additional Diagnostic Testing

• Liver function tests to assess for hepatic involvement (common in Still's disease). 1
• Platelet count and fibrinogen (typically elevated in Still's disease, but decreased in MAS). 1
• Consider screening for infections that could trigger Still's disease or mimic it. 1

Alternative Diagnoses to Exclude

Anaphylaxis

• If urticaria developed suddenly after known allergen exposure with any systemic symptoms (throat tightness, difficulty breathing, wheezing), treat as anaphylaxis with immediate intramuscular epinephrine. 1
• Generalized acute urticaria in the context of known allergen exposure could progress to life-threatening symptoms—err on the side of epinephrine injection if uncertain. 1

Drug Hypersensitivity

• Obtain complete medication history for all drugs taken in the preceding 5-28 days. 2
• Drug reactions can present with urticarial rash and facial erythema, though typically lack the spiking fever pattern of Still's disease. 1

Viral Exanthem

• Acute urticaria is often triggered by viruses in the pediatric population. 3
• However, viral exanthems typically do not present with the specific pattern of urticarial wheals on trunk plus intensely red cheeks combined with high spiking fevers. 1

Management Algorithm

If Still's Disease is Confirmed

• Hospitalization is warranted for severe presentations, especially with systemic symptoms or concern for MAS. 1
• Treatment decisions should be made rapidly for severe flares, favoring options that act quickly. 1
• Monitor closely for MAS development with serial laboratory workups throughout treatment course. 1

If Acute Urticaria Without Systemic Features

• First-line treatment is second-generation H1 antihistamines at standard doses. 1, 4, 5
• If inadequate response, titrate antihistamines up to 4 times the standard dose before adding additional agents. 1, 5
• Avoid oral antihistamines alone if there is any concern for progression to anaphylaxis—severe respiratory/cardiovascular symptoms can appear suddenly even after hives disappear. 1

Critical Pitfalls to Avoid

• Do not dismiss this as simple urticaria if fever is present—the combination of urticarial rash, facial erythema, and fever strongly suggests Still's disease. 1
• Do not delay workup for inflammatory markers and ferritin, as early diagnosis of Still's disease allows for prompt treatment and MAS surveillance. 1
• Do not assume benign etiology without documenting the complete clinical picture including fever pattern, joint symptoms, and inflammatory markers. 1
• If uncertain whether this represents anaphylaxis versus urticaria, err on the side of administering epinephrine—it is safer to inject epinephrine and observe than to withhold it in a potentially life-threatening situation. 1