Case Report: Pediatric B-cell acute lymphoblastic leukemia presenting with recurrent seizures and persistent peripheral blood blast negativity

Background Central nervous system-predominant pediatric B-cell acute lymphoblastic leukemia (B-ALL) may present without peripheral blood abnormalities, and corticosteroid exposure can cause false-negative CSF cytomorphology. Case presentation A 5-year-and-7-month-old boy presented with recurrent seizures and cranial nerve deficits. Initial peripheral blood smears and CSF examinations were blast-negative. The interval between symptom onset and definitive diagnosis was 8 months; whether early symptoms were leukemia-attributable remains uncertain. In December 2019, repeated lumbar puncture revealed CSF pleocytosis (89 × 10 6 /L) with suspicious blasts. Bone marrow showed 81.5% blasts; flow cytometry confirmed B-lymphoid phenotype. CSF flow cytometry confirmed CNS3 status. KRAS G12 V (10.2% VAF) was detected. Peripheral blood remained blast-negative throughout. He achieved complete remission (morphological blasts <5%; MRD <0.01%) on the CCCG-ALL-2015 HR protocol. In March 2021, he developed isolated bone marrow relapse (morphological blasts 56%; MRD 50%) with negative CSF. Sequential CD19/CD22 CAR T therapy induced sustained remission. At the last follow-up (52 months post-CAR T), MRD remained below detection. Conclusions CNS-predominant B-ALL may present with recurrent seizures despite normal peripheral blood. Repeated CSF analysis and flow cytometry are essential when cytomorphology is negative. Sequential CD19/CD22 CAR T achieved durable remission in relapsed B-ALL with prior CNS involvement.