Vincristine-Induced Unilateral Foot Drop in a Patient With Diffuse Large B-Cell Lymphoma: A Rare Presentation of Severe Motor Axonopathy.

Vincristine is a cornerstone chemotherapeutic agent in combination regimens for lymphoid malignancies; however, its neurotoxicity remains a major dose-limiting adverse effect. While vincristine-induced peripheral neuropathy (VIPN) commonly presents as a symmetric, sensory-predominant polyneuropathy, isolated unilateral motor neuropathy causing foot drop is exceedingly rare. We report a case of a 64-year-old male with extranodal diffuse large B-cell lymphoma (DLBCL) of the left calf muscle, staged as Ann Arbor stage IE with Revised International Prognostic Index (R-IPI) score of 2, who developed left-sided foot drop following the second cycle of dose-adjusted R-DA-EPOCH chemotherapy. Neurological examination confirmed isolated left peroneal nerve palsy with mild sensory involvement, and nerve conduction studies revealed motor axonal neuropathy consistent with vincristine toxicity. MRI excluded local compressive or infiltrative pathology and demonstrated interval tumor response. Vincristine dosing did not exceed the capped dose of 2 mg during the first two cycles; from the third cycle onward, the dose was reduced by approximately 60%. Early physiotherapy, ankle-foot orthosis support, and symptomatic pharmacologic therapy were initiated, leading to gradual neurological recovery. This case highlights the importance of early recognition of vincristine-induced neuropathy and timely dose adjustment to prevent irreversible deficits while maintaining therapeutic efficacy.