Atypical Adult-Onset Still's Disease Presenting With Seizures and Myocarditis Complicated by Macrophage Activation Syndrome.

Adult-onset Still's disease (AOSD) is a rare multi-systemic inflammatory disorder characterized by high spiking fevers, nonpruritic, salmon-colored rash, and severe polyarthralgia. Laboratory features typically include elevation in white blood cells, liver enzymes, and ferritin. Central nervous system and cardiac involvements, particularly myocarditis, are rare. Macrophage activation syndrome (MAS) is a well-described complication of AOSD, leading to a high mortality rate. Herein, we describe a case of AOSD complicated by MAS in a 32-year-old male presenting with atypical clinical manifestations, including recurrent seizures, scaly, pruritic, and hyperpigmented rash, and right heart failure due to lymphocytic myocarditis. The patient exhibited a delayed onset of fever, leukocytosis, and transaminitis that initially deterred eligibility for Yamaguchi criteria for AOSD. Bone marrow and lymph node biopsies did not show malignancy, infection, or hemophagocytosis. However, soluble interleukin-2 receptor alpha or soluble CD-25 was elevated. The patient experienced significant improvement on combination therapy of anakinra, methotrexate, and stress-dose steroids. HScore was later indicative of a high probability for MAS. Outpatient management involved prednisone, cyclosporine, and canakinumab for MAS. Seizure and myocarditis are possible presenting features of atypical AOSD. Early recognition of non-criteria AOSD and MAS and prompt initiation of therapy may prevent mortality.