Fever of unknown origin, muscle weakness, and an adrenal nodule—the cat we chased like a hare: a case report

Abstract

Abstract Background Fever of unknown origin remains a significant clinical challenge in hospitalized patients worldwide. In this report, we describe a case of fever of unknown origin in which the principal radiological finding was an adrenal nodule exhibiting a hyperintense signal on contrast-enhanced T2-weighted magnetic resonance imaging. While such description is characteristic of pheochromocytoma or malignant adrenal lesions, this report aims to explore alternative diagnostic considerations in light of the clinical context. Case presentation A 70-year-old white male Brazilian was admitted to a university hospital for investigation of persistent fever and lack of response to systemic antibiotic therapy. He reported experiencing daily episodes of evening fever, reaching up to 39 °C, for approximately 6 months. He also experienced significant unintentional weight loss of approximately 15 kg, accompanied by progressive asthenia that evolved into marked lower limb weakness (muscle strength grade 4 in lower limbs, deep reflexes preserved). An extensive infectious investigation was performed: serial blood, urine, and otologic secretion cultures were repeatedly negative. Transesophageal echocardiography excluded infectious endocarditis. Screening for rheumatologic disease, including Antinuclear antibodies, anti-cyclic citrullinated peptide antibodies, anti-neutrophil cytoplasmic antibody, and screening for antiphospholipid syndrome were negative. Subsequent abdominal magnetic resonance imaging identified a 0.7 cm left adrenal nodule with T2 hyperintensity and arterial/venous enhancement. A diagnostic adrenalectomy was performed, and histopathological analysis revealed necrotizing vasculitis with thrombosis of peri-adrenal fat vessels, accompanied by lymphocytic and neutrophilic infiltrates, without granulomas or giant cells—findings consistent with vasculitis, most compatible with polyarteritis nodosa. Following an extensive diagnostic workup by the clinical team, the patient achieved full recovery after treatment with high-dose pulse corticosteroid therapy and immunosuppressive agent. Conclusions This case report underscores the critical importance of continuously revisiting diagnostic hypotheses in complex cases, maintaining vigilance for atypical and uncommon etiologies to prevent misdiagnosis and delays in appropriate therapy.