Bilateral Facial Nerve Palsy Revealing Diffuse Large B-Cell Lymphoma With Leptomeningeal Involvement.

We present a diagnostically challenging case of a 54-year-old woman who developed progressive bilateral lower motor neuron (LMN) facial nerve palsy, later accompanied by lower limb weakness and impaired dexterity. The initial presentation was misattributed to idiopathic Bell's palsy, which delayed recognition of a more serious underlying condition. Neuroimaging revealed enhancement of multiple cranial nerves and diffuse leptomeningeal involvement. Cerebrospinal fluid (CSF) analysis demonstrated lymphocytic pleocytosis, elevated protein levels, low glucose, and increased lactate dehydrogenase (LDH), findings suggestive of a malignant infiltrative process. Additional imaging of the spine and plexus showed diffuse marrow signal abnormalities, cauda equina enhancement, and a right C5 nerve root mass with bony erosion. A diagnosis of stage IVA diffuse large B-cell lymphoma (DLBCL) with central nervous system (CNS) and leptomeningeal involvement was established during admission. The patient was referred to hematology, and a multidisciplinary team recommended systemic chemotherapy with methotrexate, cytarabine, thiotepa, and rituximab (MATRix) or rituximab, ifosfamide, carboplatin, and etoposide (RICE), followed by autologous stem cell transplantation for consolidation. This case highlights a rare presentation of CNS lymphoma and underscores the importance of early consideration of malignancy in patients presenting with progressive cranial neuropathies and polyradiculopathy, particularly in the absence of constitutional or systemic symptoms. Timely neuroimaging and CSF analysis were instrumental in achieving an accurate diagnosis, allowing for early initiation of appropriate therapy in this aggressive disease.