1525: Anti-Crmp5 Paraneoplastic Neurological Syndrome With Mac Pneumonia and Occult Small Cell Lung Cancer

Introduction: Anti-collapsin response-mediator protein-5 (CRMP5) antibody-associated paraneoplastic encephalitis is rare and classically linked to small cell lung cancer (SCLC) or thymoma. Presentations are heterogeneous and may include limbic encephalitis, seizures, ataxia, polyneuropathy, ophthalmoplegia, and choreiform movements. We present a diagnostically challenging case complicated by both delayed antibody detection and coexisting pulmonary Mycobacterium Avium Complex (MAC) infection, which initially obscured a diagnosis of SCLC. Description: A 66-year-old woman with COPD and polysubstance use disorder developed new-onset refractory status epilepticus requiring aggressive treatment with anesthetics. Brain imaging revealed multifocal non-enhancing T2 hyperintensities. Cerebrospinal fluid (CSF) was notable for elevated protein with negative infectious and autoimmune panels. She received IV steroids and plasma exchange with minimal improvement. Chest imaging identified a right upper lobe cavitary lesion with a positive acid-fast bacillus smear, and cultures ultimately confirmed MAC. After one month, she was discharged to a long-term acute care hospital (LTACH) with tracheostomy and PEG. At LTACH, arousal improved but she developed worsening choreiform movements and ataxia. Her third CSF autoimmune panel, performed prior to discharge, returned positive for anti-CRMP5 antibodies (1:256), prompting readmission. Exam revealed an alert, elderly woman with tracheostomy, following commands with ophthalmoplegia, asymmetric choreiform movements and diffuse dysmetria. MRI brain showed progression of T2 hyperintensities and a new enhancing lesion. Given the strong association of this antibody with SCLC, lymph node biopsy was pursued and confirmed SCLC. She was diagnosed with CRMP5 paraneoplastic syndrome, SCLC, and superimposed MAC pneumonia. She underwent repeat immunotherapy and started chemotherapy. Discussion: This case illustrates the diagnostic complexity of anti-CRMP5 encephalitis with delayed antibody detection and underlying malignancy masked by MAC pneumonia. The presence of anti-CRMP5 antibodies led to re-evaluation for malignancy despite an existing diagnosis. Clinicians should maintain high suspicion for SCLC in patients with CRMP5 encephalitis, even if alternate systemic processes are present.