Diagnosis: Adult-Onset Still's Disease (AOSD)
This clinical presentation is most consistent with Adult-Onset Still's Disease, characterized by the classic triad of high-spiking fever, salmon-pink evanescent rash, and arthralgia/myalgia, along with mouth ulcers. 1, 2
Clinical Features Supporting AOSD Diagnosis
The patient presents with the hallmark manifestations of AOSD:
Fever: Daily high-spiking fever (typically ≥39°C) is present in 95-100% of AOSD cases and represents a cardinal feature 1, 2
Characteristic Rash: The pruritic generalized rash with palmar involvement is consistent with the salmon-pink, evanescent maculopapular eruption seen in 72.7% of AOSD patients (range 51-87% across major series) 1. The rash is typically pruritic, predominantly affects proximal limbs and trunk, and can be confused with drug allergy 1
Joint Involvement: Arthralgia and myalgia are present in 64-100% of AOSD cases, with polyarticular involvement being typical 1
Myalgia: Generalized myalgia occurs in 56-84% of cases and typically appears with fever exacerbations 1
Mouth Ulcers: Oral manifestations can occur in AOSD, though sore throat is more commonly reported (38-92% of cases) 1
Diagnostic Workup
Immediate laboratory investigations should include:
Complete blood count: Look for leukocytosis (often >15×10⁹/L in 50% of patients, with 37% having WBC >20×10⁹/L), neutrophilia, anemia of chronic disease, and thrombocytosis 1, 2
Inflammatory markers: ESR and CRP are elevated in virtually all patients 1, 2
Serum ferritin: Markedly elevated levels (often 4,000-30,000 ng/mL, sometimes up to 250,000 ng/mL) with 80% sensitivity when elevated 5-fold above normal 1, 2
Glycosylated ferritin fraction: When <20% combined with 5-fold ferritin elevation, sensitivity is 43% but specificity rises to 93% 1
Liver function tests: Hepatomegaly and abnormal liver biochemistry occur in 50-75% of patients 1, 2
IL-18 and/or S100 proteins (calprotectin): Recommended by 2024 EULAR/PReS guidelines to support diagnosis 2
Critical Differential Diagnoses to Exclude
Rocky Mountain Spotted Fever (RMSF) must be ruled out urgently given the overlapping features of fever, rash with palmar involvement, myalgia, and potential mortality of 5-10% if untreated 1. However, RMSF typically:
- Has a 3-12 day incubation period after tick exposure 1
- Rash begins on ankles/wrists/forearms and spreads centrally, appearing 2-4 days after fever onset 1
- Becomes petechial by day 5-6 (not pruritic) 1
- Causes thrombocytopenia and hyponatremia 1
Other important exclusions:
Systemic Lupus Erythematosus: Check ANA, anti-dsDNA, complement levels. SLE typically has positive ANA with diverse systemic manifestations 3, 4
Infections: Blood cultures, throat culture for Group A Streptococcus (to exclude acute rheumatic fever, though erythema marginatum is non-pruritic) 5, 6
Malignancy and other immune-mediated diseases: Must be carefully excluded per 2024 EULAR/PReS guidelines 2
Management Approach
Initial treatment for AOSD:
NSAIDs: First-line therapy for mild disease, though liver dysfunction may be a cofactor requiring monitoring 1
Corticosteroids: Oral prednisolone 0.5-1 mg/kg/day for moderate to severe disease 1
Monitor for Macrophage Activation Syndrome (MAS): A life-threatening complication that can occur at onset, during treatment, or in remission. Watch for pancytopenia, which necessitates prompt immunosuppressive treatment 1, 2
Critical Pitfalls to Avoid
Do not wait for arthritis to develop: The Yamaguchi criteria do not require arthritis for AOSD diagnosis, and delaying diagnosis while waiting for arthritis can lead to unnecessary and potentially deleterious delays 2
Do not dismiss the diagnosis based on age: While called "adult-onset," this can occur in young adults like this 25-year-old patient 1
Do not attribute liver dysfunction solely to NSAIDs: Hepatic involvement is part of AOSD itself in 50-75% of cases 1
Monitor for MAS continuously: This complication requires vigilance throughout the disease course, not just at presentation 1, 2