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Introduction: Pediatric-onset systemic lupus erythematosus (SLE) may be particularly difficult to diagnose when the initial presentation is dominated by cerebrovascular events and fulfilment of other classification criteria may be secondary to infection or parainfectious immune activation.

Methods: We report an 11-year-old girl with 18p deletion syndrome and immune dysregulation who initially presented with progressive gait disturbance and hemiparesis. Brain MRI revealed multiple subacute multifocal cerebral infarcts. Echocardiography showed a small vegetation on the mitral valve. Differential diagnoses included Bartonella endocarditis, Libman–Sacks endocarditis in the context of SLE, and antiphospholipid (AP) antibody-associated cerebrovascular disease. In parallel, leukopenia with lymphopenia, thrombocytopenia, hypocomplementemia, antinuclear antibodies, initially low-level anti-dsDNA/nucleosome antibodies and AP antibodies were detected, while Crithidia lucillae Immunofluorescence testing (CLIFT) remained negative. There was no fever, arthritis, serositis, nephritis, or typical cutaneous lupus manifestation. At presentation, the fulfilment of SLE classification criteria were interpreted cautiously because they could also be attributed to secondary or parainfectious immune activation.

Results: Because the initial working diagnosis remained stroke related either to Bartonella endocarditis or to SLE with Libman–Sacks endocarditis and AP syndrome, treatment was initially directed against Bartonella and combined with anticoagulation. Serial reassessment changed the interpretation fundamentally. A follow-up MRI did not reveal definitive findings and failed to detect signs of small vessel vasculitis. The diagnosis was ultimately established by digital subtraction angiography (DSA), which clearly demonstrated small vessel involvement. Notably, CLIFT remained negative despite positive anti-dsDNA by enzyme-linked immunosorbent assay, possibly reflecting impaired antibody avidity maturation in the context of 18p deletion-associated immune dysregulation. Persistent lupus activity, papilledema with visual impairment, and a high Systemic Lupus Eryhtematosus Activity Index supported the diagnosis of neuropsychiatric SLE with central nervous system (CNS) vasculitis. Immunosuppressive treatment with high-dose methylprednisolone, mycophenolate mofetil, hydroxychloroquine, and subsequently rituximab led to clinical improvement. The mitral lesion remained unchanged on serial echocardiography, arguing against an active infectious or an active lupus-related valvular process.

Conclusion: This case highlights the diagnostic complexity of pediatric stroke when infectious, thrombotic, and autoimmune features overlap. Faced with a plethora of abnormal findings but no definitive diagnosis, treatment was initially directed at the most acute threat while systematically reassessing competing hypotheses. Ultimately, DSA – detecting small vessel vasculitis not visible on conventional MRI – revealed CNS vasculitis as the dominant mechanism, leading to reinterpretation as neuropsychiatric SLE and a change in treatment strategy.