Early-Onset Medullocervical Low-Grade Glioma With FGFR1 Mutation and Leptomeningeal Spread in an Infant: A Case Report.

Pediatric low-grade gliomas (PLGGs) are generally slow-growing tumors associated with favorable long-term outcomes. However, their occurrence in early infancy is rare, particularly when arising in the posterior fossa with extensive dissemination and hydrocephalus. Advances in molecular profiling have identified specific genetic subtypes, including FGFR1-mutated gliomas, which may demonstrate more aggressive clinical behavior than suggested by histology alone. We report a four-month-old female who presented with signs of increased intracranial pressure and sunsetting eyes. Imaging revealed a heterogeneously enhancing exophytic medullary mass with a large cystic component causing tetraventricular hydrocephalus. Following ventriculoperitoneal shunt placement and surgical decompression, histology confirmed a low-grade glioneuronal tumor with low proliferative activity. Molecular analysis identified an FGFR1 mutation and 18q13 deletion, and methylation profiling classified the tumor within the MYB(L1)-family subtype B. Despite benign histologic features, the tumor progressed with cervical cord extension and diffuse spinal leptomeningeal metastases. Targeted therapy with trametinib achieved partial radiologic response before further progression. The patient remains clinically stable under ongoing therapy and multidisciplinary care. This case underscores the critical role of molecular diagnostics in risk stratification and treatment selection, particularly in infants with atypically aggressive PLGG.