Rheumatoid Meningoencephalitis Diagnosed with CSF ACPA Index: A Case Report

Background: Rheumatoid meningitis is a rare but serious extra-articular manifestation of rheumatoid arthritis. Its diagnosis is challenging due to nonspecific clinical features and the need to exclude infectious and neoplastic causes. We present this case to underscore the diagnostic utility of the cerebrospinal fluid (CSF) anti-cyclic citrullinated peptide antibody (ACPA) index, a biomarker seldom reported in Latin America. Case presentation: A 77-year-old Peruvian woman with long-standing rheumatoid arthritis on immunosuppressive therapy developed progressive headache, paroxysmal movements, altered mental status, dysphagia, and motor weakness. CSF analysis revealed elevated protein and a markedly increased ACPA index (26.3; normal <1.3), strongly suggestive of rheumatoid meningitis, while extensive infectious and neoplastic workup was negative. High-dose intravenous methylprednisolone followed by rituximab led to rapid seizure control, improvement in consciousness and swallowing, and progressive functional recovery over three months. Conclusions: This case highlights the need to consider rheumatoid meningitis in the differential diagnosis of chronic meningitis in immunosuppressed patients, especially in regions with high prevalence of infectious diseases. The CSF ACPA index emerges as a promising diagnostic tool that may enable timely immunosuppressive treatment and favorable neurological outcomes.